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lib/libc/amd64/string/memchr.S: fix behaviour with overly long buffers

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When memchr(buf, c, len) is called with a phony len (say, SIZE_MAX),
buf + len overflows and we have buf + len < buf.  This confuses the
implementation and makes it return incorrect results.  Neverthless we
must support this case as memchr() is guaranteed to work even with
phony buffer lengths, as long as a match is found before the buffer
actually ends.

Sponsored by:	The FreeBSD Foundation
Reported by:	yuri, des
Tested by:	des
Approved by:	mjg (blanket, via IRC)
MFC after:	1 week
MFC to:		stable/14
PR:		273652
This commit is contained in:
Robert Clausecker 2023-09-10 00:11:07 -04:00
parent 4fc08109fe
commit b2618b651b
1 changed files with 6 additions and 3 deletions
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9
lib/libc/amd64/string/memchr.S
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@ -44,7 +44,9 @@ ARCHENTRY(__memchr, scalar)
je .Lnomatch
lea (, %rdi, 8), %ecx
add %rdi, %rdx # pointer to end of buffer
mov $-1, %rax
add %rdi, %rdx # pointer to end of buffer or to end of
cmovc %rax, %rdx # address space (whichever comes first)
and $~7, %rdi # align to 8 bytes
mov (%rdi), %rax # load first word
movzbl %sil, %esi # clear stray high bits
@ -118,14 +120,15 @@ ARCHENTRY(__memchr, baseline)
movd %esi, %xmm2
mov %edi, %ecx
add %rdi, %rdx # pointer to end of buffer
mov $-1, %r9
add %rdi, %rdx # pointer to end of buffer or to end of
cmovc %r9, %rdx # address space (whichever comes first)
and $~0x1f, %rdi # align to 32 bytes
movdqa (%rdi), %xmm0 # load first 32 bytes
movdqa 16(%rdi), %xmm1
punpcklbw %xmm2, %xmm2 # c -> cc
mov $-1, %r9d
shl %cl, %r9d # mask with zeroes before the string
punpcklwd %xmm2, %xmm2 # cc -> cccc