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SubjectRe: [PATCH v4 2/5] fuse: Call vfs_get_tree() for submounts
On Fri, 21 May 2021 10:19:48 +0200
Miklos Szeredi <miklos@szeredi.hu> wrote:

> On Thu, May 20, 2021 at 05:46:51PM +0200, Greg Kurz wrote:
> > We don't set the SB_BORN flag on submounts superblocks. This is wrong
> > as these superblocks are then considered as partially constructed or
> > dying in the rest of the code and can break some assumptions.
> >
> > One such case is when you have a virtiofs filesystem and you try to
> > mount it again : virtio_fs_get_tree() tries to obtain a superblock
> > with sget_fc(). The matching criteria in virtio_fs_test_super() is
> > the pointer of the underlying virtiofs device, which is shared by
> > the root mount and its submounts. This means that any submount can
> > be picked up instead of the root mount. This is itself a bug :
> > submounts should be ignored in this case. But, most importantly, it
> > then triggers an infinite loop in sget_fc() because it fails to grab
> > the superblock (very easy to reproduce).
> >
> > The only viable solution is to set SB_BORN at some point. This
> > must be done with vfs_get_tree() because setting SB_BORN requires
> > special care, i.e. a memory barrier for super_cache_count() which
> > can check SB_BORN without taking any lock.
>
> Looks correct, but...
>
> as an easily backportable and verifiable bugfix I'd still go with the
> simple two liner:
>
> --- a/fs/fuse/dir.c
> +++ b/fs/fuse/dir.c
> @@ -351,6 +351,9 @@ static struct vfsmount *fuse_dentry_automount(struct path *path)
> list_add_tail(&fm->fc_entry, &fc->mounts);
> up_write(&fc->killsb);
>
> + smp_wmb();
> + sb->s_flags |= SB_BORN;
> +

plus the mandatory comment one must put to justify the
need for a memory barrier.

> /* Create the submount */
> mnt = vfs_create_mount(fsc);
> if (IS_ERR(mnt)) {
>
> And have this patch be the cleanup.
>

Fair enough.

> Also we need Fixes: and a Cc: stable@... tags on that one.
>

Oops, I'll add these in the next round.

> Thanks,
> Miklos

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