linux-kernel - Re: Is s390's new generic-using syscall code actually correct?

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Date:   Wed, 24 Mar 2021 18:38:50 +0100
From:   Vasily Gorbik <gor@...ux.ibm.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Sven Schnelle <svens@...ux.ibm.com>, X86 ML <x86@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>
Subject: Re: Is s390's new generic-using syscall code actually correct?

Hi Andy,

On Sat, Mar 20, 2021 at 08:48:34PM -0700, Andy Lutomirski wrote:
> Hi all-
> 
> I'm working on my kentry patchset, and I encountered:
> 
> commit 56e62a73702836017564eaacd5212e4d0fa1c01d
> Author: Sven Schnelle <svens@...ux.ibm.com>
> Date:   Sat Nov 21 11:14:56 2020 +0100
> 
>     s390: convert to generic entry
> 
> As part of this work, I was cleaning up the generic syscall helpers,
> and I encountered the goodies in do_syscall() and __do_syscall().
> 
> I'm trying to wrap my head around the current code, and I'm rather confused.
> 
> 1. syscall_exit_to_user_mode_work() does *all* the exit work, not just
> the syscall exit work.  So a do_syscall() that gets called twice will
> do the loopy part of the exit work (e.g. signal handling) twice.  Is
> this intentional?  If so, why?
> 
> 2. I don't understand how this PIF_SYSCALL_RESTART thing is supposed
> to work.  Looking at the code in Linus' tree, if a signal is pending
> and a syscall returns -ERESTARTSYS, the syscall will return back to
> do_syscall().  The work (as in (1)) gets run, calling do_signal(),
> which will notice -ERESTARTSYS and set PIF_SYSCALL_RESTART.
> Presumably it will also push the signal frame onto the stack and aim
> the return address at the svc instruction mentioned in the commit
> message from "s390: convert to generic entry".  Then __do_syscall()
> will turn interrupts back on and loop right back into do_syscall().
> That seems incorrect.
> 
> Can you enlighten me?  My WIP tree is here:
> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/log/?h=x86/kentry
> 

For all the details to that change we'd have to wait for Sven, who is back
next week.

> Here are my changes to s390, and I don't think they're really correct:
> 
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/diff/arch/s390/kernel/syscall.c?h=x86/kentry&id=58a459922be0fb8e0f17aeaebcb0ac8d0575a62c

Couple of things: syscall_exit_to_user_mode_prepare is static,
and there is another code path in arch/s390/kernel/traps.c using
enter_from_user_mode/exit_to_user_mode.

Anyhow I gave your branch a spin and got few new failures on strace test
suite, in particular on restart_syscall test. I'll try to find time to
look into details.