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

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Message-ID: <20110822151305.GV2203@ZenIV.linux.org.uk>
Date:	Mon, 22 Aug 2011 16:13:05 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Borislav Petkov <bp@...64.org>
Cc:	Andrew Lutomirski <luto@....edu>, Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"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>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re:
 [RFC] weird crap with vdso on uml/i386)

On Mon, Aug 22, 2011 at 04:40:51PM +0200, Borislav Petkov wrote:

> Just to make sure I'm grokking this correctly - we want to use int $0x80
> only for the SYSCALL variant in __kernel_vsyscall, right? Not for all
> 32-bit syscalls on a 64-bit kernel.

Um...  The problem is, syscall restart with SYSCALL insn is badly broken;
we end up with arg2 (expected by SYSCALL in %ebp) overwritten with arg6
by IRET path.  With obvious nasty results.

In __kernel_vsyscall() the problem is possible to deal with; there we control
the code around that sucker.  It's SYSCALL in 32bit binary outside of
vdso32 that causes real PITA...
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