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

[Sven Schnelle]

Hi Andy,

Andy Lutomirski <luto@xxxxxxxxxx> writes:

> On Wed, Mar 24, 2021 at 10:39 AM Vasily Gorbik <gor@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
>> > 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.

PIF_SYSCALL_RESTART is set in arch_do_signal_or_restart(), but only if
there's no signal handler registered. In that case we don't need a
signal frame, so that should be fine.

The problem why your branch is not working is that arch_do_signal_or_restart()
gets called from exit_to_user_mode_prepare(), and that is after the
check whether PIF_SYSCALL_RESTART is set in __do_syscall().

So i'm wondering how to fix that. x86 simply rewinds the pc, so the
system call instruction gets re-executed when returning to user
space. For s390 that doesn't work, as the s390 svc instruction might
have the syscall number encoded. If we would have to restart a system
call with restart_systemcall(), we need to change the syscall number to
__NR_restart_syscall. As we cannot change the hardcoded system call
number, we somehow need to handle that inside of the kernel.

So i wonder whether we should implement the PIF_SYSCALL_RESTART check in
entry.S after all the return to user space entry code was run but before
doing the real swtch back to user space. If PIF_SYSCALL_RESTART is set
we would then just jump back to the entry code and pretend we came from
user space.

That would have the benefit that the entry C code looks the same like
other architectures and that amount of code to add in entry.S shouldn't
be much.

Any thoughts?

Regards
Sven