Re: [RFC] entry: Untangle the return value of syscall_enter_from_user_mode from syscall NR

From: Thomas Gleixner <tglx@kernel.org>
To: "Michal Suchánek" <msuchanek@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Jonathan Corbet <corbet@lwn.net>,
	Shuah Khan <skhan@linuxfoundation.org>,
	Huacai Chen <chenhuacai@kernel.org>,
	WANG Xuerui <kernel@xen0n.name>,
	Madhavan Srinivasan <maddy@linux.ibm.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Nicholas Piggin <npiggin@gmail.com>,
	"Christophe Leroy (CS GROUP)" <chleroy@kernel.org>,
	Paul Walmsley <pjw@kernel.org>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Alexandre Ghiti <alex@ghiti.fr>,
	Heiko Carstens <hca@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Alexander Gordeev <agordeev@linux.ibm.com>,
	Christian Borntraeger <borntraeger@linux.ibm.com>,
	Sven Schnelle <svens@linux.ibm.com>,
	Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
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	Rui Qi <qirui.001@bytedance.com>, Kees Cook <kees@kernel.org>,
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	loongarch@lists.linux.dev, linuxppc-dev@lists.ozlabs.org,
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Subject: Re: [RFC] entry: Untangle the return value of syscall_enter_from_user_mode from syscall NR
Date: Fri, 03 Jul 2026 11:34:39 +0200	[thread overview]
Message-ID: <87ik6wmn68.ffs@fw13> (raw)
In-Reply-To: <akdqlO0eJ6jKH-wU@kunlun.suse.cz>

On Fri, Jul 03 2026 at 09:53, Michal Suchánek wrote:
> On Thu, Jul 02, 2026 at 10:45:54PM +0200, Thomas Gleixner wrote:
>> > When the syscall number was -1 to start with it's not possible to
>> > determine if the syscall was fileterd from the return value. s390
>> > returns the filtered state in a flag it sets on the regs structure,
>> > avoiding this problem.
>> 
>> What needs to determine whether the syscall was filtered or not?
>
> The code that executes syscall_enter_from_user_mode() needs to determine
> that.
>
> After syscall_enter_from_user_mode() returns the syscall needs to be
> executed or skipped.
>
> 'Executing' an invalid syscall boils down to setting the return value to
> -ENOSYS.
>
> But if the syscall number returned is -1 was the syscall filtered and
> the return value set by syscall_enter_from_user_mode() or should it be
> set by the caller to -ENOSYS?

See the explanation I gave to Peter. It's conclusive, but admittedly not
intuitive.

The fact, that s390 decided to have the syscall number and the return
value in the same register is a different problem, but as I pointed out
to Sven it could have been solved completely without that flag.

Thanks,

        tglx