Re: Linux doesn't follow x86/x86-64 ABI wrt direction flag

["H.J. Lu" <hjl.tools@gmail.com>]

Hi,

According to i386 psABI,

---
The direction flag must be set "forward" direction before entry and
upon exit from
a function.
---

So, asm statement should make sure that the direction flag is cleared before
function returns and kernel should make sure that the direction flag is cleared
when calling a signal handler.

H.J.
On Wed, Mar 5, 2008 at 7:30 AM, Aurelien Jarno <aurelien@aurel32.net> wrote:
> Hi all,
>
>  Since version 4.3, gcc changed its behaviour concerning the x86/x86-64
>  ABI and the direction flag, that is it now assumes that the direction
>  flag is cleared at the entry of a function and it doesn't clear once
>  more if needed.
>
>  This causes some problems with the Linux kernel which does not clear
>  the direction flag when entering a signal handler. The small code below
>  (for x86-64) demonstrates that.
>
>  If the signal handler is using code that need the direction flag cleared
>  (for example bzero() or memset()), the code is incorrectly executed.
>
>  I guess this has to be fixed on the kernel side, but also gcc-4.3 could
>  revert back to the old behaviour, that is clearing the direction flag
>  when entering a routine that touches it until most people are running a
>  fixed kernel.
>
>  Kind regards,
>  Aurelien
>
>  [1] http://gcc.gnu.org/gcc-4.3/changes.html
>
>
>  #include <stdint.h>
>  #include <stdlib.h>
>  #include <stdio.h>
>  #include <signal.h>
>
>  void handler(int signal) {
>         uint64_t rflags;
>
>         asm volatile("pushfq ; popq %0" : "=g" (rflags));
>
>         if (rflags & (1 << 10))
>                 printf("DF = 1\n");
>         else
>                 printf("DF = 0\n");
>  }
>
>  int main() {
>         signal(SIGUSR1, handler);
>
>         while(1)
>         {
>                 asm volatile("std\r\n");
>         }
>
>         return 0;
>  }
>
>  --
>   .''`.  Aurelien Jarno             | GPG: 1024D/F1BCDB73
>   : :' :  Debian developer           | Electrical Engineer
>   `. `'   aurel32@debian.org         | aurelien@aurel32.net
>    `-    people.debian.org/~aurel32 | www.aurel32.net
>