Re: [PATCH] efivarfs: fix NULL dereference on resume

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Mar 18, 2025 at 08:04:59AM +0100, Ard Biesheuvel wrote:

> the latter is only needed when it is mounted to begin with, and as a
> VFS non-expert, I struggle to understand why it is a) ok and b)
> preferred to create a new mount to pass to kernel_file_open(). Could
> we add a paragraph to the commit log that explains this?

I'm not at all convinced that iterate_dir() is the right thing to use
there, but *IF* we go that way, yes, we need a reference to struct
mount.  We are not going to introduce a very special kind of struct
file, along with the arseloads of checking for that crap all over the
place - not for the sake of one weird case in one weird filesystem.

file->f_path is a valid struct path, which means that ->mnt->mnt_sb == ->dentry->d_sb
and refcount of ->mnt is positive as long as struct file exists.

Keeping a persistent internal struct mount is, of course, possible,
but it will make the damn thing impossible to rmmod, etc. - it will
remain in place until the reboot.

It might be possible to put together something like "grab a reference
to superblock and allocate a temporary struct mount refering to it"
(which is what that vfs_kern_mount() boils down to).  But I would
very much prefer to have it go over the list of children of ->s_root
manually, instead of playing silly buggers with iterate_dir().

And yes, it would require exclusion with dissolving dentry tree on
umount, for obvious reasons.  Which might be done with ->s_active
or simply by unregistering that notifier chain as the very first step
in ->kill_sb() there.