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From: Andy Lutomirski <luto@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Brian Gerst <brgerst@gmail.com>, Juergen Gross <jgross@suse.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Arnd Bergmann <arnd@arndb.de>
Subject: Re: [patch 08/10] x86/entry/32: Remove the 0/-1 distinction from exception entries
Date: Wed, 26 Feb 2020 10:57:16 -0800	[thread overview]
Message-ID: <CALCETrXbNQJyvDEkfi0f0P3r+zrz8h7cPMaWB0PM_eTkFEAF0w@mail.gmail.com> (raw)
In-Reply-To: <87blpli40i.fsf@nanos.tec.linutronix.de>

On Wed, Feb 26, 2020 at 10:42 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Andy Lutomirski <luto@kernel.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.

I *think* this is plumbed much more directly to userspace:

$ cat /proc/$$/syscall
61 0xffffffff 0x7ffccf734ed0 0xa 0x0 0x1 0x0 0x7ffccf734eb8 0x7f0667465eda

That entire feature is highly dubious and I suppose we could just
delete it.  But right now, we at least pretend that we can tell,
totally asynchronously, whether another task is in a syscall.  Unless
we do *something*, though, I think you shouldn't make this change.

Sticking 0 in the error_code field in ucontext for a signal with no
error code seems entirely harmless to me in contrast.

  reply	other threads:[~2020-02-26 18:57 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-25 21:36 [patch 00/10] x86/entry: Consolidation - Part I Thomas Gleixner
2020-02-25 21:36 ` [patch 01/10] x86/entry/32: Add missing ASM_CLAC to general_protection entry Thomas Gleixner
2020-02-26  1:00   ` Frederic Weisbecker
2020-02-27 14:15   ` [tip: x86/entry] " tip-bot2 for Thomas Gleixner
2020-02-25 21:36 ` [patch 02/10] x86/mce: Disable tracing and kprobes on do_machine_check() Thomas Gleixner
2020-02-26  1:13   ` Frederic Weisbecker
2020-02-26  5:29     ` Andy Lutomirski
2020-02-26 13:28       ` Peter Zijlstra
2020-02-26 15:10         ` Andy Lutomirski
2020-02-26 16:08           ` Peter Zijlstra
2020-02-26 17:28             ` Andy Lutomirski
2020-02-26 18:42               ` Borislav Petkov
2020-02-26 18:59                 ` Peter Zijlstra
2020-02-26 19:09                   ` Andy Lutomirski
2020-02-26 20:59                     ` Steven Rostedt
2020-02-26 11:18   ` Borislav Petkov
2020-02-27 14:15   ` [tip: x86/entry] " tip-bot2 for Andy Lutomirski
2020-02-25 21:36 ` [patch 03/10] x86/entry/32: Force MCE through do_mce() Thomas Gleixner
2020-02-26  1:11   ` Frederic Weisbecker
2020-02-27 14:15   ` [tip: x86/entry] " tip-bot2 for Thomas Gleixner
2020-02-25 21:36 ` [patch 04/10] x86/traps: Remove pointless irq enable from do_spurious_interrupt_bug() Thomas Gleixner
2020-02-26  1:19   ` Frederic Weisbecker
2020-02-25 21:36 ` [patch 05/10] x86/traps: Document do_spurious_interrupt_bug() Thomas Gleixner
2020-02-26 17:08   ` Frederic Weisbecker
2020-02-27 14:15   ` [tip: x86/entry] " tip-bot2 for Thomas Gleixner
2020-02-25 21:36 ` [patch 06/10] x86/traps: Remove redundant declaration of do_double_fault() Thomas Gleixner
2020-02-27 14:15   ` [tip: x86/entry] " tip-bot2 for Thomas Gleixner
2020-02-25 21:36 ` [patch 07/10] x86/irq: Remove useless return value from do_IRQ() Thomas Gleixner
2020-02-27 14:15   ` [tip: x86/entry] " tip-bot2 for Thomas Gleixner
2020-02-25 21:36 ` [patch 08/10] x86/entry/32: Remove the 0/-1 distinction from exception entries Thomas Gleixner
2020-02-26  5:34   ` Andy Lutomirski
2020-02-26 18:42     ` Thomas Gleixner
2020-02-26 18:57       ` Andy Lutomirski [this message]
2020-02-26 19:15         ` Thomas Gleixner
2020-02-27 14:24           ` [patch V2 " Thomas Gleixner
2020-02-29 11:49             ` [tip: x86/entry] " tip-bot2 for Thomas Gleixner
2020-02-25 21:36 ` [patch 09/10] x86/entry/entry_32: Route int3 through common_exception Thomas Gleixner
2020-02-26 17:35   ` Frederic Weisbecker
2020-02-27 14:15   ` [tip: x86/entry] " tip-bot2 for Thomas Gleixner
2020-02-25 21:36 ` [patch 10/10] x86/traps: Stop using ist_enter/exit() in do_int3() Thomas Gleixner
2020-02-27 14:15   ` [tip: x86/entry] " tip-bot2 for Thomas Gleixner
2020-02-27 14:33   ` tip-bot2 for Andy Lutomirski
2020-02-26  5:26 ` [patch 00/10] x86/entry: Consolidation - Part I Andy Lutomirski
2020-02-26  5:35 ` Andy Lutomirski
2020-02-27 11:01 ` Alexandre Chartre

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