linux-kernel - Re: SYSCALL, ptrace and syscall restart breakages (Re: [RFC] weird crap with vdso on uml/i386)

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Date:	Sun, 21 Aug 2011 21:11:54 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Lutomirski <luto@....edu>, mingo@...hat.com,
	Richard Weinberger <richard@....at>,
	user-mode-linux-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: SYSCALL, ptrace and syscall restart breakages (Re: [RFC] weird
 crap with vdso on uml/i386)

On 08/21/2011 09:07 PM, Al Viro wrote:
> On Sun, Aug 21, 2011 at 06:41:16PM -0700, Linus Torvalds wrote:
>> On Sun, Aug 21, 2011 at 6:16 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>>>
>>> Is that ability a part of userland ABI or are we declaring that hopelessly
>>> wrong and require to go through the function in vdso32? ?Linus?
>>
>> If people are using syscall directly, we're pretty much stuck. No
>> amount of "that's hopelessly wrong" will ever matter. We don't break
>> existing binaries.
> 
> There's a funny part, though - such binary won't work on 32bit kernel.
> AFAICS, we never set MSR_*STAR on 32bit kernels (and native 32bit vdso
> doesn't provide a SYSCALL-based variant).
> 
> So if we really consider such SYSCALL outside of vdso32 kosher, shouldn't
> we do something with entry_32.S as well?  I don't think it's worth doing,
> TBH...
> 
> Again, I very much hope that binaries with such stray SYSCALL simply do
> not exist.  In theory it's possible to write one, but...
> 
> IIRC, the reason we never had SYSCALL support in 32bit kernel was the utter
> lack of point - the *only* CPU where it would matter would be K6-2, IIRC,
> and (again, IIRC) it had some differences in SYSCALL semantics compared to
> K7 (which supports SYSENTER as well).  Bugger if I remember what those
> differences might've been...  Some flag not cleared?

The most likely reason for a binary to execute a stray SYSCALL is
because they read it out of the vdso.  Totally daft, but we certainly
see a lot of stupid things as evidenced by the JIT thread earlier this
month.

In that sense, a "safe" thing would be to drop use of SYSCALL for 32-bit
processes... I just sent Borislav a query about the cost.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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