On Mon, Jun 23, 2025 at 2:25 PM Christian Brauner <brauner@xxxxxxxxxx> wrote: > > 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? I think the AT prefix of AT_FDCWD may have been a mistake because it is quite easy to confuse this value with the completely unrelated namespace of AT_ flags. This is a null dirfd value. Is it not? FD_NULL, FD_NONE? You could envision that an *at() syscalls could in theory accept (FD_NONE , "/an/absolute/path/only", ... or MOUNTFD_NONE if we want to define a constant specifically for this open_by_handle_at() extension. Thanks, Amir.
