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 12:18:15 -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>,
	Borislav Petkov <bp@...64.org>, Ingo Molnar <mingo@...nel.org>,
	"user-mode-linux-devel@...ts.sourceforge.net" 
	<user-mode-linux-devel@...ts.sourceforge.net>,
	Richard Weinberger <richard@....at>,
	"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 08/23/2011 09:48 AM, Al Viro wrote:
> 
> Um...  How would it know which syscall variant had that been, to start
> with?  For int 0x80 it would need to use registers as-is.  For SYSENTER
> it also could use them as-is - ebp will differ from what we put there
> when entering the sucker, but not critically so; on the way out of
> syscall we'll overwrite it anyway immediately (either by pop or mov).
> For SYSCALL... we don't really care about ecx contents prior to entering
> the kernel (and it'll be blown out anyway), and ebp one could be found in
> regs.ecx.  So yes, we can do it that way, but... how to tell what variant
> had been triggered?  Examining two bytes prior to user eip?  Sounds bloody
> brittle...

We could drop that information in a metaregister.  It's not backward
compatible, but at least it will be obvious when that information is
available and not.

	-hpa
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