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Date: Sun, 21 Aug 2011 15:43:52 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Andrew Lutomirski <luto@mit.edu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        "H. Peter Anvin" <hpa@zytor.com>, mingo@redhat.com,
        Richard Weinberger <richard@nod.at>,
        user-mode-linux-devel@lists.sourceforge.net,
        linux-kernel@vger.kernel.org
Subject: Re: SYSCALL, ptrace and syscall restart breakages (Re: [RFC] weird
 crap with vdso on uml/i386)
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On Sun, Aug 21, 2011 at 07:24:35AM -0400, Andrew Lutomirski wrote:

> I don't see the point of all this hackery at all.  sysenter/sysexit
> indeed screws up some registers, but we can return on the iret path in
> the case of restart.

We *do* return on iret path in case of restart, TYVM.

> So why do we lie to ptrace (and iret!) at all?  Why not just fill in
> pt_regs with the registers as they were (at least the
> non-clobbered-by-sysenter ones), set the actual C parameters correctly
> to contain the six arguments (in rdi, rsi, etc.), do the syscall, and
> return back to userspace without any funny business?  Is there some
> ABI reason that, once we've started lying to tracers, we have to keep
> doing so?

We do not lie to ptrace and iret.  At all.  We do just what you have
described.  And fuck up when restart returns us to the SYSCALL / SYSENTER
instruction again, which expects the different calling conventions,
so the values arranged in registers in the way int 0x80 would expect
do us no good.