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 <[EMAIL PROTECTED]> 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 > `. `' [EMAIL PROTECTED] | [EMAIL PROTECTED] > `- people.debian.org/~aurel32 | www.aurel32.net >
