Re: [PATCH] kernel/sys.c: use RCU when accessing task_struct->real_parent

[Alex Elder]

On 8/11/26 8:17 PM, Andrew Morton wrote:
> On Tue, 11 Aug 2026 14:05:00 -0500 Alex Elder <elder@xxxxxxxxxxxx> 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;