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Fri, 03 Jul 2026 06:17:00 +0000 (GMT) Received: from smtpav02.fra02v.mail.ibm.com (smtpav02.fra02v.mail.ibm.com [10.20.54.101]) by smtprelay06.fra02v.mail.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 6636GvOd23724404 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 3 Jul 2026 06:16:57 GMT Received: from smtpav02.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id E084220040; Fri, 3 Jul 2026 06:16:56 +0000 (GMT) Received: from smtpav02.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 5DC3D20043; Fri, 3 Jul 2026 06:16:56 +0000 (GMT) Received: from tuxmaker.linux.ibm.com (unknown [9.87.85.9]) by smtpav02.fra02v.mail.ibm.com (Postfix) with ESMTPS; Fri, 3 Jul 2026 06:16:56 +0000 (GMT) From: Sven Schnelle To: Michal =?utf-8?Q?Such=C3=A1nek?= Cc: Peter Zijlstra , Jonathan Corbet , Shuah Khan , Huacai Chen , WANG Xuerui , Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , "Christophe Leroy (CS GROUP)" , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Christian Borntraeger , Andy Lutomirski , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Andrew Donnellan , Mark Rutland , Arnd Bergmann , Jiaxun Yang , Ryan Roberts , Greg Kroah-Hartman , Mukesh Kumar Chaurasiya , Shrikanth Hegde , Zong Li , Nam Cao , Deepak Gupta , Lukas Gerlach , Rui Qi , Kees Cook , 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 In-Reply-To: References: Date: Fri, 03 Jul 2026 08:16:56 +0200 Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Authority-Analysis: v=2.4 cv=RYqgzVtv c=1 sm=1 tr=0 ts=6a4753de cx=c_pps a=bLidbwmWQ0KltjZqbj+ezA==:117 a=bLidbwmWQ0KltjZqbj+ezA==:17 a=IkcTkHD0fZMA:10 a=RAioF0-LDSMA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=uAbxVGIbfxUO_5tXvNgY:22 a=PxzjjWZHAImlhUGC5A8A:9 a=QEXdDO2ut3YA:10 X-Proofpoint-Spam-Info: AW1haW4tMjYwNzAzMDA1MiBTYWx0ZWRfX3KioJ0aE0IJK 6GCXJHhUp1F2Z1YveNR5roy+VlB9mYaqqPi6ZnW5aiC5GW26VV2+HS77GgwsK/LiTuT7b05HLTJ 7bqNBO0gAyrISbMT0IKHCUvnIc+Bc/c= X-Proofpoint-GUID: MZHualSITsNhKNRwX3VbQpZ3GLr3BMiU X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNzAzMDA1MiBTYWx0ZWRfX1xdOF6m0TbsV RRdovO7OWz9x9ZJwMBitj+Ms2sSe6kn4owwgid7NsQ+hw6RMIxUbpMSid4pQ0mJnsYrCDGpBWuN ettr7U4yik8mEnZt/EICOdu8qFM5H9i+b35dllBVZzrNmsQ0B2SVeUuCOeJ8TraSiMkYJUJO+Nc BJnyRM01Jj4QXANp0Io9tTz6Hj1vAd7lh+mkuzGmIJAfH0VdAS260qxw2YxoCTBvcir1LIuYlPw LOfBvQ9Z5gu2kgM3zQKXqocxBvBylQvTG6K1R0ZF5Oy59AM0zBTpy13iio6Fdsb3nIFuT345/oZ uzPcE5TicueH+0AQBY6ur7FPfScbrnIFQO95r+GaiM+KZ/cE3mSboUBKGGWCtZHTxfM/3ubcCX4 lKnp6tSsvLMOKZt/p9bvHcVJ7wXQ4gz33Og1uvgzaO9B74gsTFQTPb0/f/1a5u1GN5Ijss+/Nej V3bAm3cpL0fOCvtJW1g== X-Proofpoint-ORIG-GUID: UKYrWbQA3xg_QgCJ6b-fW3NtJr_lLQLl X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.125,FMLib:17.12.100.49 definitions=2026-07-03_01,2026-06-26_01,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 phishscore=0 impostorscore=0 malwarescore=0 spamscore=0 lowpriorityscore=0 adultscore=0 priorityscore=1501 suspectscore=0 bulkscore=0 clxscore=1015 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2607030052 Michal Such=C3=A1nek writes: > On Thu, Jul 02, 2026 at 02:01:02PM +0200, Sven Schnelle wrote: >> Michal Such=C3=A1nek writes: >>=20 >> > On Thu, Jul 02, 2026 at 10:12:35AM +0200, Sven Schnelle wrote: >> >> Michal Such=C3=A1nek writes: >> >>=20 >> >> > The return value of syscall_enter_from_user_mode is used both for t= he >> >> > adjusted syscall number and the indicator that a syscall should be >> >> > skipped. >> >> > >> >> > As seccomp can be invoked on any syscall, including invalid ones th= is >> >> > 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 t= his >> >> > 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 syscal= l is >> >> > already handled. >> >> > >> >> > This should avoid the need for the s390 PIF_SYSCALL_RET_SET which i= s the >> >> > workaround for exactly this deficiency. >> >>=20 >> >> 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 qui= te >> >> a while since I touched all that ptrace stuff. >> > >> > When the syscall return value is set (in the registers) the return val= ue >> > which is also the modified syscall number is set to -1 indicating the >> > syscall was handled. At least that's how the API is described. >> > >> > So yes, if the syscall number range is restricted or the syscall number >> > is returned through a path different from the function return value the >> > flag should not be needed in the entry path because the case can be >> > detected through the return value alone. >>=20 >> I'm still failing to see how this would work without an additional >> flag. Assume a program (the tracee) is stopped because of a syscall >> entry. The tracer then decides to skip the syscall and changes >> regs->gpr2 (which contains either the syscall number or return value) >> to contain 42. When the tracer than restarts the syscall, how does >> do_syscall() know that gpr2 is now a return value and not a syscall numb= er? > > Because then the return value from the syscall_enter_from_user_mode > machinery would be -1 indicating the syscall should be skipped. That is > how the return value of syscall_enter_from_user_mode is documented, I > did not verify that it actually works that way for the tracing case on > s390. I read the code and tested - I think I confused the way how syscall are intercepted by seccomp vs. ptrace. The PIF_SYSCALL_RET_SET is indeed only required to indicate syscalls skipped via seccomp and not ptrace.