Re: [Qemu-devel] [PULL v3 47/55] linux headers: update against Linux 5.2-rc1

[Aleksandar Markovic]

On May 22, 2019 3:42 PM, "Alex Bennée" <address@hidden> wrote:
>
>
> Aleksandar Markovic <address@hidden> writes:
>
> > On May 22, 2019 2:24 PM, "Cornelia Huck" <address@hidden> wrote:
> >>
> >> On Wed, 22 May 2019 14:10:39 +0200
> >> Laurent Vivier <address@hidden> wrote:
> >>
> >> > On 22/05/2019 14:07, Cornelia Huck wrote:
> >> > > On Wed, 22 May 2019 13:47:25 +0200
> >> > > Philippe Mathieu-Daudé <address@hidden> wrote:
> >> > >
> >> > >> On 5/21/19 5:28 PM, Cornelia Huck wrote:
> >> > >>> commit a188339ca5a396acc588e5851ed7e19f66b0ebd9
> >> > >>>
> >> > >>> Signed-off-by: Cornelia Huck <address@hidden>
> >> > >>> ---
> >> > >> [...]
> >> > >>>   #define __NR_mq_notify 184
> >> > >>>   __SC_COMP(__NR_mq_notify, sys_mq_notify, compat_sys_mq_notify)
> >> > >>>   #define __NR_mq_getsetattr 185
> >> > >>> @@ -536,8 +567,10 @@ __SC_COMP(__NR_msgsnd, sys_msgsnd,
> > compat_sys_msgsnd)
> >> > >>>   __SYSCALL(__NR_semget, sys_semget)
> >> > >>>   #define __NR_semctl 191
> >> > >>>   __SC_COMP(__NR_semctl, sys_semctl, compat_sys_semctl)
> >> > >>> +#if defined(__ARCH_WANT_TIME32_SYSCALLS) || __BITS_PER_LONG !=
32
> >> > >
> >> > > Eww. It seems only aarch64 sets __ARCH_WANT_TIME32_SYSCALLS, and
the
> >> > > second condition probably catches others but not mipsel.
> >> > >
> >> > >>>   #define __NR_semtimedop 192
> >> > >>> -__SC_COMP(__NR_semtimedop, sys_semtimedop,
compat_sys_semtimedop)
> >> > >>> +__SC_COMP(__NR_semtimedop, sys_semtimedop,
sys_semtimedop_time32)
> >> > >>> +#endif
> >> > >>>   #define __NR_semop 193
> >> > >>>   __SYSCALL(__NR_semop, sys_semop)
> >> > >> [...]
> >> > >>
> >> > >>
https://app.shippable.com/github/qemu/qemu/runs/1703/summary/console
> >> > >>
> >> > >> It seems this commit introduce a regression on mips32:
> >> > >>
> >> > >>    CC      mipsel-linux-user/linux-user/syscall.o
> >> > >> ./linux-user/syscall.c: In function 'safe_semtimedop':
> >> > >> ./linux-user/syscall.c:697:25: error: '__NR_semtimedop' undeclared
> >> > >> (first use in this function)
> >> > >>       return safe_syscall(__NR_##name, arg1, arg2, arg3, arg4); \
> >> > >
> >> > > So, we unconditionally deal with this syscall, i.e. we assume it is
> >> > > always present? (I'm not sure of the logic in linux-user code.)
> >> > >
> >> >
> >> > linux-user assumes it is present if __NR_msgsnd is present.
> >>
> >> Hm. The kernel change seems to break that assumption. Does anyone with
> >> mips knowledge have an idea whether that was intentional (and the
> >> linux-user code needs to be changed), or whether that's an issue on the
> >> kernel side?
> >>
> >
> > Hi, Cornelia.
> >
> > Thanks for your involving into this issue!
> >
> > It could be that (not-originating-from-MIPS) kernel commit:
> >
> >
https://github.com/torvalds/linux/commit/1a787fc5ba18ac767e635c58d06a0b46876184e3
> >
> > made a mess with system call availability for MIPS (I will forward this
to
> > MIPS kernel maintainer Paul Burton). My impression is that this was not
> > intentional, and is a temporary instability of kernel interface.
>
> I think this stems from 2038 time bomb work. Eventually they want it to
> be possible to build non-legacy kernels that don't expose time32 to the
> outside world. As part of that new system calls are being introduced
> where needed. The IPC syscall which orignally multiplexed a bunch of
> these operations on some systems would eventually be potentially phased
> out.
>
> > However, I think that QEMU nevertheless should not make the assumption
that
> > if __NR_MSGSND, than semtimedop() is present. It could be true, but it
is
> > still just self-imposed belief in QEMU, kernel never guarantied such
things.
> >
> > The alternative way of invoking via IPCV6 (else part of “ifdef
> > __NR_MSGSND”) should work for MIPS in the present stage of headers and
> > kernel.
>
> Yeah I think #ifndef __NR_ipc would work for now. It shouldn't affect
> architectures that never had the IPC call.
>
> > As a side note, perhaps we shoul update kernel headers only off of
stable
> > kernel releases.
>
> I guess that's a part of the tension for supporting new kernel APIs
> quickly. At least 5.2-rc1 wasn't a random tree - you would expect the
> external facing ABI to be stable after the merge window closed. It would
> be nice to know what new features were being exposed though.
>

Yes, one would expect no intentional changes in ABI kernel headers would
happen after RC1. However, one must expect that there could certainly be
bugs in RC1 - and there is a larger risk of propagating these bugs to QEMU
with header updates from non-stable code.

Sincerely,
Aleksandar

> >
> > Regards,
> > Aleksandar
>
>
> --
> Alex Bennée