cvs commit: src/sys/i386/include in_cksum.h

[David E. O'Brien]

obrien      2004-08-25 18:28:15 UTC

  FreeBSD src repository

  Modified files:
    sys/i386/include     in_cksum.h 
  Log:
  Fix a bug in in_cksum_hdr w/o -O.
  
  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 at urgle.com>, Qing Li <qing.li at bluecoat.com>
  
  Revision  Changes    Path
  1.16      +14 -16    src/sys/i386/include/in_cksum.h