handle_bytes On Thu, Apr 27, 2023 at 7:36 PM Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > On Thu, 2023-04-27 at 18:52 +0300, Amir Goldstein wrote: > > On Thu, Apr 27, 2023 at 6:13 PM Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > > > > > On Tue, 2023-04-25 at 16:01 +0300, Amir Goldstein wrote: > > > > Jan, > > > > > > > > Following up on the FAN_REPORT_ANY_FID proposal [1], here is a shot at an > > > > alternative proposal to seamlessly support more filesystems. > > > > > > > > While fanotify relaxes the requirements for filesystems to support > > > > reporting fid to require only the ->encode_fh() operation, there are > > > > currently no new filesystems that meet the relaxed requirements. > > > > > > > > I will shortly post patches that allow overlayfs to meet the new > > > > requirements with default overlay configurations. > > > > > > > > The overlay and vfs/fanotify patch sets are completely independent. > > > > The are both available on my github branch [2] and there is a simple > > > > LTP test variant that tests reporting fid from overlayfs [3], which > > > > also demonstrates the minor UAPI change of name_to_handle_at(2) for > > > > requesting a non-decodeable file handle by userspace. > > > > > > > > Thanks, > > > > Amir. > > > > > > > > [1] https://lore.kernel.org/linux-fsdevel/20230417162721.ouzs33oh6mb7vtft@quack3/ > > > > [2] https://github.com/amir73il/linux/commits/exportfs_encode_fid > > > > [3] https://github.com/amir73il/ltp/commits/exportfs_encode_fid > > > > > > > > Amir Goldstein (4): > > > > exportfs: change connectable argument to bit flags > > > > exportfs: add explicit flag to request non-decodeable file handles > > > > exportfs: allow exporting non-decodeable file handles to userspace > > > > fanotify: support reporting non-decodeable file handles > > > > > > > > Documentation/filesystems/nfs/exporting.rst | 4 +-- > > > > fs/exportfs/expfs.c | 29 ++++++++++++++++++--- > > > > fs/fhandle.c | 20 ++++++++------ > > > > fs/nfsd/nfsfh.c | 5 ++-- > > > > fs/notify/fanotify/fanotify.c | 4 +-- > > > > fs/notify/fanotify/fanotify_user.c | 6 ++--- > > > > fs/notify/fdinfo.c | 2 +- > > > > include/linux/exportfs.h | 18 ++++++++++--- > > > > include/uapi/linux/fcntl.h | 5 ++++ > > > > 9 files changed, 67 insertions(+), 26 deletions(-) > > > > > > > > > > This set looks fairly benign to me, so ACK on the general concept. > > > > Thanks! > > > > > > > > I am starting to dislike how the AT_* flags are turning into a bunch of > > > flags that only have meanings on certain syscalls. I don't see a cleaner > > > way to handle it though. > > > > Yeh, it's not great. > > > > There is also a way to extend the existing API with: > > > > Perhstruct file_handle { > > unsigned int handle_bytes:8; > > unsigned int handle_flags:24; > > int handle_type; > > unsigned char f_handle[]; > > }; > > > > AFAICT, this is guaranteed to be backward compat > > with old kernels and old applications. > > > > That could work. It would probably look cleaner as a union though. > Something like this maybe? > > union { > unsigned int legacy_handle_bytes; > struct { > u8 handle_bytes; > u8 __reserved; > u16 handle_flags; > }; > } I have no problem with the union, but does this struct guarantee that the lowest byte of legacy_handle_bytes is in handle_bytes for all architectures? That's the reason I went with struct { unsigned int handle_bytes:8; unsigned int handle_flags:24; } Is there a problem with this approach? > > unsigned int handle_bytes:8; > > unsigned int handle_flags:24; > > __reserved must be zeroed (for now). You could consider using it for > some other purpose later. > > It's a little ugly as an API but it would be backward compatible, given > that we never use the high bits today anyway. > > Callers might need to deal with an -EINVAL when they try to pass non- > zero handle_flags to existing kernels, since you'd trip the > MAX_HANDLE_SZ check that's there today. > Exactly. > > It also may not be a bad idea that the handle_flags could > > be used to request specific fh properties (FID) and can also > > describe the properties of the returned fh (i.e. non-decodeable) > > that could also be respected by open_by_handle_at(). > > > > For backward compact, kernel will only set handle_flags in > > response if new flags were set in the request. > > > > Do you consider this extension better than AT_HANDLE_FID > > or worse? At least it is an API change that is contained within the > > exportfs subsystem, without polluting the AT_ flags global namespace. > > > > Personally, yes. I think adding a struct file_handle_v2 would be cleaner > and allows for expanding the API later through new flags. I agree. I will give it a try. Thanks, Amir.
