linux-kernel - Re: [PATCH 14/15] arch: add split IPC system calls where needed

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Date:   Thu, 10 Jan 2019 21:32:53 +0100
From:   Heiko Carstens <heiko.carstens@...ibm.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     y2038@...ts.linaro.org, linux-api@...r.kernel.org,
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Subject: Re: [PATCH 14/15] arch: add split IPC system calls where needed

On Thu, Jan 10, 2019 at 05:24:34PM +0100, Arnd Bergmann wrote:
> The IPC system call handling is highly inconsistent across architectures,
> some use sys_ipc, some use separate calls, and some use both.  We also
> have some architectures that require passing IPC_64 in the flags, and
> others that set it implicitly.
> 
> For the additon of a y2083 safe semtimedop() system call, I chose to only
> support the separate entry points, but that requires first supporting
> the regular ones with their own syscall numbers.
> 
> The IPC_64 is now implied by the new semctl/shmctl/msgctl system
> calls even on the architectures that require passing it with the ipc()
> multiplexer.
> 
> I'm not adding the new semtimedop() or semop() on 32-bit architectures,
> those will get implemented using the new semtimedop_time64() version
> that gets added along with the other time64 calls.
> Three 64-bit architectures (powerpc, s390 and sparc) get semtimedop().
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> One aspect here that might be a bit controversial is the use of
> the same system call numbers across all architectures, synchronizing
> all of them with the x86-32 numbers. With the new syscall.tbl
> files, I hope we can just keep doing that in the future, and no
> longer require the architecture maintainers to assign a number.
> 
> This is mainly useful for implementers of the C libraries: if
> we can add future system calls everywhere at the same time, using
> a particular version of the kernel headers also guarantees that
> the system call number macro is visible.

> diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
> index 022fc099b628..428cf512a757 100644
> --- a/arch/s390/kernel/syscalls/syscall.tbl
> +++ b/arch/s390/kernel/syscalls/syscall.tbl
> @@ -391,3 +391,15 @@
>  381  common	kexec_file_load		sys_kexec_file_load		compat_sys_kexec_file_load
>  382  common	io_pgetevents		sys_io_pgetevents		compat_sys_io_pgetevents
>  383  common	rseq			sys_rseq			compat_sys_rseq
> +# room for arch specific syscalls
> +392	64	semtimedop		sys_semtimedop			-
> +393  common	semget			sys_semget			sys_semget
...
> +395  common	shmget			sys_shmget			sys_shmget
...
> +398  common	shmdt			sys_shmdt 			sys_shmdt
> +399  common	msgget			sys_msgget			sys_msgget

These four need compat system call wrappers, unfortunately... (well,
actually only shmget and shmdt require them, but let's add them for
all four). See arch/s390/kernel/compat_wrapper.c

I'm afraid this compat special handling will be even more annoying in
the future, since s390 will be the only architecture which requires
this special handling.

_Maybe_ it would make sense to automatically generate a weak compat
system call wrapper for s390 with the SYSCALL_DEFINE macros, but that
probably won't work in all cases.