On Mon, Jun 23, 2025 at 02:06:43PM +0200, Jan Kara wrote: > On Mon 23-06-25 11:01:30, Christian Brauner wrote: > > Various filesystems such as pidfs (and likely drm in the future) have a > > use-case to support opening files purely based on the handle without > > having to require a file descriptor to another object. That's especially > > the case for filesystems that don't do any lookup whatsoever and there's > > zero relationship between the objects. Such filesystems are also > > singletons that stay around for the lifetime of the system meaning that > > they can be uniquely identified and accessed purely based on the file > > handle type. Enable that so that userspace doesn't have to allocate an > > object needlessly especially if they can't do that for whatever reason. > > > > Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx> > > Hmm, maybe we should predefine some invalid fd value userspace should pass > when it wants to "autopick" fs root? Otherwise defining more special fd > values like AT_FDCWD would become difficult in the future. Or we could just Fwiw, I already did that with: #define PIDFD_SELF_THREAD -10000 /* Current thread. */ #define PIDFD_SELF_THREAD_GROUP -20000 /* Current thread group leader. */ I think the correct thing to do would have been to say anything below #define AT_FDCWD -100 /* Special value for dirfd used to is reserved for the kernel. But we can probably easily do this and say anything from -10000 to -40000 is reserved for the kernel. I would then change: #define PIDFD_SELF_THREAD -10000 /* Current thread. */ #define PIDFD_SELF_THREAD_GROUP -10001 /* Current thread group leader. */ since that's very very new and then move PIDFD_SELF_THREAD/PIDFD_SELF_THREAD_GROUP to include/uapi/linux/fcntl.h and add that comment about the reserved range in there. The thing is that we'd need to enforce this on the system call level. Thoughts? > define that FILEID_PIDFS file handles *always* ignore the fd value and > auto-pick the root. I see the issue I don't think it's a big deal but I'm open to adding: #define AT_EBADF -10009 /* -10000 - EBADF */ and document that as a stand-in for a handle that can't be resolved. Thoughts?
