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Date: Tue, 23 Aug 2011 22:17:44 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Andrew Lutomirski <luto@mit.edu>,
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Subject: Re: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re:
 [RFC] weird crap with vdso on uml/i386)
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On Tue, Aug 23, 2011 at 12:43:28PM -0700, Linus Torvalds wrote:
> On Tue, Aug 23, 2011 at 12:41 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > And it's not cheap - doing that on each syscall will be unpleasant...
> > Frankly, I'd rather stopped telling the uml userland about vdso in such
> > setups. ?And anything that plays with SYSCALL outside of vdso... <shrug>
> > we already have a "don't run it native on 32bit", adding "don't run
> > it on 32bit uml on amd64 host" is not too serious. ?At least for now...
> 
> I do agree that the solution might well be to just stop using the
> non-int80 vdsos for UML. That may just solve everything in practice.

SYSENTER works fine, actually...  And we can easily check if we have an
affected SYSCALL, simply by forking a child, tracing it into a syscall
and doing POKEUSER to ebp on the second stop (i.e. on the way out).
If the value ends up in ecx after __kernel_vsyscall(), we have SYSCALL-based
variant on amd64 host, if it's lost completely - it's SYSENTER, if it shows
up in ebp - int 0x80.