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Unpoison fiber stack after context switch on AArch64 #46

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Unpoison fiber stack after context switch on AArch64 #46

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a6da369
Unpoison fiber stack after context switch on AArch64
alexey-milovidov Apr 9, 2026
24df31b
Add missing `__msan_unpoison` declaration to externc.hpp
alexey-milovidov Apr 10, 2026
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1 change: 1 addition & 0 deletions boost/context/detail/externc.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -19,6 +19,7 @@ void __sanitizer_finish_switch_fiber( void *, const void **, size_t *);
extern "C" {
void __msan_start_switch_fiber( const void *, size_t);
void __msan_finish_switch_fiber( const void **, size_t *);
void __msan_unpoison( const volatile void *, size_t);
}
#endif

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17 changes: 17 additions & 0 deletions boost/context/fiber_ucontext.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -172,6 +172,15 @@ struct BOOST_CONTEXT_DECL fiber_activation_record {
#if defined(BOOST_USE_MSAN)
__msan_finish_switch_fiber( (const void **) & current()->from->stack_bottom,
& current()->from->stack_size);
// On AArch64, LLVM loses MSan shadow for struct-padding bytes in return values
// (llvm/llvm-project#54476). On heap-allocated fiber stacks the dirty shadow
// accumulates across context switches and propagates via stack-slot reuse.
// Unpoison the whole fiber stack to prevent these false positives.
#if defined(__aarch64__)
if ( ! current()->main_ctx) {
__msan_unpoison( current()->stack_bottom, current()->stack_size);
}
#endif
#endif
#if defined(BOOST_NO_CXX14_STD_EXCHANGE)
return exchange( current()->from, nullptr);
Expand Down Expand Up @@ -241,6 +250,11 @@ struct BOOST_CONTEXT_DECL fiber_activation_record {
#if defined(BOOST_USE_MSAN)
__msan_finish_switch_fiber( (const void **) & current()->from->stack_bottom,
& current()->from->stack_size);
#if defined(__aarch64__)
if ( ! current()->main_ctx) {
__msan_unpoison( current()->stack_bottom, current()->stack_size);
}
#endif
#endif
#if defined(BOOST_NO_CXX14_STD_EXCHANGE)
return exchange( current()->from, nullptr);
Expand Down Expand Up @@ -302,6 +316,9 @@ class fiber_capture_record : public fiber_activation_record {
#if defined(BOOST_USE_MSAN)
__msan_finish_switch_fiber( (const void **) & from->stack_bottom,
& from->stack_size);
#if defined(__aarch64__)
__msan_unpoison( stack_bottom, stack_size);
#endif
#endif
Ctx c{ from };
try {
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