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: <CA+55aFygGyi8HRr8wNVDw5xk46D5QDOhis18u2qV-9+h0-4ReA@mail.gmail.com>
Date:	Mon, 22 Aug 2011 18:59:48 -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 6:13 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> *UGH*.  OK,
>        1) I'm an idiot; int_ret_from_sys_call does *not* usually step on
> rbp (it's callee-saved).  So normally ebp is left as is on the way out,
> which is why we don't see stuff getting buggered left, right and center.

Check.

And the system call restart should actually work fine too, because at
syscall entry we save %ebp *both* in the slot for ebp and ecx when we
enter the first time. So the second time, we'll re-load the third
argument from ebp again, but that's fine - it's still going to be the
right value. Yes? No?

However, I note that the cstar entrypont has a comment about not saving ebp:

 * %ebp Arg2    [note: not saved in the stack frame, should not be touched]

which sounds odd. Why don't we save it? If we take a signal handler
there, don't we want %ebp on the kernel stack in pt_regs, in order to
do everything right?

Now I'm *really* confused.

                       Linus
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