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Pin the whole SharedBlock layout with static_asserts (ABI tripwire)

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The cross-process block's layout is a wire protocol shared by the x64 host and the
x86 hook, but only three front offsets were asserted, and hook_selftest's
dump_layout merely printed the rest. A field reordered/resized inside HookStatus
(which precedes control/video/mkb) would silently shift everything with no
compile-time tripwire and, if the developer forgot to bump kProtocolVersion, ship
a silent host<->DLL mismatch.

- protocol.hpp now static_asserts sizeof(SharedBlock) and every sub-channel offset
  (status/control/video/mkb) plus each sub-struct size (HookStatus/HookControl/
  VideoShare/AudioStreamInfo/HookEntry/MkbRing). protocol.hpp is compiled for both
  arches, so a cross-bitness divergence fails to compile on the one that disagrees.
- hook_selftest's dump_layout now ASSERTS the same numbers instead of only
  printing, so hook_selftest_x86 confirms the x86 layout at runtime too.

Verified: x64 and x86 builds both compile (identical layout) and both selftests
pass; sizeof(SharedBlock)=3936 on both.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
BlackMark
2026-06-24 01:47:19 +02:00
parent 1a93fcf196
commit 5e9b1cde4c
3 changed files with 31 additions and 7 deletions
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23
common/include/coop/protocol.hpp
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@@ -270,14 +270,27 @@ static_assert(std::atomic<std::uint32_t>::is_always_lock_free,
static_assert(std::atomic<std::uint64_t>::is_always_lock_free,
"status counters need a lock-free 64-bit atomic for cross-process use");
// The x64 host and the x86 hook map this same block, so its layout must be
// byte-identical across bitness. These offsets (verified equal on both arches)
// lock the front of the block -- the seqlock + pad state the input hot path reads;
// a future field reorder that diverges between x86 and x64 fails to compile on the
// arch that disagrees. (Fixed-width POD + no pointers is what keeps it stable.)
// The x64 host and the x86 hook map this same block, so its layout must be byte-identical across
// bitness, and any change is a wire-protocol change. These pin the WHOLE block -- total size, every
// sub-channel offset, and each sub-struct's size -- so an accidental field reorder/resize fails to
// COMPILE (on whichever arch disagrees, since protocol.hpp is built for both) until the numbers here
// AND kProtocolVersion (the runtime handshake in ipc_client) are both revisited. The version gate only
// guards against a *known* change; this is the tripwire for an *unintended* one. (Fixed-width POD + no
// pointers is what keeps it stable.) If one of these fails: confirm the layout change is intended,
// bump kProtocolVersion, and update the number here.
static_assert(sizeof(SharedBlock) == 3936, "SharedBlock size changed -- this is a wire-protocol change");
static_assert(offsetof(SharedBlock, sequence) == 12, "cross-bitness: sequence offset moved");
static_assert(offsetof(SharedBlock, pads) == 16, "cross-bitness: pad-state offset moved");
static_assert(offsetof(SharedBlock, status) == 96, "cross-bitness: status offset moved");
static_assert(offsetof(SharedBlock, control) == 1816, "cross-bitness: control offset moved");
static_assert(offsetof(SharedBlock, video) == 1840, "cross-bitness: video offset moved");
static_assert(offsetof(SharedBlock, mkb) == 1880, "cross-bitness: mkb offset moved");
static_assert(sizeof(HookStatus) == 1720, "HookStatus size changed -- wire-protocol change");
static_assert(sizeof(HookControl) == 24, "HookControl size changed -- wire-protocol change");
static_assert(sizeof(VideoShare) == 40, "VideoShare size changed -- wire-protocol change");
static_assert(sizeof(AudioStreamInfo) == 32, "AudioStreamInfo size changed -- wire-protocol change");
static_assert(sizeof(HookEntry) == 56, "HookEntry size changed -- wire-protocol change");
static_assert(sizeof(MkbRing) == 2056, "MkbRing size changed -- wire-protocol change");
// --- Seqlock helpers -------------------------------------------------------