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[libc][bug] Fix out of bound write in memcpy w/ software prefetching #90613

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[libc][bug] Fix out of bound write in memcpy w/ software prefetching #90613

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[libc][bug] Fix out of bound write in memcpy wi software prefetching
gchatelet Apr 30, 2024
2f7a53c
Fix code.
gchatelet May 14, 2024
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16 changes: 14 additions & 2 deletions libc/src/string/memory_utils/x86_64/inline_memcpy.h
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Original file line number Diff line number Diff line change
Expand Up @@ -107,7 +107,13 @@ inline_memcpy_x86_sse2_ge64_sw_prefetching(Ptr __restrict dst,
offset += K_THREE_CACHELINES;
}
}
return builtin::Memcpy<32>::loop_and_tail_offset(dst, src, count, offset);
// We don't use 'loop_and_tail_offset' because it assumes at least one
// iteration of the loop.
while (offset + 32 <= count) {
builtin::Memcpy<32>::block_offset(dst, src, offset);
offset += 32;
}
return builtin::Memcpy<32>::tail(dst, src, count);
}

[[maybe_unused]] LIBC_INLINE void
Expand Down Expand Up @@ -139,7 +145,13 @@ inline_memcpy_x86_avx_ge64_sw_prefetching(Ptr __restrict dst,
builtin::Memcpy<K_THREE_CACHELINES>::block_offset(dst, src, offset);
offset += K_THREE_CACHELINES;
}
return builtin::Memcpy<64>::loop_and_tail_offset(dst, src, count, offset);
// We don't use 'loop_and_tail_offset' because it assumes at least one
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This is dead code. (return above). I'm surprised this was not caught by tests ?

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@gchatelet gchatelet May 14, 2024 •
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The code is obvioulsy wrong here. It's actually not fixing the out-of-bound write with software prefetchers for AVX. This code path is not tested because it depends on the LIBC_COPT_MEMCPY_X86_USE_SOFTWARE_PREFETCHING flag that is not activated for tests. We should be testing it but it depends on host runner hardware capabilities and so requires some more scaffolding.

Created #92089 to track this.

// iteration of the loop.
while (offset + 64 <= count) {
builtin::Memcpy<64>::block_offset(dst, src, offset);
offset += 64;
}
return builtin::Memcpy<64>::tail(dst, src, count);
}

[[maybe_unused]] LIBC_INLINE void
Expand Down