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Tue, 11 Aug 2026 19:06:31 -0700 (PDT) Message-ID: Date: Tue, 11 Aug 2026 21:06:30 -0500 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] kernel/sys.c: use RCU when accessing task_struct->real_parent To: Andrew Morton Cc: david@kernel.org, pjw@kernel.org, brauner@kernel.org, ljs@kernel.org, demiobenour@gmail.com, tglx@kernel.org, debug@rivosinc.com, oleg@redhat.com, thomas.weissschuh@linutronix.de, linux-kernel@vger.kernel.org References: <20260811190501.1486442-1-elder@riscstar.com> <20260811181721.f9e383fdb84f963520c664b6@linux-foundation.org> Content-Language: en-US From: Alex Elder In-Reply-To: <20260811181721.f9e383fdb84f963520c664b6@linux-foundation.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 8/11/26 8:17 PM, Andrew Morton wrote: > On Tue, 11 Aug 2026 14:05:00 -0500 Alex Elder wrote: > >> In the setpgid() syscall definition, a check is made to determine >> whether the target process is in the same thread group as the >> current process. The check accesses the target process's real_parent >> pointer directly, however that field is supposed to be accessed via >> RCU. Use rcu_dereference() to avoid this C=1 build warning: >> >> kernel/sys.c:1144:32: warning: incorrect type in argument 1 (different address spaces) > > Oh. > > Why don't all those other uses of ->real_parent produce this warning? Looks like there are others; this was just the one that kept showing up in my builds. Maybe some cases simply don't need the rcu_dereference() (like initializing init_task.real_parent in init/init_task.c). It's inconsistent. Some files use rcu_dereference() always for task_struct->real_parent: - drivers/connector/cn_proc.c - fs/binfmt_elf.c - fs/nfs/dir.c - fs/binfmt_elf_fdpic.c - kernel/sched/core.c - kernel/acct.c - security/keys/keyctl.c - security/selinux/hooks.c - security/yama/yama_lsm.c But others do not. And there are lots of other fields annotated with __rcu that don't always use rcu_dereference(): - task_struct->parent - task_struct->sighand - mm_struct->owner and so on. I could go dig a little deeper and solve this more comprehensively. It would probably have to be a long-running background effort. Do you think it's worth it? If not, I don't mind just retracting this suggestion for now. -Alex > > >> --- a/kernel/sys.c >> +++ b/kernel/sys.c >> @@ -1114,6 +1114,7 @@ COMPAT_SYSCALL_DEFINE1(times, struct compat_tms __user *, tbuf) >> SYSCALL_DEFINE2(setpgid, pid_t, pid, pid_t, pgid) >> { >> struct task_struct *p; >> + struct task_struct *real_parent; >> struct task_struct *group_leader = current->group_leader; >> struct pid *pids[PIDTYPE_MAX] = { 0 }; >> struct pid *pgrp; >> @@ -1141,7 +1142,8 @@ SYSCALL_DEFINE2(setpgid, pid_t, pid, pid_t, pgid) >> if (!thread_group_leader(p)) >> goto out; >> >> - if (same_thread_group(p->real_parent, group_leader)) { >> + real_parent = rcu_dereference(p->real_parent); >> + if (same_thread_group(real_parent, group_leader)) { >> err = -EPERM; >> if (task_session(p) != task_session(group_leader)) >> goto out; >> > >