Glibc change exposing bugs - a bug in proposed memcpy

Glibc change exposing bugs - a bug in proposed memcpy

Posted Nov 16, 2010 16:45 UTC (Tue) by promotion-account (guest, #70778)
In reply to: Glibc change exposing bugs by MisterIO
Parent article: Glibc change exposing bugs

Look at the one proposed by Linus:
void *memcpy(void *dst, const void *src, size_t size)
{
      void *orig = dst;
      asm volatile("rep ; movsq"
          :"=D" (dst), "=S" (src)
          :"0" (dst), "1" (src), "c" (size >> 3)
          :"memory");
      asm volatile("rep ; movsb"
          :"=D" (dst), "=S" (src)
          :"0" (dst), "1" (src), "c" (size & 7)
          :"memory");
      return orig;
}

For completeness, this should have an "rcx" clobber, or GCC may believe that this important register will not change after each assembly snippet. Such a bug may get triggered if GCC aggressively inlined the code, which occurs in a good number of cases given its optimizer competency.

--Darwish

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