linux-kernel - Re: [PATCH] vfs: avoid spurious dentry ref/unref cycle on open

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Message-ID: <20240807063350.GV5334@ZenIV>
Date: Wed, 7 Aug 2024 07:33:50 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: brauner@...nel.org, jack@...e.cz, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] vfs: avoid spurious dentry ref/unref cycle on open

On Wed, Aug 07, 2024 at 07:23:00AM +0100, Al Viro wrote:
> 	After having looked at the problem, how about the following
> series:
> 
> 1/5) lift path_get() *AND* path_put() out of do_dentry_open()
> into the callers.  The latter - conditional upon "do_dentry_open()
> has not set FMODE_OPENED".  Equivalent transformation.
> 
> 2/5) move path_get() we'd lifted into the callers past the
> call of do_dentry_open(), conditionally collapse it with path_put().
> You'd get e.g.
> int vfs_open(const struct path *path, struct file *file)
> {
>         int ret;
> 
>         file->f_path = *path;
>         ret = do_dentry_open(file, NULL);
>         if (!ret) {
>                 /*
>                  * Once we return a file with FMODE_OPENED, __fput() will call
>                  * fsnotify_close(), so we need fsnotify_open() here for
>                  * symmetry.
>                  */
>                 fsnotify_open(file);
>         }
> 	if (file->f_mode & FMODE_OPENED)
> 		path_get(path);
>         return ret;
> }
> 
> Equivalent transformation, provided that nobody is playing silly
> buggers with reassigning ->f_path in their ->open() instances.
> They *really* should not - if anyone does, we'd better catch them
> and fix them^Wtheir code.  Incidentally, if we find any such,
> we have a damn good reason to add asserts in the callers.  As
> in, "if do_dentry_open() has set FMODE_OPENED, it would bloody
> better *not* modify ->f_path".  <greps> Nope, nobody is that
> insane.
> 
> 3/5) split vfs_open_consume() out of vfs_open() (possibly
> named vfs_open_borrow()), replace the call in do_open() with
> calling the new function.
> 
> Trivially equivalent transformation.
> 
> 4/5) Remove conditional path_get() from vfs_open_consume()
> and finish_open().  Add
> 		if (file->f_mode & FMODE_OPENED)
> 			path_get(&nd->path);
> before terminate_walk(nd); in path_openat().
> 
> Equivalent transformation - see
>         if (file->f_mode & (FMODE_OPENED | FMODE_CREATED)) {
>                 dput(nd->path.dentry);
>                 nd->path.dentry = dentry;
>                 return NULL;
>         }
> in lookup_open() (which is where nd->path gets in sync with what
> had been given to do_dentry_open() in finish_open()); in case
> of vfs_open_consume() in do_open() it's in sync from the very
> beginning.  And we never modify nd->path after those points.
> So we can move grabbing it downstream, keeping it under the
> same condition (which also happens to be true only if we'd
> called do_dentry_open(), so for all other paths through the
> whole thing it's a no-op.
> 
> 5/5) replace
> 		if (file->f_mode & FMODE_OPENED)
> 			path_get(&nd->path);
> 		terminate_walk(nd);
> with
> 		if (file->f_mode & FMODE_OPENED) {
> 			nd->path.mnt = NULL;
> 			nd->path.dentry = NULL;
> 		}
> 		terminate_walk(nd);
> Again, an obvious equivalent transformation.

BTW, similar to that, with that we could turn do_o_path()
into

        struct path path;
        int error = path_lookupat(nd, flags, &path);
        if (!error) {
                audit_inode(nd->name, path.dentry, 0);
                error = vfs_open_borrow(&path, file);
		if (!(file->f_mode & FMODE_OPENED))
			path_put(&path);
        }
        return error;
}

and perhaps do something similar in the vicinity of
vfs_tmpfile() / do_o_tmpfile().