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

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Date:	Mon, 22 Aug 2011 05:07:59 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Lutomirski <luto@....edu>, "H. Peter Anvin" <hpa@...or.com>,
	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 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?
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