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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id z4sm3040338pfz.99.2021.10.14.09.14.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 14 Oct 2021 09:14:22 -0700 (PDT) Date: Thu, 14 Oct 2021 09:14:21 -0700 From: Kees Cook To: Yafang Shao Cc: Mathieu Desnoyers , acme , rostedt , kernel test robot , 0day robot , Petr Mladek , Peter Zijlstra , Alexander Viro , linux-kernel , lkp , Andrew Morton , Valentin Schneider , Qiang Zhang , robdclark , christian , Dietmar Eggemann , Ingo Molnar , Juri Lelli , Vincent Guittot , Ben Segall , Mel Gorman , bristot , aubrey li , yu c chen Subject: Re: [sched.h] 317419b91e: perf-sanity-tests.Parse_sched_tracepoints_fields.fail Message-ID: <202110140910.295E856@keescook> References: <20211010102429.99577-4-laoar.shao@gmail.com> <20211014072707.GA18719@xsang-OptiPlex-9020> <1529739526.13983.1634215325995.JavaMail.zimbra@efficios.com> <173454728.14036.1634216949862.JavaMail.zimbra@efficios.com> <1171592945.14099.1634219447199.JavaMail.zimbra@efficios.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 14, 2021 at 10:40:04PM +0800, Yafang Shao wrote: > On Thu, Oct 14, 2021 at 9:50 PM Mathieu Desnoyers > wrote: > > > > ----- On Oct 14, 2021, at 9:11 AM, Yafang Shao laoar.shao@gmail.com wrote: > > > > > On Thu, Oct 14, 2021 at 9:09 PM Mathieu Desnoyers > > > wrote: > > >> > > >> ----- On Oct 14, 2021, at 9:05 AM, Yafang Shao laoar.shao@gmail.com wrote: > > >> [...] > > >> >> If it happens that this ABI break is noticed by more than an in-tree test > > >> >> program, then > > >> >> the kernel's ABI rules will require that this trace field size stays unchanged. > > >> >> This brings > > >> >> up once more the whole topic of "Tracepoints ABI" which has been discussed > > >> >> repeatedly in > > >> >> the past. > > >> >> > > >> > > > >> > I will check if any other in-tree tools depends on TASK_COMM_LEN. > > >> > > >> That's a start, but given this is a userspace ABI, out-of-tree userland > > >> tools which depend of this to be fixed-size are also relevant. > > >> > > > > > > TASK_COMM_LEN isn't defined in include/uapi/ directory, so it seems > > > that it isn't the uerspace ABI? > > > > One case where this 16 bytes size is expected by userspace is prctl(2) PR_GET_NAME > > and PR_SET_NAME. > > > > the prctl(2) man page says that: > : PR_SET_NAME > : If the length of the string, including the terminating > : null byte, exceeds 16 bytes, the string is silently truncated. > : PR_GET_NAME > : The buffer should allow space for up to 16 bytes > : the returned string will be null-terminated. > > As the string returned to user space is null-terminated, extending > task comm won't break the prctl(2) user code. If userspace was: char comm[16]; int important_values; ... prctl(PR_GET_NAME, pid, comm); the kernel will clobber "important_values", so prctl() must enforce the old size (and terminate it correctly). It's not okay for us to expect that userspace get recompiled -- old binaries must continue to work correctly on kernel kernels. case PR_GET_NAME: get_task_comm(comm, me); if (copy_to_user((char __user *)arg2, comm, sizeof(comm))) return -EFAULT; break; This looks like it needs to be explicitly NUL terminated at 16 as well. I'd say we need a TASK_COMM_LEN_V1 to use in all the old hard-coded places. -Kees > > > The other case I am referring to is with ftrace and perf: > > > > mount -t tracefs nodev /sys/kernel/tracing > > cat /sys/kernel/tracing/events/sched/sched_switch/format > > > > name: sched_switch > > ID: 314 > > format: > > [...] > > field:char prev_comm[16]; offset:8; size:16; signed:1; > > [...] > > field:char next_comm[16]; offset:40; size:16; signed:1; > > > > Both of those fields expose a fixed-size of 16 bytes. > > > > AFAIK Steven's intent was that by parsing this file, trace viewers could adapt to > > changes in the event field layout. Unfortunately, there have been cases where > > trace viewers had a hard expectation on the field layout. Hopefully those have > > all been fixed a while ago. > > > > I don't have a clear idea what will happen to trace viewers if we > extend task comm. > > Steven, do you have any suggestions ? > > -- > Thanks > Yafang -- Kees Cook