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

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Date:   Wed, 24 Mar 2021 11:06:54 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Vasily Gorbik <gor@...ux.ibm.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        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?

On Wed, Mar 24, 2021 at 10:39 AM Vasily Gorbik <gor@...ux.ibm.com> wrote:
>
> 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.

I refreshed the branch, but I confess I haven't compile tested it. :)

I would guess that the new test case failures are a result of the
buggy syscall restart logic.  I think that all of the "restart" cases
except execve() should just be removed.  Without my patch, I suspect
that signal delivery with -ERESTARTSYS would create the signal frame,
do an accidental "restarted" syscall that was a no-op, and then
deliver the signal.  With my patch, it may simply repeat the original
interrupted signal forever.

--Andy