[llvm] [NFC] [HWASan] add example for ring buffer wrap (PR #87029)
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Thu Mar 28 18:18:09 PDT 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-compiler-rt-sanitizer
@llvm/pr-subscribers-llvm-transforms
Author: Florian Mayer (fmayer)
<details>
<summary>Changes</summary>
Also another move comment to correct place.
---
Full diff: https://github.com/llvm/llvm-project/pull/87029.diff
1 Files Affected:
- (modified) llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp (+16-2)
``````````diff
diff --git a/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp b/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp
index f7d4803ded155a..0bc866164540ea 100644
--- a/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp
@@ -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) {
@@ -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 NN = 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));
``````````
</details>
https://github.com/llvm/llvm-project/pull/87029
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