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

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On Thu, 2023-04-27 at 22:11 +0300, Amir Goldstein wrote:
> 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 is a very good point. 

> 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?
> 

I just have a natural aversion to bitfields.

What you're proposing would work fine, I think. You won't be able to
take a pointer into the bitfield of course, but that's not necessarily a
showstopper for an "interface struct" like file_handle.



> > >         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.

Cool.
-- 
Jeff Layton <jlayton@xxxxxxxxxx>




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