linux-kernel - Re: [PATCH 1/2] VFS: change kern_path_locked() and user_path_locked

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Message-id: <173890403265.22054.8267826472424760232@noble.neil.brown.name>
Date: Fri, 07 Feb 2025 15:53:52 +1100
From: "NeilBrown" <neilb@...e.de>
To: "Kent Overstreet" <kent.overstreet@...ux.dev>
Cc: "Christian Brauner" <brauner@...nel.org>,
 "Alexander Viro" <viro@...iv.linux.org.uk>, "Jan Kara" <jack@...e.cz>,
 "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
 "Rafael J. Wysocki" <rafael@...nel.org>, "Danilo Krummrich" <dakr@...nel.org>,
 "Trond Myklebust" <trondmy@...nel.org>, "Anna Schumaker" <anna@...nel.org>,
 "Namjae Jeon" <linkinjeon@...nel.org>, "Steve French" <sfrench@...ba.org>,
 "Sergey Senozhatsky" <senozhatsky@...omium.org>,
 "Tom Talpey" <tom@...pey.com>, "Paul Moore" <paul@...l-moore.com>,
 "Eric Paris" <eparis@...hat.com>, linux-kernel@...r.kernel.org,
 linux-bcachefs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-nfs@...r.kernel.org, linux-cifs@...r.kernel.org, audit@...r.kernel.org
Subject: Re: [PATCH 1/2] VFS: change kern_path_locked() and
 user_path_locked_at() to never return negative dentry

On Fri, 07 Feb 2025, Kent Overstreet wrote:
> On Fri, Feb 07, 2025 at 02:36:47PM +1100, NeilBrown wrote:
> > No callers of kern_path_locked() or user_path_locked_at() want a
> > negative dentry.  So change them to return -ENOENT instead.  This
> > simplifies callers.
> > 
> > This results in a subtle change to bcachefs in that an ioctl will now
> > return -ENOENT in preference to -EXDEV.  I believe this restores the
> > behaviour to what it was prior to
> 
> I'm not following how the code change matches the commit message?

Maybe it doesn't.  Let me checked.

Two of the possible error returns from bch2_ioctl_subvolume_destroy(),
which implements the BCH_IOCTL_SUBVOLUME_DESTROY ioctl, are -ENOENT and
-EXDEV.

-ENOENT is returned if the path named in arg.dst_ptr cannot be found.
-EXDEV is returned if the filesystem on which that path exists is not
 the one that the ioctl is called on.

If the target filesystem is "/foo" and the path given is "/bar/baz" and
/bar exists but /bar/baz does not, then user_path_locked_at or
user_path_at will return a negative dentry corresponding to the
(non-existent) name "baz" in /bar.

In this case the dentry exists so the filesystem on which it was found
can be tested, but the dentry is negative.  So both -ENOENT and -EXDEV
are credible return values.

- before bbe6a7c899e7 the -EXDEV is tested immediately after the call
  to user_path_att() so there is no chance that ENOENT will be returned.
  I cannot actually find where ENOENT could be returned ...  but that
  doesn't really matter now.

- after that patch .... again the -EXDEV test comes first. That isn't
  what I remember.  I must have misread it.

- after my patch user_path_locked_at() will return -ENOENT if the whole
  name cannot be found.  So now you get -ENOENT instead of -EXDEV.

So with my patch, ENOENT always wins, and it was never like that before.
Thanks for challenging me!

Do you think there could be a problem with changing the error returned
in this circumstance? i.e. if you try to destroy a subvolume with a
non-existant name on a different filesystem could getting -ENOENT
instead of -EXDEV be noticed?

Thanks,
NeilBrown