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

From: Sven Schnelle <svens@linux.ibm.com>
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>,
	Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@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>,
	Andrew Donnellan <andrew+kernel@donnellan.id.au>,
	Mark Rutland <mark.rutland@arm.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Jiaxun Yang <jiaxun.yang@flygoat.com>,
	Ryan Roberts <ryan.roberts@arm.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Mukesh Kumar Chaurasiya <mkchauras@linux.ibm.com>,
	Shrikanth Hegde <sshegde@linux.ibm.com>,
	Zong Li <zong.li@sifive.com>, Nam Cao <namcao@linutronix.de>,
	Deepak Gupta <debug@rivosinc.com>,
	Lukas Gerlach <lukas.gerlach@cispa.de>,
	Rui Qi <qirui.001@bytedance.com>, Kees Cook <kees@kernel.org>,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	loongarch@lists.linux.dev, linuxppc-dev@lists.ozlabs.org,
	linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org
Subject: Re: [RFC] entry: Untangle the return value of syscall_enter_from_user_mode from syscall NR
Date: Thu, 02 Jul 2026 10:12:35 +0200	[thread overview]
Message-ID: <yt9dechlbyj0.fsf@linux.ibm.com> (raw)
In-Reply-To: <akVRcPsD_R_CE1qW@kunlun.suse.cz>

Michal Suchánek <msuchanek@suse.de> writes:

> The return value of syscall_enter_from_user_mode is used both for the
> adjusted syscall number and the indicator that a syscall should be
> skipped.
>
> As seccomp can be invoked on any syscall, including invalid ones this
> somewhat undermines seccomp.
>
> While the seccomp variants that terminate the process do not need to
> care about this for the filter that sets the syscall return value this
> disctinction is required.
>
> Pass the syscall number as a pointer to the inline entry functions, and
> use the return value exclusively for the indication that the syscall is
> already handled.
>
> This should avoid the need for the s390 PIF_SYSCALL_RET_SET which is the
> workaround for exactly this deficiency.

I'm not sure whether PIF_SYSCALL_RET_SET can be removed - the syscall
return might still get set by PTRACE_SET_SYSCALL_INFO when the tracee is
stopped. This might be a positive number which can't be distinguished
from a syscall number. But maybe i'm missing something? It's been quite
a while since I touched all that ptrace stuff.