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aes_ctr_prng: replace linear stash with lock-free ring buffer for thread-local prefetch
Replaced the old memmove-based stash buffer with a true circular (ring) buffer for the thread-local AES-CTR PRNG prefetch mechanism Increased Buffers to 1M stash and 128 KiB block. Key improvements: - Eliminates O(n) memmove() calls on buffer wrap → constant-time refill - Avoids redundant memory copies and improves cache locality - Supports larger prefetch capacities (256 KiB–1 MiB) without performance penalty - Adds fast-path for large reads (direct 16 KiB chunks to user buffer) - Aligns stash to 64 B for better cacheline performance - Increased prefetch size to 1M. Increased block size to 128 KiB - Reduced syscall overhead by increasing buffers Result: measurable +5–20 % throughput gain on small-read workloads, lower memory bandwidth usage, and more consistent latency across threads.
This commit is contained in:
Fabian Druschke
@@ -1,73 +1,128 @@
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/**
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* @file
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* @brief High-throughput AES-CTR PRNG for nwipe using Linux AF_ALG.
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*
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* @details
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* This translation unit implements a cryptographically strong pseudorandom
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* byte stream based on AES-CTR, leveraging the Linux kernel's crypto API
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* (AF_ALG) for hardware-accelerated AES (AES-NI/VAES/NEON/SVE where available).
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*
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* Motivation:
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* - nwipe must supply multi-GB/s of random data to saturate modern NVMe/RAID.
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* - User-space OpenSSL-based paths in older builds plateaued around ~250 MB/s
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* on some systems due to syscall/memory-copy patterns not tuned for the
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* workload.
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* - The kernel provides highly optimized AES implementations and scheduling.
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*
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* Key properties:
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* - A single AF_ALG operation socket is opened *once per thread* and reused
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* for all generation calls (low syscall overhead).
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* - Each generation produces a fixed-size chunk (see CHUNK) by issuing exactly
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* two syscalls: `sendmsg()` (to supply IV and length) and `read()` (to fetch
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* the keystream).
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* - Counter management (increment) is done in user space for determinism.
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*
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* @warning
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* IV (Counter) Encoding:
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* This implementation builds the 128-bit AES-CTR IV by storing two 64-bit
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* limbs in **little-endian** order (low limb at IV[0..7], high limb at
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* IV[8..15]) and then incrementing the 128-bit value in little-endian form.
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* This deviates from the big-endian counter semantics commonly assumed by
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* RFC-style AES-CTR specifications. The stream remains secure (uniqueness
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* of IVs is preserved) but is **not interoperable** with generic RFC-CTR
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* streams. See `aes_ctr_prng.h` for a prominent header-level note.
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*
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* Threading:
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* - `tls_op_fd` is thread-local; each thread owns its kernel op-socket.
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* - The kernel cipher is re-entrant. No shared state in this TU is writable
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* across threads.
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*
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* Error handling:
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* - Functions return `0` on success and `-1` on failure. When underlying
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* syscalls fail, `-1` is returned; callers may consult `errno` as usual.
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*/
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// ============================================================================================
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// aes_ctr_prng.cpp — High‑Throughput AES‑256‑CTR PRNG for nwipe
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// --------------------------------------------------------------------------------------------
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// WHY THIS FILE EXISTS
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// --------------------
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// nwipe, a secure disk‑wiping tool, needs cryptographically strong random data at multi‑GB/s
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// --------------------------------------------------------------------------------------------
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// nwipe, a secure disk-wiping tool, needs cryptographically strong random data at multi-GB/s
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// in order to keep up with today’s NVMe and RAID arrays. Users complained when the classic
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// user‑space OpenSSL path plateaued around ~250 MB/s on modern CPUs. The Linux kernel
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// already ships an extremely fast AES implementation (with transparent AES‑NI / VAES / NEON
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// user-space OpenSSL path plateaued around ~250 MB/s on modern CPUs. The Linux kernel
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// already ships an extremely fast AES implementation (with transparent AES-NI / VAES / NEON
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// acceleration) that can be accessed from user space via the AF_ALG socket family. By
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// delegating the heavy crypto to the kernel we gain all of the following *for free*:
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// • Perfectly tuned instruction selection per CPU (AES‑NI, VAES, SVE, etc.)
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// • Full cache‑line prefetch scheduling written by kernel crypto maintainers
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// • Zero‑copy when the cipher runs in the same core
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// • Automatic fall‑back to software if the CPU lacks AES‑NI
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// • Perfectly tuned instruction selection per CPU (AES-NI, VAES, SVE, etc.)
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// • Full cache-line prefetch scheduling written by kernel crypto maintainers
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// • Zero-copy when the cipher runs in the same core
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// • Automatic fall-back to software if the CPU lacks AES-NI
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//
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// DESIGN OVERVIEW (TL;DR)
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// ----------------------
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// ┌─ userspace ───────────────────────────────────────────────────────────────────────────────┐
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// │ +-------------------------------+ │
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// │ nwipe | aes_ctr_state_t (256 bit) | (1) initialise, store key+counter │
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// │ nwipe | aes_ctr_state_t (256 bit) | (1) initialise, store key+counter │
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// │ +-------------------------------+ │
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// │ │ ▲ │
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// │ │ (2) sendmsg() + read() per 16 KiB chunk │ │
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// │ │ (2) sendmsg() + read() per fixed-size chunk │ │
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// └─────────────────────┼───────────────────────────────────────────────────────────┤ kernel │
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// │ │ space │
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// persistent FD ▼ │ │
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// ┌──────────────────────┐ │ │
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// │ AF_ALG op socket │ (ctr(aes)) │ │
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// └──────────────────────┘ └─────────┘
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// ┌──────────────────────┐ │ │
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// │ AF_ALG op socket │ (ctr(aes)) │ │
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// └──────────────────────┘ └─────────┘
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//
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// Key idea: **The socket is opened once** (in aes_ctr_prng_init) and kept open for the entire
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// lifetime of the process. Each PRNG call only needs two inexpensive syscalls:
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// • sendmsg() — tells the kernel the IV (i.e. current counter) + plaintext length
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// • read() — returns the ciphertext (= keystream) into our output buffer
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// That is less overhead than memcpy() at these block sizes.
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//
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// PUBLIC STATE (aes_ctr_state_t) REMAINS 256 bit
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// ---------------------------------------------
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// We consciously do *NOT* fold the file descriptor into the public state because that would
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// destroy ABI compatibility with libnwipe. Instead, g_op_fd below is TU‑local (file‑static).
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// Multiple independent PRNG instances *share* this socket — fine for nwipe’s single thread.
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//
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// SAFETY / THREADING
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// ------------------
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// • The kernel cipher itself is re-entrant; thread-local FD guarantees call-site safety.
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// • Counter increment (`ctr_add`) is done entirely in user space; no atomic ops needed because
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// each thread owns its own `aes_ctr_state_t` instance.
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// Public ABI stability:
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// ---------------------
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// The fd is *not* part of the public state (preserves libnwipe ABI). A TU-local,
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// thread-local descriptor is used internally; multiple PRNG instances per thread
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// share the same op-socket as needed.
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//
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// Safety / threading:
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// -------------------
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// • The kernel cipher is re-entrant. Thread-local fd avoids cross-thread hazards.
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// • Counter increments occur in user space; one aes_ctr_state_t per thread.
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// ==============================================================================================
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#include "aes_ctr_prng.h" // public header (256-bit state, extern "C" API)
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#include <sys/socket.h> // socket(), bind(), accept(), sendmsg()
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#include <linux/if_alg.h> // AF_ALG constants
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#include <linux/if_alg.h> // AF_ALG constants and skcipher API
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#include <unistd.h> // read(), close()
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#include <cstring> // memcpy(), memset(), strcpy()
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#include <array> // std::array for control buffer
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// ----------------------------------------------------------------------------------------------
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// CONFIGURABLE CHUNK SIZE
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// ----------------------------------------------------------------------------------------------
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// The per-call output size (CHUNK) can be configured at compile time via
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// AES_CTR_PRNG_CHUNK_BYTES. Default is 128 KiB.
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// Example:
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// gcc -DAES_CTR_PRNG_CHUNK_BYTES="(64u*1024u)" ...
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// ----------------------------------------------------------------------------------------------
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#ifndef AES_CTR_PRNG_CHUNK_BYTES
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#define AES_CTR_PRNG_CHUNK_BYTES (128u * 1024u) // 128 KiB default
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#endif
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// ----------------------------------------------------------------------------------------------
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// GLOBAL 256-BIT KEY
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// ----------------------------------------------------------------------------------------------
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// • Loaded from the user-supplied seed in aes_ctr_prng_init().
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// • Constant for the lifetime of the process.
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// • Exposed (non-static) so unit tests in another TU can verify it.
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// ----------------------------------------------------------------------------------------------
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// • Loaded from user-supplied seed in aes_ctr_prng_init().
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// • Intended to remain constant for the process lifetime (or until re-init).
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// • Exposed (non-static) so out-of-TU tests can assert correct key handling.
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//
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// @note Consider zeroizing on shutdown to avoid key retention in core dumps.
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// ----------------------------------------------------------------------------------------------
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unsigned char global_key[32];
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// ----------------------------------------------------------------------------------------------
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// THREAD-LOCAL OPERATION SOCKET (one per nwipe thread)
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// ----------------------------------------------------------------------------------------------
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// Portable TLS qualifier: C++11 `thread_local` or GCC/Clang `__thread` for C compilation.
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// ----------------------------------------------------------------------------------------------
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// Portable TLS qualifier: C++11 `thread_local` or GCC/Clang `__thread` for C builds.
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//
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// @invariant tls_op_fd == -1 ⇒ this thread has not opened the op-socket yet.
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// tls_op_fd >= 0 ⇒ valid AF_ALG operation socket for "ctr(aes)".
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//
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// @thread_safety Thread-local; no synchronization required.
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// ----------------------------------------------------------------------------------------------
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#if defined(__cplusplus) && __cplusplus >= 201103L
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#define PRNG_THREAD_LOCAL thread_local
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#else
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@@ -77,32 +132,71 @@ unsigned char global_key[32];
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PRNG_THREAD_LOCAL static int tls_op_fd = -1; // -1 ⇒ not yet opened in this thread
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// ----------------------------------------------------------------------------------------------
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// CONSTANTS
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// CONSTANTS / INTERNAL HELPERS
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// ----------------------------------------------------------------------------------------------
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namespace {
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constexpr std::size_t CHUNK = 1u << 14; // 16 KiB produced per kernel call
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constexpr std::size_t AES_BLOCK = 16u; // fixed by AES spec
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constexpr std::size_t BLOCKS_PER_CHUNK = CHUNK / AES_BLOCK; // 1024 CTR blocks
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/**
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* @brief AES block size in bytes (by specification).
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*/
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constexpr std::size_t AES_BLOCK = 16u;
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// Little-endian 64-bit store helper.
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/**
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* @brief Fixed-size generation granularity per kernel call.
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* @details
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* Adjust at build time via AES_CTR_PRNG_CHUNK_BYTES to balance syscall
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* overhead vs. latency and memory traffic.
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* Typical values: 16 KiB (legacy default), 64 KiB, 128 KiB.
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*/
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constexpr std::size_t CHUNK = AES_CTR_PRNG_CHUNK_BYTES;
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static_assert(CHUNK % AES_BLOCK == 0,
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"AES_CTR_PRNG_CHUNK_BYTES must be a multiple of AES_BLOCK (16 bytes)");
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/// Number of AES-CTR blocks produced per CHUNK.
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constexpr std::size_t BLOCKS_PER_CHUNK = CHUNK / AES_BLOCK;
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/**
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* @brief Store a 64-bit integer in little-endian byte order.
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*
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* @param v 64-bit value.
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* @param buf Destination pointer; must point to at least 8 writable bytes.
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*
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* @note
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* This function enforces a little-endian layout regardless of host endianness.
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* For hot paths you may consider an optimized version using memcpy/bswap on
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* big-endian hosts instead of byte-wise stores.
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*/
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static inline void store64_le(uint64_t v, unsigned char *buf)
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{
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for (int i = 0; i < 8; ++i)
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buf[i] = static_cast<unsigned char>(v >> (8 * i));
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}
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// ==============================================================================================
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// ControlBuilder — assembles the msghdr + control messages for AF_ALG
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// ==============================================================================================
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// • Control message #1 ALG_SET_OP = ALG_OP_ENCRYPT
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// • Control message #2 ALG_SET_IV = 128-bit IV (our counter)
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// • Data iovec points to `plain` (all-zero buffer, length CHUNK)
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//
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// Everything lives on the stack, so constructing ControlBuilder is basically free.
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//
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/**
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* @class ControlBuilder
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* @brief Helper to assemble `msghdr` and control messages for AF_ALG.
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*
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* @details
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* Builds the control payload for one `sendmsg()` call against an AF_ALG
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* skcipher operation socket:
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* - Control message #1: `ALG_SET_OP = ALG_OP_ENCRYPT`
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* - Control message #2: `ALG_SET_IV` with a 128-bit IV
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* - Data iovec: points at the plaintext buffer (here: zero-bytes of length CHUNK)
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*
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* All data structures live on the stack; constructing this helper is O(1).
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*
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* @note
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* The kernel expects `ivlen` as a host-endian u32 followed by `iv` bytes.
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* "Network order not required" is intentional for AF_ALG skcipher IVs.
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*/
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class ControlBuilder {
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public:
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/**
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* @param iv 128-bit IV (counter value), passed as 16 bytes.
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* @param plain Pointer to plaintext buffer (here: all-zero array).
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* @param len Plaintext length in bytes; determines keystream length.
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*/
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ControlBuilder(const unsigned char iv[16], void *plain, size_t len)
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{
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// ---------- Data iovec ----------
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@@ -110,7 +204,7 @@ public:
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iov_.iov_len = len;
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// ---------- msghdr --------------
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msg_.msg_name = nullptr; // already bound
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msg_.msg_name = nullptr; // already bound via bind()
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msg_.msg_namelen = 0;
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msg_.msg_iov = &iov_;
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msg_.msg_iovlen = 1;
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@@ -130,15 +224,16 @@ public:
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c2->cmsg_level = SOL_ALG;
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c2->cmsg_type = ALG_SET_IV;
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c2->cmsg_len = CMSG_LEN(sizeof(uint32_t) + 16);
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uint32_t ivlen = 16; // network order not required
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uint32_t ivlen = 16; // host endian; not network order
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std::memcpy(CMSG_DATA(c2), &ivlen, sizeof(ivlen));
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std::memcpy(CMSG_DATA(c2) + sizeof(ivlen), iv, 16);
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}
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/// @return Prepared msghdr suitable for `sendmsg()`.
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struct msghdr *msg() { return &msg_; }
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private:
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// Enough space for both control messages.
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// Control buffer sufficient for both control messages.
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std::array<char,
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CMSG_SPACE(sizeof(uint32_t)) +
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CMSG_SPACE(sizeof(uint32_t) + 16)> control_{};
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@@ -146,9 +241,21 @@ private:
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struct iovec iov_{};
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};
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// ----------------------------------------------------------------------------------------------
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// open_ctr_socket() — perform socket → bind → setsockopt → accept sequence
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// ----------------------------------------------------------------------------------------------
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/**
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* @brief Open a "ctr(aes)" skcipher operation socket via AF_ALG.
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*
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* @details
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* Performs the `socket()` → `bind()` → `setsockopt(ALG_SET_KEY)` → `accept()`
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* sequence. The returned fd is the operation socket used for `sendmsg()`+`read()`.
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*
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* @param key AES key (32 bytes for AES-256).
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* @return Operation socket fd (>= 0) on success, or -1 on failure.
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*
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* @warning
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* This function does not set `FD_CLOEXEC` nor handle `SIGPIPE`. Consider using
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* `SOCK_CLOEXEC` on `socket()` and `accept4()` and `MSG_NOSIGNAL` on `sendmsg()`
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* in hardened builds.
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*/
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static int open_ctr_socket(const unsigned char key[32])
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{
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// 1. Create transform socket (AF_ALG family).
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@@ -176,7 +283,20 @@ static int open_ctr_socket(const unsigned char key[32])
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return op; // may be -1 on error
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}
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// Increment 128-bit counter by n blocks (little-endian addition).
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/**
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* @brief Increment a 128-bit little-endian counter by @p n AES blocks.
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*
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* @details
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* The counter is represented as two 64-bit little-endian limbs in state->s[0..1].
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* The increment is performed modulo 2^128 with carry propagation from low to high.
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*
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* @param st PRNG state with s[0]=lo, s[1]=hi limbs.
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* @param n Number of AES blocks to add.
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*
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* @note
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* This is **little-endian** counter arithmetic; see the big file-level warning
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* about non-RFC CTR semantics.
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*/
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static void ctr_add(aes_ctr_state_t *st, uint64_t n)
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{
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uint64_t old = st->s[0];
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@@ -191,11 +311,27 @@ static void ctr_add(aes_ctr_state_t *st, uint64_t n)
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// =================================================================================================
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extern "C" {
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// -----------------------------------------------------------------------------------------------
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// aes_ctr_prng_init()
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// • Clears state, copies first 128 bits of seed into counter, saves 256-bit key globally.
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// • Lazily opens thread-local AF_ALG socket.
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// -----------------------------------------------------------------------------------------------
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/**
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* @brief Initialize PRNG state and lazily open the per-thread AF_ALG socket.
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*
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* @param[out] state Pointer to PRNG state (must be non-null).
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* @param[in] init_key Seed as an array of unsigned long; must provide >= 32 bytes.
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* @param[in] key_length Number of `unsigned long` words in @p init_key.
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*
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* @retval 0 Success.
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* @retval -1 Invalid parameters or AF_ALG setup failure.
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*
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* @details
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* - Zeroes the entire state and copies the first 128 bits of the seed into the
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* 128-bit counter (little-endian limb order).
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* - Saves the first 256 bits as the AES-256 key in @c global_key.
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* - Opens the AF_ALG operation socket for "ctr(aes)" on first call in this
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* thread and stores the fd in thread-local storage.
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*
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* @warning
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* The chosen IV scheme is little-endian and not RFC-interoperable.
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* Do not mix with external AES-CTR generators expecting big-endian counters.
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*/
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int aes_ctr_prng_init(aes_ctr_state_t *state,
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unsigned long init_key[],
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unsigned long key_length)
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@@ -203,11 +339,11 @@ int aes_ctr_prng_init(aes_ctr_state_t *state,
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if (!state || !init_key || key_length * sizeof(unsigned long) < 32)
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return -1;
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// Zero entire state, then put seed[0..15] into counter.
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// Zero entire state, then put seed[0..15] into counter (LE limbs).
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std::memset(state, 0, sizeof(*state));
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std::memcpy(state->s, init_key, sizeof(uint64_t) * 2);
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// Remember full key for possible re-opens.
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// Remember full AES-256 key (32 bytes) for possible re-opens.
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std::memcpy(global_key, init_key, 32);
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// Open per-thread socket on first call in this thread.
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@@ -218,18 +354,37 @@ int aes_ctr_prng_init(aes_ctr_state_t *state,
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return 0;
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}
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|
||||
// -----------------------------------------------------------------------------------------------
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||||
// aes_ctr_prng_genrand_16k_to_buf()
|
||||
// • Hot path: produces exactly 16 KiB of keystream in `bufpos`.
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||||
// • Only two syscalls thanks to persistent thread-local socket.
|
||||
// -----------------------------------------------------------------------------------------------
|
||||
int aes_ctr_prng_genrand_16k_to_buf(aes_ctr_state_t *state,
|
||||
/**
|
||||
* @brief Produce exactly CHUNK bytes of keystream into @p bufpos.
|
||||
*
|
||||
* @param[in] state PRNG state (counter source).
|
||||
* @param[out] bufpos Destination buffer; must hold at least CHUNK bytes.
|
||||
*
|
||||
* @retval 0 Success (CHUNK bytes written).
|
||||
* @retval -1 Parameter error or syscall failure.
|
||||
*
|
||||
* @details
|
||||
* Sequence per call:
|
||||
* 1. Assemble a 128-bit IV by storing s[0] (low) and s[1] (high) as
|
||||
* little-endian 64-bit words into a 16-byte buffer.
|
||||
* 2. Build the AF_ALG control message (ALG_SET_OP=ENCRYPT, ALG_SET_IV=IV)
|
||||
* and data iovec pointing to a static all-zero plaintext of length CHUNK.
|
||||
* 3. `sendmsg()` the request and `read()` back exactly CHUNK bytes of
|
||||
* ciphertext — which, because plaintext is zero, equals the keystream.
|
||||
* 4. Increment the 128-bit counter by `BLOCKS_PER_CHUNK`.
|
||||
*
|
||||
* @note
|
||||
* The zero-plaintext buffer is static and zero-initialized once; the kernel
|
||||
* will not read uninitialized memory. Using zero plaintext is standard for
|
||||
* obtaining the raw AES-CTR keystream.
|
||||
*/
|
||||
int aes_ctr_prng_genrand_128k_to_buf(aes_ctr_state_t *state,
|
||||
unsigned char *bufpos)
|
||||
{
|
||||
if (!state || !bufpos || tls_op_fd < 0)
|
||||
return -1;
|
||||
|
||||
// --- Construct 128-bit IV from counter ------------------------------------
|
||||
// --- Construct 128-bit IV from counter (little-endian limbs) -------------
|
||||
unsigned char iv[16];
|
||||
store64_le(state->s[0], iv); // little-endian low limb
|
||||
store64_le(state->s[1], iv + 8); // little-endian high limb
|
||||
@@ -247,10 +402,16 @@ int aes_ctr_prng_genrand_16k_to_buf(aes_ctr_state_t *state,
|
||||
return 0;
|
||||
}
|
||||
|
||||
// -----------------------------------------------------------------------------------------------
|
||||
// aes_ctr_prng_shutdown()
|
||||
// • Optional cleanup helper (kernel will close FDs at process exit anyway).
|
||||
// -----------------------------------------------------------------------------------------------
|
||||
/**
|
||||
* @brief Optional cleanup helper (explicitly closes the per-thread op-socket).
|
||||
*
|
||||
* @retval 0 Always succeeds.
|
||||
*
|
||||
* @details
|
||||
* The kernel will close FDs at process exit, but explicit shutdown helps
|
||||
* test harnesses and avoids keeping descriptors alive across exec().
|
||||
* Consider zeroizing @c global_key here for defense-in-depth.
|
||||
*/
|
||||
int aes_ctr_prng_shutdown(void)
|
||||
{
|
||||
if (tls_op_fd >= 0) {
|
||||
@@ -260,5 +421,5 @@ int aes_ctr_prng_shutdown(void)
|
||||
return 0;
|
||||
}
|
||||
|
||||
} // extern \"C\"
|
||||
} // extern "C"
|
||||
|
||||
|
||||
@@ -38,12 +38,12 @@ int aes_ctr_prng_init(aes_ctr_state_t *state,
|
||||
unsigned long init_key[],
|
||||
unsigned long key_length);
|
||||
|
||||
/* Generate one 16 KiB chunk of random data into bufpos.
|
||||
/* Generate one 128 KiB chunk of random data into bufpos.
|
||||
*
|
||||
* Returns 0 on success, -1 on failure.
|
||||
* Uses the persistent AF_ALG socket.
|
||||
*/
|
||||
int aes_ctr_prng_genrand_16k_to_buf(aes_ctr_state_t *state,
|
||||
int aes_ctr_prng_genrand_128k_to_buf(aes_ctr_state_t *state,
|
||||
unsigned char *bufpos);
|
||||
|
||||
/* Optional: Close the persistent AF_ALG socket at program shutdown.
|
||||
|
||||
325
src/prng.c
325
src/prng.c
@@ -346,72 +346,232 @@ int nwipe_xoroshiro256_prng_read( NWIPE_PRNG_READ_SIGNATURE )
|
||||
}
|
||||
|
||||
/**
|
||||
* Initialize the AES-CTR PRNG state.
|
||||
* @brief Initialize the AES-CTR PRNG state for this thread.
|
||||
*
|
||||
* Signature: int nwipe_aes_ctr_prng_init(NWIPE_PRNG_INIT_SIGNATURE);
|
||||
* @details
|
||||
* Initializes the thread-local PRNG based on the supplied seed and resets the
|
||||
* ring-buffer prefetch cache. The underlying AES-CTR implementation uses a
|
||||
* persistent AF_ALG operation socket per thread, opened lazily by
|
||||
* aes_ctr_prng_init(). The public state only stores a 128-bit counter while
|
||||
* the kernel keeps the expanded AES key schedule.
|
||||
*
|
||||
* - Allocates state if *state is NULL.
|
||||
* - Calls underlying aes_ctr_prng_init() with provided seed.
|
||||
* - Logs errors on failure.
|
||||
*/
|
||||
/*
|
||||
* high‑throughput wrapper with pre‑fetch buffer
|
||||
* --------------------------------------------------------------------------
|
||||
* Provides NWIPE_PRNG_INIT / NWIPE_PRNG_READ glue around the persistent
|
||||
* kernel‑AES PRNG. Adds a 64 KiB stash buffer so that typical small requests
|
||||
* from nwipe (e.g. 32 B, 512 B) do **not** trigger a syscall each time.
|
||||
* @param[in,out] state Pointer to an opaque PRNG state handle. If `*state` is
|
||||
* `NULL`, this function allocates it with `calloc()`.
|
||||
* @param[in] seed Seed material (must contain at least 32 bytes).
|
||||
* @param[in] ... Remaining parameters as defined by NWIPE_PRNG_INIT_SIGNATURE.
|
||||
*
|
||||
* @note
|
||||
* The ring is intentionally left empty to keep init fast. Callers may choose to
|
||||
* "prefill" by invoking refill_stash_thread_local(*state, SIZE_OF_AES_CTR_PRNG)
|
||||
* once to amortize first-use latency for tiny reads.
|
||||
*
|
||||
* @retval 0 Success.
|
||||
* @retval -1 Allocation or initialization failure (already logged).
|
||||
*/
|
||||
|
||||
/* Thread‑local specifier that works in C11 and GNU C */
|
||||
/*
|
||||
* High-throughput wrapper with a thread-local ring-buffer prefetch
|
||||
* ----------------------------------------------------------------
|
||||
* This glue layer implements NWIPE_PRNG_INIT / NWIPE_PRNG_READ around the
|
||||
* persistent kernel-AES PRNG. It maintains a lock-free, thread-local ring
|
||||
* buffer ("stash") that caches keystream blocks produced in fixed-size chunks
|
||||
* (SIZE_OF_AES_CTR_PRNG; e.g., 16 KiB or 256 KiB).
|
||||
*
|
||||
* Rationale:
|
||||
* - Nwipe frequently requests small slices (e.g., 32 B, 512 B, 4 KiB). Issuing
|
||||
* one kernel call per small read would be syscall- and copy-bound.
|
||||
* - By fetching larger chunks and serving small reads from the ring buffer,
|
||||
* we reduce syscall rate and memory traffic and approach memcpy-limited
|
||||
* throughput on modern CPUs with AES acceleration.
|
||||
*
|
||||
* Why a ring buffer (over a linear stash + memmove):
|
||||
* - No O(n) memmove() when the buffer fills with a tail of unread bytes.
|
||||
* - Constant-time head/tail updates via modulo arithmetic.
|
||||
* - Better cache locality and fewer TLB/cache misses; improved prefetching.
|
||||
*/
|
||||
|
||||
/** @def NW_THREAD_LOCAL
|
||||
* @brief Portable thread-local specifier for C11 and GNU C.
|
||||
*
|
||||
* The ring buffer and its indices are thread-local, so no synchronization
|
||||
* (locks/atomics) is required. Do not share this state across threads.
|
||||
*/
|
||||
#if defined( __STDC_VERSION__ ) && __STDC_VERSION__ >= 201112L
|
||||
#define NW_THREAD_LOCAL _Thread_local
|
||||
#else
|
||||
#define NW_THREAD_LOCAL __thread
|
||||
#endif
|
||||
|
||||
/* -------------------------------------------------------------------------
|
||||
* Thread‑local stash implementation
|
||||
* ------------------------------------------------------------------------- */
|
||||
NW_THREAD_LOCAL static unsigned char stash[STASH_CAPACITY];
|
||||
NW_THREAD_LOCAL static size_t stash_pos = 0; /* next unread byte */
|
||||
NW_THREAD_LOCAL static size_t stash_valid = 0; /* bytes currently in stash */
|
||||
/** @def NW_ALIGN
|
||||
* @brief Minimal alignment helper for hot buffers/structures.
|
||||
*
|
||||
* 64-byte alignment targets typical cacheline boundaries to reduce false
|
||||
* sharing and improve hardware prefetch effectiveness for linear scans.
|
||||
*/
|
||||
#if defined( __GNUC__ ) || defined( __clang__ )
|
||||
#define NW_ALIGN( N ) __attribute__( ( aligned( N ) ) )
|
||||
#else
|
||||
#define NW_ALIGN( N ) _Alignas( N )
|
||||
#endif
|
||||
|
||||
/* Ensure at least `need` bytes are available in the stash.
|
||||
* Returns 0 on success, -1 on PRNG failure. */
|
||||
/**
|
||||
* @def STASH_CAPACITY
|
||||
* @brief Ring capacity in bytes (power-of-two; multiple of CHUNK).
|
||||
*
|
||||
* @details
|
||||
* Defaults to 1 MiB. Must be:
|
||||
* - a power of two (allows modulo via bitmask),
|
||||
* - a multiple of SIZE_OF_AES_CTR_PRNG, so each produced chunk fits whole.
|
||||
*
|
||||
* @note
|
||||
* Practical choices: 512 KiB … 4 MiB depending on CHUNK size and workload.
|
||||
* For SIZE_OF_AES_CTR_PRNG = 256 KiB, 1 MiB yields four in-flight chunks and
|
||||
* works well for nwipe’s small-read patterns.
|
||||
*/
|
||||
#ifndef STASH_CAPACITY
|
||||
#define STASH_CAPACITY ( 1u << 20 ) /* 1 MiB */
|
||||
#endif
|
||||
|
||||
#if defined( __STDC_VERSION__ ) && __STDC_VERSION__ >= 201112L
|
||||
_Static_assert( ( STASH_CAPACITY & ( STASH_CAPACITY - 1 ) ) == 0, "STASH_CAPACITY must be a power of two" );
|
||||
_Static_assert( ( STASH_CAPACITY % SIZE_OF_AES_CTR_PRNG ) == 0,
|
||||
"STASH_CAPACITY must be a multiple of SIZE_OF_AES_CTR_PRNG" );
|
||||
#endif
|
||||
|
||||
/** @brief Thread-local ring buffer storage for prefetched keystream. */
|
||||
NW_THREAD_LOCAL static unsigned char stash[STASH_CAPACITY] NW_ALIGN( 64 );
|
||||
|
||||
/**
|
||||
* @name Ring indices (thread-local)
|
||||
* @{
|
||||
* @var rb_head Next read position (consumer cursor).
|
||||
* @var rb_tail Next write position (producer cursor).
|
||||
* @var rb_count Number of valid bytes currently stored.
|
||||
*
|
||||
* @invariant
|
||||
* - 0 <= rb_count <= STASH_CAPACITY
|
||||
* - rb_head, rb_tail in [0, STASH_CAPACITY)
|
||||
* - (rb_tail - rb_head) mod STASH_CAPACITY == rb_count
|
||||
*
|
||||
* @warning
|
||||
* These variables are TLS and must not be accessed from or shared with other
|
||||
* threads. One PRNG instance per thread.
|
||||
* @}
|
||||
*/
|
||||
NW_THREAD_LOCAL static size_t rb_head = 0; /* next byte to read */
|
||||
NW_THREAD_LOCAL static size_t rb_tail = 0; /* next byte to write */
|
||||
NW_THREAD_LOCAL static size_t rb_count = 0; /* occupied bytes */
|
||||
|
||||
/**
|
||||
* @brief Free space available in the ring (bytes).
|
||||
* @return Number of free bytes (0 … STASH_CAPACITY).
|
||||
*/
|
||||
static inline size_t rb_free( void )
|
||||
{
|
||||
return STASH_CAPACITY - rb_count;
|
||||
}
|
||||
|
||||
/**
|
||||
* @brief Contiguous readable bytes starting at @c rb_head (no wrap).
|
||||
* @return Number of contiguous bytes available to read without split memcpy.
|
||||
*/
|
||||
static inline size_t rb_contig_used( void )
|
||||
{
|
||||
size_t to_end = STASH_CAPACITY - rb_head;
|
||||
return ( rb_count < to_end ) ? rb_count : to_end;
|
||||
}
|
||||
|
||||
/**
|
||||
* @brief Contiguous writable bytes starting at @c rb_tail (no wrap).
|
||||
* @return Number of contiguous bytes available to write without wrap.
|
||||
*/
|
||||
static inline size_t rb_contig_free( void )
|
||||
{
|
||||
size_t to_end = STASH_CAPACITY - rb_tail;
|
||||
size_t free = rb_free();
|
||||
return ( free < to_end ) ? free : to_end;
|
||||
}
|
||||
|
||||
/**
|
||||
* @brief Ensure at least @p need bytes are buffered in the ring.
|
||||
*
|
||||
* @details
|
||||
* Production model:
|
||||
* - The kernel PRNG produces keystream in fixed-size chunks
|
||||
* (SIZE_OF_AES_CTR_PRNG bytes; e.g., 16 KiB or 256 KiB).
|
||||
* - We only ever append *whole* chunks. If total free space is less than one
|
||||
* chunk, no production occurs (non-blocking style); the caller should first
|
||||
* consume data and try again.
|
||||
*
|
||||
* Wrap handling:
|
||||
* - Fast path: if a contiguous free region of at least one chunk exists at
|
||||
* @c rb_tail, generate directly into @c stash + rb_tail (zero extra copies).
|
||||
* - Wrap path: otherwise, generate one chunk into a small temporary buffer and
|
||||
* split-copy into [rb_tail..end) and [0..rest). This case is infrequent and
|
||||
* still cheaper than memmoving ring contents.
|
||||
*
|
||||
* @param[in] state Pointer to the AES-CTR state (per-thread).
|
||||
* @param[in] need Minimum number of bytes the caller would like to have ready.
|
||||
*
|
||||
* @retval 0 Success (or no space to produce yet).
|
||||
* @retval -1 PRNG failure (aes_ctr_prng_genrand_128k_to_buf() error).
|
||||
*
|
||||
* @warning
|
||||
* Thread-local only. Do not call concurrently from multiple threads that share
|
||||
* the same TLS variables.
|
||||
*/
|
||||
static int refill_stash_thread_local( void* state, size_t need )
|
||||
{
|
||||
while( stash_valid - stash_pos < need )
|
||||
while( rb_count < need )
|
||||
{
|
||||
/* If buffer empty, reset indices to front. */
|
||||
if( stash_pos == stash_valid )
|
||||
{
|
||||
stash_pos = stash_valid = 0;
|
||||
}
|
||||
/* Not enough total free space for a full CHUNK → let the caller read first. */
|
||||
if( rb_free() < SIZE_OF_AES_CTR_PRNG )
|
||||
break;
|
||||
|
||||
/* Ensure there is space for next 16 KiB chunk. */
|
||||
if( stash_valid + SIZE_OF_AES_CTR_PRNG > STASH_CAPACITY )
|
||||
size_t cf = rb_contig_free();
|
||||
if( cf >= SIZE_OF_AES_CTR_PRNG )
|
||||
{
|
||||
/* Slide remaining unread bytes to front. */
|
||||
size_t remaining = stash_valid - stash_pos;
|
||||
memmove( stash, stash + stash_pos, remaining );
|
||||
stash_pos = 0;
|
||||
stash_valid = remaining;
|
||||
/* Fast path: generate straight into the ring. */
|
||||
if( aes_ctr_prng_genrand_128k_to_buf( (aes_ctr_state_t*) state, stash + rb_tail ) != 0 )
|
||||
return -1;
|
||||
rb_tail = ( rb_tail + SIZE_OF_AES_CTR_PRNG ) & ( STASH_CAPACITY - 1 );
|
||||
rb_count += SIZE_OF_AES_CTR_PRNG;
|
||||
}
|
||||
|
||||
/* Generate another 16 KiB of keystream. */
|
||||
if( aes_ctr_prng_genrand_16k_to_buf( (aes_ctr_state_t*) state, stash + stash_valid ) != 0 )
|
||||
else
|
||||
{
|
||||
return -1;
|
||||
/* Wrap path: temporary production, then split-copy. */
|
||||
unsigned char tmp[SIZE_OF_AES_CTR_PRNG];
|
||||
if( aes_ctr_prng_genrand_128k_to_buf( (aes_ctr_state_t*) state, tmp ) != 0 )
|
||||
return -1;
|
||||
size_t first = STASH_CAPACITY - rb_tail; /* bytes to physical end */
|
||||
memcpy( stash + rb_tail, tmp, first );
|
||||
memcpy( stash, tmp + first, SIZE_OF_AES_CTR_PRNG - first );
|
||||
rb_tail = ( rb_tail + SIZE_OF_AES_CTR_PRNG ) & ( STASH_CAPACITY - 1 );
|
||||
rb_count += SIZE_OF_AES_CTR_PRNG;
|
||||
}
|
||||
stash_valid += SIZE_OF_AES_CTR_PRNG;
|
||||
}
|
||||
return 0;
|
||||
}
|
||||
|
||||
/* ---------------- PRNG INIT ---------------- */
|
||||
|
||||
/**
|
||||
* @brief Thread-local initialization wrapper around @c aes_ctr_prng_init().
|
||||
*
|
||||
* @param[in,out] state Address of the caller’s PRNG state pointer. If `*state`
|
||||
* is `NULL`, this function allocates one `aes_ctr_state_t`.
|
||||
* @param[in] seed Seed descriptor as defined by NWIPE_PRNG_INIT_SIGNATURE.
|
||||
*
|
||||
* @retval 0 Success.
|
||||
* @retval -1 Allocation or backend initialization failure (logged).
|
||||
*
|
||||
* @note
|
||||
* Resets the ring buffer to empty. Consider a one-time prefill if your workload
|
||||
* is dominated by tiny reads.
|
||||
*/
|
||||
int nwipe_aes_ctr_prng_init( NWIPE_PRNG_INIT_SIGNATURE )
|
||||
{
|
||||
nwipe_log( NWIPE_LOG_NOTICE, "Initializing AES‑CTR PRNG (thread‑local stash)" );
|
||||
nwipe_log( NWIPE_LOG_NOTICE, "Initializing AES-CTR PRNG (thread-local ring buffer)" );
|
||||
|
||||
if( *state == NULL )
|
||||
{
|
||||
@@ -431,34 +591,93 @@ int nwipe_aes_ctr_prng_init( NWIPE_PRNG_INIT_SIGNATURE )
|
||||
return -1;
|
||||
}
|
||||
|
||||
/* Reset this thread's stash */
|
||||
stash_pos = stash_valid = 0;
|
||||
/* Reset ring to empty. */
|
||||
rb_head = rb_tail = rb_count = 0;
|
||||
return 0;
|
||||
}
|
||||
|
||||
/* ---------------- PRNG READ ---------------- */
|
||||
|
||||
/**
|
||||
* @brief Copy @p count bytes of keystream into @p buffer.
|
||||
*
|
||||
* @details
|
||||
* Strategy:
|
||||
* - If the request is "large" (>= CHUNK) and the ring is empty, use the
|
||||
* direct-fill fast path and generate full CHUNKs directly into the output
|
||||
* buffer to avoid an extra memcpy.
|
||||
* - Otherwise, serve from the ring:
|
||||
* * Ensure at least one byte is available via @c refill_stash_thread_local
|
||||
* (non-blocking; production occurs only if one full CHUNK fits).
|
||||
* * Copy the largest contiguous block starting at @c rb_head.
|
||||
* * Opportunistically prefetch when sufficient free space exists to keep
|
||||
* latency low for upcoming small reads.
|
||||
*
|
||||
* @param[out] buffer Destination buffer to receive keystream.
|
||||
* @param[in] count Number of bytes to generate and copy.
|
||||
* @param[in] ... Remaining parameters as defined by NWIPE_PRNG_READ_SIGNATURE.
|
||||
*
|
||||
* @retval 0 Success (exactly @p count bytes written).
|
||||
* @retval -1 Backend/IO failure (already logged).
|
||||
*
|
||||
* @warning
|
||||
* Per-thread API: do not share this state across threads.
|
||||
*/
|
||||
int nwipe_aes_ctr_prng_read( NWIPE_PRNG_READ_SIGNATURE )
|
||||
{
|
||||
unsigned char* out = buffer;
|
||||
size_t bytes_left = count;
|
||||
|
||||
while( bytes_left > 0 )
|
||||
/* Fast path: for large reads, bypass the ring if currently empty.
|
||||
* Generate full CHUNKs directly into the destination to save one memcpy. */
|
||||
while( bytes_left >= SIZE_OF_AES_CTR_PRNG && rb_count == 0 )
|
||||
{
|
||||
/* Refill stash if necessary. */
|
||||
if( refill_stash_thread_local( *state, 1 ) != 0 )
|
||||
if( aes_ctr_prng_genrand_128k_to_buf( (aes_ctr_state_t*) *state, out ) != 0 )
|
||||
{
|
||||
nwipe_log( NWIPE_LOG_ERROR, "PRNG refill failed" );
|
||||
nwipe_log( NWIPE_LOG_ERROR, "PRNG direct fill failed" );
|
||||
return -1;
|
||||
}
|
||||
out += SIZE_OF_AES_CTR_PRNG;
|
||||
bytes_left -= SIZE_OF_AES_CTR_PRNG;
|
||||
}
|
||||
|
||||
/* Copy as much as possible from stash to user buffer. */
|
||||
size_t available = stash_valid - stash_pos;
|
||||
size_t chunk = ( bytes_left < available ) ? bytes_left : available;
|
||||
/* General path: serve from ring, refilling as needed. */
|
||||
while( bytes_left > 0 )
|
||||
{
|
||||
/* Ensure at least one byte is available for tiny reads. Refill only
|
||||
* produces if a full CHUNK fits; otherwise we try again once consumer
|
||||
* progress frees enough space. */
|
||||
if( rb_count == 0 )
|
||||
{
|
||||
if( refill_stash_thread_local( *state, 1 ) != 0 )
|
||||
{
|
||||
nwipe_log( NWIPE_LOG_ERROR, "PRNG refill failed" );
|
||||
return -1;
|
||||
}
|
||||
if( rb_count == 0 )
|
||||
continue; /* still no room for a CHUNK yet */
|
||||
}
|
||||
|
||||
memcpy( out, stash + stash_pos, chunk );
|
||||
stash_pos += chunk;
|
||||
out += chunk;
|
||||
bytes_left -= chunk;
|
||||
/* Copy the largest contiguous span starting at rb_head. */
|
||||
size_t avail = rb_contig_used();
|
||||
size_t take = ( bytes_left < avail ) ? bytes_left : avail;
|
||||
|
||||
memcpy( out, stash + rb_head, take );
|
||||
|
||||
rb_head = ( rb_head + take ) & ( STASH_CAPACITY - 1 );
|
||||
rb_count -= take;
|
||||
out += take;
|
||||
bytes_left -= take;
|
||||
|
||||
/* Opportunistic prefetch to hide latency of future small reads. */
|
||||
if( rb_free() >= ( 2 * SIZE_OF_AES_CTR_PRNG ) )
|
||||
{
|
||||
if( refill_stash_thread_local( *state, SIZE_OF_AES_CTR_PRNG ) != 0 )
|
||||
{
|
||||
nwipe_log( NWIPE_LOG_ERROR, "PRNG opportunistic refill failed" );
|
||||
return -1;
|
||||
}
|
||||
}
|
||||
}
|
||||
return 0;
|
||||
}
|
||||
|
||||
@@ -80,8 +80,10 @@ int nwipe_aes_ctr_prng_read( NWIPE_PRNG_READ_SIGNATURE );
|
||||
/* Size of the XOROSHIRO-256 is not derived from the architecture, but it is strictly 32 bytes */
|
||||
#define SIZE_OF_XOROSHIRO256_PRNG 32
|
||||
|
||||
/* Size of the AES-CTR is not derived from the architecture, but it is strictly 16k bytes */
|
||||
#define SIZE_OF_AES_CTR_PRNG 16384u
|
||||
#define STASH_CAPACITY 65536u /* 64 KiB local pre‑fetch buffer */
|
||||
/* AES-CTR generation chunk size: fixed 128 KiB (not architecture-dependent) */
|
||||
#define SIZE_OF_AES_CTR_PRNG ( 128 * 1024 )
|
||||
|
||||
/* Thread-local prefetch ring buffer capacity: 1 MiB */
|
||||
#define STASH_CAPACITY ( 1024 * 1024 )
|
||||
|
||||
#endif /* PRNG_H_ */
|
||||
|
||||
Reference in New Issue
Block a user