linux-kernel - Re: [patch 08/10] x86/entry/32: Remove the 0/-1 distinction from exception entries

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Date:   Wed, 26 Feb 2020 19:42:53 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, Steven Rostedt <rostedt@...dmis.org>,
        Brian Gerst <brgerst@...il.com>,
        Juergen Gross <jgross@...e.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [patch 08/10] x86/entry/32: Remove the 0/-1 distinction from exception entries

Andy Lutomirski <luto@...nel.org> writes:

> On 2/25/20 1:36 PM, Thomas Gleixner wrote:
>> Nothing cares about the -1 "mark as interrupt" in the errorcode anymore. Just
>> use 0 for all excpetions which do not have an errorcode consistently.
>> 
>
> I sincerely wish this were the case.  But look at collect_syscall() in
> lib/syscall.c.
>
> It would be really quite nice to address this for real in some
> low-overhead way.  My suggestion would be to borrow a trick from 32-bit:
> split regs->cs into ->cs and ->__csh, and stick CS_FROM_SYSCALL into
> __csh for syscalls.  This will only add any overhead at all to the int80
> case.  Then we could adjust syscall_get_nr() to look for CS_FROM_SYSCALL.
>
> What do you think?  An alternative would be to use the stack walking
> machinery in collect_syscall(), since the mere existence of that
> function is abomination and we may not care about performance.

Looking deeper. The code in common_exception does:

	movl	PT_ORIG_EAX(%esp), %edx		# get the error code
	movl	$-1, PT_ORIG_EAX(%esp)		# no syscall to restart

So whatever the exception pushed on the stack the thing what
collect_syscall finds is -1.

The pushed value is used as the error_code argument for the exception
handler and I really can't find a single one which cares (anymore).

But darn and I overlooked that, it's propagated to do_trap() and
friends, but even if this causes a user visible change, I doubt that
anything cares about it today simply because for giggles a 64bit kernel
unconditionally pushes 0 for all exceptions which do not have a hardware
error code on stack. So any 32bit application which excpects a
particular error code (0/-1) in the signal would have been broken on the
first day it ran on a x64 bit kernel.

If someone yells regression, then that's really trivial to fix in
C-code.

Let me rephrase the changelog for this.

Thanks,

        tglx