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Date: Tue, 23 Aug 2011 19:04:18 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Lutomirski <luto@mit.edu>, "H. Peter Anvin" <hpa@zytor.com>,
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Subject: Re: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re:
 [RFC] weird crap with vdso on uml/i386)
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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.