Re: [PATCH v4 3/3] pidfd: Add task path ioctls

[Christian Brauner]

On Thu, Nov 05, 2026 at 10:16:44PM +0100, Christian Brauner wrote:
> On Tue, Oct 27, 2026 at 04:46:09PM +0800, Chen Linxuan via B4 Relay wrote:
> > +	case PIDFD_GET_EXE:
> > +	case PIDFD_GET_CWD:
> > +	case PIDFD_GET_ROOT:
> > +		error = pidfd_get_task_path(pidfd_pid(file), cmd, arg, &path);
> > +		if (error)
> > +			return error;
> > +
> > +		return FD_ADD(O_CLOEXEC,
> > +			      dentry_open(&path, O_PATH, current_cred()));
> 
> So the path we take from the task is a jump target, just like
> /proc/<pid>/cwd and /proc/<pid>/root are. So complete_walk() runs
> ->d_weak_revalidate() on them. Without that a stale dentry on nfs or
> overlayfs turns into a valid O_PATH fd.
> 
> And failfs relies on ->d_weak_revalidate() to refuse pinning its root. A
> task that fchdir()ed into the failfs root can now hand it out through
> PIDFD_GET_CWD. Which would be breaking a guarantee we give. So we need
> the same check here. I'm just not yet sure how we best do this.

Ok, just saw that Sashiko also reported that. Hm, so I think we need a
dedicated "task open" helper. Something like: