Message-id: <173891006340.22054.12479197204884946914@noble.neil.brown.name>
Date: Fri, 07 Feb 2025 17:34:23 +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 03:53:52PM +1100, 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_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!
> 
> How do you always manage to be unfailingly polite? :)

My dad impressed the value of politeness on me at an early age.  It is
an effective way of manipulating people!

> 
> > 
> > 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.

Thanks,
NeilBrown

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