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: <20110823180418.GJ2203@ZenIV.linux.org.uk>
Date:	Tue, 23 Aug 2011 19:04:18 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Lutomirski <luto@....edu>, "H. Peter Anvin" <hpa@...or.com>,
	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 Tue, Aug 23, 2011 at 06:33:17PM +0100, Al Viro wrote:
> 	* SYSCALL is not terminally broken wrt restarts.  My apologies for
> misreading what was going on.
> 	* SYSENTER with Linus' patch does work just fine wrt restarts + ptrace
> 	* SYSCALL is losing ptrace-made changes to arguments when it restarts.
> Might or might not be a problem for somebody.

BTW, that one (irrelevant to UML even if we do end up coping with SYSCALL
there) might be worth spelling it out:

tracer:
	ptrace(tracee, PTRACE_SYSCALL);
tracee:
	recvfrom(..., &addrlen);
tracer:
	ptrace(tracee, PTRACE_POKEUSER, EBP, &len2);
	ptrace(tracee, PTRACE_DETACH, 0, 0);
tracee:
	completes recvfrom(), using &len2 instead of the &addrlen

That works just fine, regardless of the way syscall is entered; yes, including
SYSCALL - there we take care to handle ptrace on the way in.  However, if it's
SYSCALL and (ex-)tracee takes a restart, the second time around we'll have the
original value of 6th argument used.  Changes made by POKEUSER are lost.

It's not a problem with int 0x80 or SYSENTER (now, with int 0x80 instead of
jmp).  It's probably not going to be a real issue for anyone, but I pity the
poor bastard stuck with debugging that if it *does* become someone's problem.
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