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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: <878q7tprau.ffs@fw13> <87jyrdnmrh.ffs@fw13> Date: Fri, 03 Jul 2026 11:34:39 +0200 Message-ID: <87ik6wmn68.ffs@fw13> 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 On Fri, Jul 03 2026 at 09:53, Michal Such=C3=A1nek 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. >>=20 >> 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