diff options
Diffstat (limited to 'lib/libc/amd64/string/memcmp.S')
| -rw-r--r-- | lib/libc/amd64/string/memcmp.S | 17 |
1 files changed, 16 insertions, 1 deletions
diff --git a/lib/libc/amd64/string/memcmp.S b/lib/libc/amd64/string/memcmp.S index d192229677b3..dc8bcff73cb9 100644 --- a/lib/libc/amd64/string/memcmp.S +++ b/lib/libc/amd64/string/memcmp.S @@ -328,13 +328,28 @@ ARCHENTRY(memcmp, baseline) movdqu 16(%rsi, %rdi, 1), %xmm1 pcmpeqb 16(%rdi), %xmm1 # compare second half of this iteration add %rcx, %rdx # pointer to last byte in buffer - pcmpeqb %xmm2, %xmm0 + jc .Loverflow # did this overflow? +0: pcmpeqb %xmm2, %xmm0 pmovmskb %xmm0, %eax xor $0xffff, %eax # any mismatch? jne .Lmismatch_head add $64, %rdi # advance to next iteration jmp 1f # and get going with the loop + /* + * If we got here, a buffer length was passed to memcmp(a, b, len) + * such that a + len < a. While this sort of usage is illegal, + * it is plausible that a caller tries to do something like + * memcmp(a, b, SIZE_MAX) if a and b are known to differ, intending + * for memcmp() to stop comparing at the first mismatch. This + * behaviour is not guaranteed by any version of ISO/IEC 9899, + * but usually works out in practice. Let's try to make this + * case work by comparing until the end of the address space. + */ +.Loverflow: + mov $-1, %rdx # compare until the end of memory + jmp 0b + /* process buffer 32 bytes at a time */ ALIGN_TEXT 0: movdqu -32(%rsi, %rdi, 1), %xmm0 |