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

H. Peter Anvin
2008-03-05T16:05:48Z

Aurelien Jarno 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.
>

Linux should definitely follow the ABI. This is a bug, and a pretty
serious such.

-hpa