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

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 11 Aug 2026 14:05:00 -0500 Alex Elder <elder@riscstar.com> 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?


> --- 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;
>