linux-kernel - Re: [PATCH 02/25] vfs: Allow fsinfo() to query what's in an fs

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Date:   Fri, 21 Jun 2019 15:50:50 +0100
From:   David Howells <dhowells@...hat.com>
To:     Christian Brauner <christian@...uner.io>
Cc:     dhowells@...hat.com, viro@...iv.linux.org.uk, raven@...maw.net,
        linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, mszeredi@...hat.com
Subject: Re: [PATCH 02/25] vfs: Allow fsinfo() to query what's in an fs_context [ver #13]

Christian Brauner <christian@...uner.io> wrote:

> > >If you tried to go through /proc/pid/fd with open(O_PATH), I think
> > >you'd get
> > >the symlink, not the target.
> > 
> > Then you should use fdget(), no? :)
> 
> That is unless you want fsinfo() to be useable on any fd and just fds
> that are returned from the new mount-api syscalls. Maybe that wasn't
> clear from my first mail.

fsinfo(), as coded, is usable on any fd, as for fstat(), statx() and
fstatfs().

I have made it such that if you do this on the fd returned by fsopen() or
fspick(), the access is diverted to the filesystem that the fs_context refers
to since querying anon_inodes is of little value.

Now, it could be argued that it should require an AT_xxx flag to cause this
diversion to happen.

> Is the information returned for:
> 
> int fd = fsopen()/fspick();
> fsinfo(fd);
> 
> int ofd = open("/", O_PATH);
> fsinfo(ofd, ...);
> 
> the same if they refer to the same mount or would they differ?

At the moment it differs.  In the former case, there may not even be a
superblock attached to the fd to query, though invariants like filesystem
parameter types and names can be queried.

David