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

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Date:	Tue, 23 Aug 2011 18:07:40 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Richard Weinberger <richard@....at>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...64.org>,
	Andrew Lutomirski <luto@....edu>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	"user-mode-linux-devel@...ts.sourceforge.net" 
	<user-mode-linux-devel@...ts.sourceforge.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>
Subject: Re: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re:
 [RFC] weird crap with vdso on uml/i386)

On Tue, Aug 23, 2011 at 06:58:18PM +0200, Richard Weinberger wrote:

> What about this hack/solution?
> While booting UML can check whether the host's vDSO contains
> a SYSCALL instruction.
> If so, UML will not make the host's vDSO available to it's
> processes...

Note that this is *only* for 32bit side of things.  64bit one works fine...

I wouldn't search for SYSCALL in vdso, BTW - not when we have a good way
to trigger that crap and recognize it.

At boot time, fork a child.  Have it traced with PTRACE_SYSCALL.  Let it
put recognizable values in registers and call __kernel_vsyscall().  Then
let the parent do one more PTRACE_SYSCALL, then PTRACE_POKEUSER and set ebp
to 0x69696969.  PTRACE_CONT the sucker and let it report what it sees in ecx.
If it's what we'd put there - fine, it looks safe.  If it's 0x69696969 -
we have a problem, no vdso for us.
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