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

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Message-ID: <7mxksfnkamzqromejfknfsat6cah4taggprj3wxdoputvvwc7f@qnjsm36bsrex>
Date: Fri, 7 Feb 2025 01:13:01 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: NeilBrown <neilb@...e.de>
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, 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? :)

> 
> 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.