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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4802dc90068sm274897235e9.7.2026.01.21.06.48.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 21 Jan 2026 06:48:03 -0800 (PST) Date: Wed, 21 Jan 2026 14:48:02 +0000 From: David Laight To: kernel test robot Cc: Ryan Roberts , Catalin Marinas , Will Deacon , Huacai Chen , Madhavan Srinivasan , Michael Ellerman , Paul Walmsley , Palmer Dabbelt , Albert Ou , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , Kees Cook , "Gustavo A. R. Silva" , Arnd Bergmann , Mark Rutland , "Jason A. Donenfeld" , Ard Biesheuvel , Jeremy Linton , llvm@lists.linux.dev, oe-kbuild-all@lists.linux.dev, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, loongarch@lists.linux.dev, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org Subject: Re: [PATCH v4 3/3] randomize_kstack: Unify random source across arches Message-ID: <20260121144802.1287ce3e@pumpkin> In-Reply-To: <20260121102017.539b5531@pumpkin> References: <20260119130122.1283821-4-ryan.roberts@arm.com> <202601210752.6Nsv9et9-lkp@intel.com> <20260121102017.539b5531@pumpkin> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: llvm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Wed, 21 Jan 2026 10:20:17 +0000 David Laight wrote: > On Wed, 21 Jan 2026 07:50:16 +0800 > kernel test robot wrote: > > > Hi Ryan, > > > > kernel test robot noticed the following build warnings: > > > > [auto build test WARNING on akpm-mm/mm-everything] > > [also build test WARNING on linus/master v6.19-rc6 next-20260119] > > [cannot apply to tip/sched/core kees/for-next/hardening kees/for-next/execve] > > [If your patch is applied to the wrong git tree, kindly drop us a note. > > And when submitting patch, we suggest to use '--base' as documented in > > https://git-scm.com/docs/git-format-patch#_base_tree_information] > > > > url: https://github.com/intel-lab-lkp/linux/commits/Ryan-Roberts/randomize_kstack-Maintain-kstack_offset-per-task/20260119-210329 > > base: https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything > > patch link: https://lore.kernel.org/r/20260119130122.1283821-4-ryan.roberts%40arm.com > > patch subject: [PATCH v4 3/3] randomize_kstack: Unify random source across arches > > config: x86_64-allmodconfig (https://download.01.org/0day-ci/archive/20260121/202601210752.6Nsv9et9-lkp@intel.com/config) > > compiler: clang version 20.1.8 (https://github.com/llvm/llvm-project 87f0227cb60147a26a1eeb4fb06e3b505e9c7261) > > reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260121/202601210752.6Nsv9et9-lkp@intel.com/reproduce) > > > > If you fix the issue in a separate patch/commit (i.e. not just a new version of > > the same patch/commit), kindly add following tags > > | Reported-by: kernel test robot > > | Closes: https://lore.kernel.org/oe-kbuild-all/202601210752.6Nsv9et9-lkp@intel.com/ > > > > All warnings (new ones prefixed by >>): > > > > >> vmlinux.o: warning: objtool: do_syscall_64+0x2c: call to preempt_count_add() leaves .noinstr.text section > > >> vmlinux.o: warning: objtool: __do_fast_syscall_32+0x3d: call to preempt_count_add() leaves .noinstr.text section > > > > When CONFIG_DEBUG_PREEMPT or CONFIG_TRACE_PREEMP_TOGGLE is set > the preempt_count_[en|dis]able() calls inside [put|get]_cpu_var() > become real functions. > > Maybe __preempt_count_[inc|dec]() can be called (with this_cpu_ptr()). Or the code could just use the per-cpu data without disabling preemption. Usually that isn't a good idea at all, but it can't matter in this case. Might give a noticeable performance gain, disabling preemption is non-trivial and/or an atomic operation on some architectures. If anyone is worried about preemption causing the output be repeated, that would be (mostly) mitigated by checking that s[1234] haven't changed prior to writing the new values. I think a 'not locked at all' compare of two of the four values will stop everything except two threads doing system calls at the same time getting the same output from the prng. The whole thing is very unlikely and there will be much easier ways to break the prng. Provided s[1234] are only written with valid values (ie ones which aren't effectively zero) it will continue generating numbers. David > > David >