Attachment 244743 Details for Bug 273652 – lib/libc/amd64/string/memchr.S: fix behaviour with overly long buffers

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

0001-lib-libc-amd64-string-memchr.S-fix-behaviour-with-ov.patch (text/plain), 1.88 KB, created by Robert Clausecker on 2023-09-10 04:42:24 UTC

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Creator: Robert Clausecker

Created: 2023-09-10 04:42:24 UTC

Size: 1.88 KB

patch

obsolete

>From e55a88d6140839dc411503b5e7c21a2330d44b77 Mon Sep 17 00:00:00 2001
>From: Robert Clausecker <fuz@FreeBSD.org>
>Date: Sun, 10 Sep 2023 00:11:07 -0400
>Subject: [PATCH] lib/libc/amd64/string/memchr.S: fix behaviour with overly
> long buffers
>
>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, throwaway_vthgwq4@protonmail.com, des
>PR:		273652
>---
> lib/libc/amd64/string/memchr.S | 9 ++++++---
> 1 file changed, 6 insertions(+), 3 deletions(-)
>
>diff --git a/lib/libc/amd64/string/memchr.S b/lib/libc/amd64/string/memchr.S
>index e10bd6c22f90..cfab9b1302de 100644
>--- a/lib/libc/amd64/string/memchr.S
>+++ b/lib/libc/amd64/string/memchr.S
>@@ -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
>-- 
>2.41.0
>

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