Re: [RFC][PATCH 0/4] Prepare for supporting more filesystems with fanotify

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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;
	};
}

__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.

> 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.
-- 
Jeff Layton <jlayton@xxxxxxxxxx>




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