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


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