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

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Message-id: <173915041509.22054.12649815796390080222@noble.neil.brown.name>
Date: Mon, 10 Feb 2025 12:20:15 +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 Sat, 08 Feb 2025, Kent Overstreet wrote:
> On Fri, Feb 07, 2025 at 06:30:00PM +1100, NeilBrown wrote:
> > 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...
> 
> Ahh...
> 
> Well, if I've scanned the series correctly (sorry, we're on different
> timezones and I haven't had much caffeine yet) I hope you don't have to
> keep that function just for bcachefs - but I do think the error code is
> important.
> 
> Userspace getting -ENOENT and reporting -ENOENT to the user will
> inevitably lead to head banging frustration by someone, somewhere, when
> they're trying to delete something and the system is tell them it
> doesn't exist when they can see it very much does exist, right there :)
> the more precise error code is a very helpful cue...
> 

???
You will only get -ENOENT if there is no ent.  There is no question of a
confusing error message.
If you ask for a non-exist name on the correct filesystem, you get -ENOENT
If you ask for an existing name of the wrong filesystem, you get -EXDEV
That all works as expected and always has.

But what if you ask for a non-existing name in a directory on the
wrong filesystem?  
The code you originally wrote in 42d237320e9817a9 would return
-ENOENT because that it what user_path_at() would return.
But using user_path_at() is "wrong" because it doesn't lock the directory
so ->d_parent is not guaranteed to be stable.
Al fixed that in bbe6a7c899e7f265c using user_path_locked_at(), but
that doesn't check for a negative dentry so Al added a check to return
-ENOENT, but that was added *after* the test that returns -EXDEV.

So now if you call subvolume_destroy on a non-existing name in a
directory on the wrong filesystem, you get -EXDEV.  I think that is
a bit weird but not a lot weird.
My patch will change it back to -ENOENT - the way you originally wrote
it.

I hope you are ok with that.

NeilBrown