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

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Message-id: <173891340026.22054.12085488968187293785@noble.neil.brown.name>
Date: Fri, 07 Feb 2025 18:30:00 +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 05:34:23PM +1100, NeilBrown wrote:
> > On Fri, 07 Feb 2025, Kent Overstreet wrote:
> > > On Fri, Feb 07, 2025 at 03:53:52PM +1100, NeilBrown wrote:
> > > > 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?
> > > 
> > > -EXDEV is the standard error code for "we're crossing a filesystem
> > > boundary and we can't or aren't supposed to be", so no, let's not change
> > > that.
> > > 
> > 
> > OK.  As bcachefs is the only user of user_path_locked_at() it shouldn't
> > be too hard.
> 
> Hang on, why does that require keeping user_path_locked_at()? Just
> compare i_sb...
> 

I changed user_path_locked_at() to not return a dentry at all when the
full path couldn't be found.  If there is no dentry, then there is no
->d_sb.
(if there was an ->i_sb, there would be an inode and this all wouldn't
be an issue).

To recap: the difference happens if the path DOESN'T exist but the
parent DOES exist on a DIFFERENT filesystem.  It is very much a corner
case and the error code shouldn't matter.  But I had to ask...

NeilBrown