d/freebsd-dev
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

Fix a bug in in_cksum_hdr w/o -O.

Browse Source 
The C code assumes that the carry bit is always kept from the previous
operation. However, the pointer indexing requires another add operation.
Thus, the carry bit from the first operation is tromped over by the
"addl" operation that ends up following it, so the "adcl" that follows
that has no effect because the carry bit is cleared before it.
The result is checksum failure on received packets.

The larger issue is that there isn't any other way of preventing the compiler
inserting arbitrary instructions between different __asm statements (and
that the commit message in revision 1.13 of in_cksum.h is wrong on
this point).  From
http://developer.apple.com/documentation/DeveloperTools/gcc-3.3/gcc/Extended-Asm.html
	---8<---8<---8<---
	You can't expect a sequence of volatile asm instructions to remain
	perfectly consecutive. If you want consecutive output, use a single
	asm.  Also, GCC will perform some optimizations across a volatile
	asm instruction; GCC does not "forget everything" when it encounters
	a volatile asm instruction the way some other compilers do.
	---8<---8<---8<---

Also, this change also makes the ASM code much easier to read.

PR:		69257
Submitted by:	Mike Bristow <mike@urgle.com>, Qing Li <qing.li@bluecoat.com>
This commit is contained in:
David E. O'Brien 2004-08-25 18:28:15 +00:00
parent 016bdcb36a
commit 2e262ac39b
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=134300
1 changed files with 14 additions and 16 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

30
sys/i386/include/in_cksum.h
View File

@ -55,22 +55,20 @@ in_cksum_hdr(const struct ip *ip)
{
register u_int sum = 0;
/* __volatile is necessary here because the condition codes are used. */
#define ADD(n) __asm __volatile ("addl %1, %0" : "+r" (sum) : \
"g" (((const u_int32_t *)ip)[n / 4]))
#define ADDC(n) __asm __volatile ("adcl %1, %0" : "+r" (sum) : \
"g" (((const u_int32_t *)ip)[n / 4]))
#define MOP __asm __volatile ("adcl $0, %0" : "+r" (sum))
ADD(0);
ADDC(4);
ADDC(8);
ADDC(12);
ADDC(16);
MOP;
#undef ADD
#undef ADDC
#undef MOP
__asm __volatile (
"addl %1, %0\n"
"adcl %2, %0\n"
"adcl %3, %0\n"
"adcl %4, %0\n"
"adcl %5, %0\n"
"adcl $0, %0"
: "+r" (sum)
: "g" (((const u_int32_t *)ip)[0]),
"g" (((const u_int32_t *)ip)[1]),
"g" (((const u_int32_t *)ip)[2]),
"g" (((const u_int32_t *)ip)[3]),
"g" (((const u_int32_t *)ip)[4])
);
sum = (sum & 0xffff) + (sum >> 16);
if (sum > 0xffff)
sum -= 0xffff;