Re: [PATCH 1/4] fs/notify: fdinfo can report unsupported file handles.

[NeilBrown <neilb@suse.com>]

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On Tue, Dec 12 2017, Amir Goldstein wrote:

> On Mon, Dec 11, 2017 at 11:52 PM, NeilBrown <neilb@suse.com> wrote:
>> On Mon, Dec 11 2017, Amir Goldstein wrote:
>>
>>> On Mon, Dec 11, 2017 at 8:04 AM, NeilBrown <neilb@suse.com> wrote:
>>>> If a filesystem does not set sb->s_export_op, then it
>>>> does not support filehandles and export_fs_encode_fh()
>>>> and exportfs_encode_inode_fh() should not be called.
>>>> They will use export_encode_fh() is which is a default
>>>> that uses inode number generation number, but in general
>>>> they may not be stable.
>>>>
>>>> So change exportfs_encode_inode_fh() to return FILEID_INVALID
>>>> if called on an unsupported Filesystem.  Currently only
>>>> notify/fdinfo can do that.
>>>>
>>>
>>> I wish you would leave this check to the caller, maybe add a helper
>>> exportfs_can_decode_fh() for callers to use.
>>>
>>> Although there are no current uses for it in-tree, there is value in
>>> being able to encode a unique file handle even when it cannot be
>>> decoded back to an open file.
>>>
>>> I am using this property in my fanotify super block watch patches,
>>> where the object identifier on the event is an encoded file handle
>>> of the object, which delegates tracking filesystem objects to
>>> userspace and prevents fanotify from keeping elevated refcounts
>>> on inodes and dentries.
>>>
>>> There are quite a few userspace tools out there that are checking
>>> that st_ino hasn't changed on a file between non atomic operations.
>>> Those tools (or others) could benefit from a unique file handle if
>>> we ever decide to provide a relaxed version of name_to_handle_at().
>>
>> If the filesystem doesn't define ->s_export_op, then you really cannot
>> trust anything beyond the inode number (and maybe not even that), and
>> the inode number is already easily available.
>> What actual value do you think you get from this pretend-file-handle
>> on filesystems that don't support file handles?
>>
>
> Sorry, I misread your patch. In my mind I thought you wanted to
> eliminate the default export_encode_fh if there was no fh_to_dentry
> operation like do_sys_name_to_handle() does. Just in my head...

I see... I would have said that fh_to_dentry was mandatory if
s_export_op was set, and Documentation/filesystems/nfs/Exporting agrees
with me.  But I do see that sys_name_to_handle() and nfsd explicitly
test for it as well as for s_export_op.

It appears that cifs sets s_export_op, but doesn't provide fh_to_dentry,
and it is unique in this.
But the CIFS_NFSD_EXPORT config option is marked as BROKEN, so
that probably doesn't matter.

So for all current filesystems, filehandles are only reported if they
are usable for dentry lookup...

>
> FWIW, according to Pavel, if fdinfo would not export file handle
> in case !fh_to_dentry op would probably be desirable, because
> criu has no need for file handles that cannot be decoded.

Yes, it was good to have that confirmed - thanks for getting that
checked.

NeilBrown

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