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[NFC] [HWASan] add example for ring buffer wrap #87029

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Merged
merged 2 commits into from
Mar 29, 2024
Merged

[NFC] [HWASan] add example for ring buffer wrap #87029

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278d49f
[𝘀𝗽𝗿] initial version
fmayer Mar 29, 2024
d1cc6b8
nit
fmayer Mar 29, 2024
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18 changes: 16 additions & 2 deletions llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1114,8 +1114,6 @@ void HWAddressSanitizer::tagAlloca(IRBuilder<> &IRB, AllocaInst *AI, Value *Tag,
// FIXME: the interceptor is not as fast as real memset. Consider lowering
// llvm.memset right here into either a sequence of stores, or a call to
// hwasan_tag_memory.
// Mechanical proof of this address calculation can be found at:
// https://github.com/google/sanitizers/blob/master/hwaddress-sanitizer/prove_hwasanwrap.smt2
if (ShadowSize)
IRB.CreateMemSet(ShadowPtr, Tag, ShadowSize, Align(1));
if (Size != AlignedSize) {
Expand Down Expand Up @@ -1312,6 +1310,22 @@ void HWAddressSanitizer::emitPrologue(IRBuilder<> &IRB, bool WithFrameRecord) {
// The use of AShr instead of LShr is due to
// https://bugs.llvm.org/show_bug.cgi?id=39030
// Runtime library makes sure not to use the highest bit.
//
// Mechanical proof of this address calculation can be found at:
// https://github.com/google/sanitizers/blob/master/hwaddress-sanitizer/prove_hwasanwrap.smt2
//
// Example of the wrap case for N = 1
// Pointer: 0x01AAAAAAAAAAAFF8
// +
// 0x0000000000000008
// =
// 0x01AAAAAAAAAAB000
// &
// WrapMask: 0xFFFFFFFFFFFFF000
// =
// 0x01AAAAAAAAAAA000
//
// Then the WrapMask will be a no-op until the next wrap case.
Value *WrapMask = IRB.CreateXor(
IRB.CreateShl(IRB.CreateAShr(ThreadLong, 56), 12, "", true, true),
ConstantInt::get(IntptrTy, (uint64_t)-1));
Expand Down