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
