Message-id: <173891122321.22054.758548441508211924@noble.neil.brown.name>
Date: Fri, 07 Feb 2025 17:53:43 +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, NeilBrown wrote:
> 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_at() 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.

No, that's not right.  user_path_at() never returns a negative dentry.
It isn't documented and I tried reading the code instead of looking at
callers.

So before that commit it DID return ENOENT rather than -EXDEV where as
after that commit it returns -EXDEV rather than -ENOENT.

So my patch IS restoring the old behaviour.  Are you *sure* you want
-EXDEV when given a non-existent path?

Thanks,
NeilBrown


> 
> - 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
> 
> 

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