linux-kernel - Re: gcc feature request / RFC: extra clobbered regs

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CALCETrW9vbWrv8=0JvWOYdoWYQ9Sy6jSMkTa30=DFVowL5sOpw@mail.gmail.com>
Date:	Wed, 1 Jul 2015 13:09:06 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Jakub Jelinek <jakub@...hat.com>
Cc:	Vladimir Makarov <vmakarov@...hat.com>,
	Andy Lutomirski <luto@...nel.org>, gcc@....gnu.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: gcc feature request / RFC: extra clobbered regs

On Wed, Jul 1, 2015 at 10:43 AM, Jakub Jelinek <jakub@...hat.com> wrote:
> On Wed, Jul 01, 2015 at 01:35:16PM -0400, Vladimir Makarov wrote:
>> Actually it raise a question for me.  If we describe that a function
>> clobbers more than calling convention and then use it as a value (assigning
>> a variable or passing as an argument) and loosing a track of it and than
>> call it.  How can RA know what the call clobbers actually.  So for the
>> function with the attributes we should prohibit use it as a value or make
>> the attributes as a part of the function type, or at least say it is unsafe.
>> So now I see this as a *bigger problem* with this extension.  Although I
>> guess it already exists as we have description of different ABI as an
>> extension.
>
> Unfortunately target attribute is function decl attribute rather than
> function type.  And having more attributes affect switchable targets will be
> non-fun.

Just to make sure we're on the same page here, if I write:

extern void normal_func(void);

void weird_func(void) __attribute__((used_regs("r12")))
{
  // do something
  normal_func();
  // do something
}

I'd want the code that calls normal_func() to be understand that
normal_func() *will* preserve r12 despite the fact that weird_func is
allowed to clobber r12.  I think this means that the attribute would
have to be an attribute of a function, not of the RA while compiling
the function.

--Andy
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/