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 09:03:04 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	"H. Peter Anvin" <hpa@...or.com>, 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 Mon, Aug 22, 2011 at 11:15 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> * it does SETREGS, setting eax to return value, eip to original return
> address of syscall insn... and ebp to what it had in regs.bp.  I.e. the
> damn arg6 value.

Ok, I think that exhaustively explains that

 (a) our system call restart has always worked correctly, and we're good.

 (b) it's simply just UML that is buggy, and doesn't understand the
subtleties about doing GETREGS at a system call.

and I think that the correct and simple solution is to just teach UML
to understand the proper logic of pt_regs during a system call (and a
'syscall' instruction in particular).

The thing is, UML emulates 'syscall' the way the *CPU* does it, not
the way *we* do it. That may make sense, but it's simply not correct.

So I would vote very strongly against actually changing anything in
arch/x86. This is very much an UML issue.

Suggested fixes:

 - instead of blindly doing SETREGS, just write the result registers
individually like you suggested

OR (and perhaps preferably):

 - teach UML that when you do 'GETREGS' after a system call trapped,
we have switched things around to match the "official system call
order", and UML should just undo our swizzling, and do a "regs.ebp =
regs.ecx" to make it be what the actual original registers were (or
whatever the actual correct swizzle is - I didn't think that through
very much).

IOW, I think the core kernel does the right thing. Our argument
register swizzling is odd, yes, but it's an implementation detail that
something like uml should just have to take into account. No?

Hmm?

                            Linus
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