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

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Date:	Wed, 05 Mar 2008 21:52:14 +0100
From:	Aurelien Jarno <aurelien@...el32.net>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Michael Matz <matz@...e.de>, Joe Buck <Joe.Buck@...opsys.COM>,
	linux-kernel@...r.kernel.org, gcc@....gnu.org
Subject: Re: Linux doesn't follow x86/x86-64 ABI wrt direction flag

H. Peter Anvin a écrit :
> Michael Matz wrote:
>> Hi,
>>
>> On Wed, 5 Mar 2008, Aurelien Jarno wrote:
>>
>>>> So I think gcc at least needs an *option* to revert to the old
>>>> behavior,
>>>> and there's a good argument to make it the default for now, at least
>>>> for
>>>> x86/x86-64 on Linux.
>>> And for other kernels. I tested OpenBSD 4.1, FreeBSD 6.3, NetBSD 4.0,
>>> they have the same behaviour as Linux, that is they don't clear DF
>>> before calling the signal handler.
>>
>> Sigh.  We could perhaps insert a cld for all functions which can be
>> recognized as possible signal handlers and call other unknown or
>> string functions.  But it's probably even faster to emit cld in front
>> of the inline copies of mem functions again :-(
>>
> 
> Well, there is a (slight) difference: you know that a called function
> will not clobber your DF state; it's only the entry condition which is
> imprecise.
> 
> The best would be if this could be controlled by a flag, which we can
> flip once kernel fixes has been around for long enough.

I have to agree there. Whatever the decision that gcc will take,
distributions will reenable the old behaviour for some time for to allow
upgrades from a previous version.

Providing a flag to switch the behaviour (whatever the default
behaviour) will help a lot.


-- 
  .''`.  Aurelien Jarno	            | GPG: 1024D/F1BCDB73
 : :' :  Debian developer           | Electrical Engineer
 `. `'   aurel32@...ian.org         | aurelien@...el32.net
   `-    people.debian.org/~aurel32 | www.aurel32.net
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