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:	Mon, 22 Aug 2011 18:18:15 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andrew Lutomirski <luto@....edu>
CC:	Al Viro <viro@...iv.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...64.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Richard Weinberger <richard@....at>,
	Ingo Molnar <mingo@...nel.org>,
	"user-mode-linux-devel@...ts.sourceforge.net" 
	<user-mode-linux-devel@...ts.sourceforge.net>
Subject: Re: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re:
 [RFC] weird crap with vdso on uml/i386)

On 08/22/2011 06:16 PM, Andrew Lutomirski wrote:
> 
> I suspect that very few things care whether syscall arguments get
> clobbered.  The only way it would matter is if gcc reuses the argument
> in the ecx slot after an inlined syscall later in the same function. 
> Any code that does that is already wrong if the syscall restarts with
> changed ecx or if something like UML changes the syscall argument.
> 

No, the glibc wrapper for the system call Al was looking at used %ecx to
hold a copy of the PIC pointer (normally %ebx)!

	-hpa

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