On Wed, Oct 14, 2020 at 01:45:28PM -0700, Andrii Nakryiko wrote:
> Fix data race in prepend_path() with re-reading mnt->mnt_ns twice without
> holding the lock. is_mounted() does check for NULL, but is_anon_ns(mnt->mnt_ns)
> might re-read the pointer again which could be NULL already, if in between
> reads one of kern_unmount()/kern_unmount_array()/umount_tree() sets mnt->mnt_ns
> to NULL.
Cute... What config/compiler has resulted in that? I agree with the analysis, but
I really hate the open-coded (and completely unexplained) use of IS_ERR_OR_NULL()
there.
> - if (is_mounted(vfsmnt) && !is_anon_ns(mnt->mnt_ns))
> + mnt_ns = READ_ONCE(mnt->mnt_ns);
> + /* open-coded is_mounted() to use local mnt_ns */
> + if (!IS_ERR_OR_NULL(mnt_ns) && !is_anon_ns(mnt_ns))
> error = 1; // absolute root
> else
> error = 2; // detached or not attached yet
Better turn that into an inlined helper in fs/mount.h, next to is_mounted(), IMO,
and kill that IS_ERR_OR_NULL garbage there. What that thing does is
if (ns == NULL || ns == MNT_NS_INTERNAL)
and it's *not* on any kind of hot path to warrant that kind of microoptimizations.
So let's make that
static inline bool is_real_ns(struct mnt_namespace *mnt_ns)
{
return mnt_ns && mnt_ns != MNT_NS_INTERNAL;
}
turn is_mounted(m) into is_real_ns(m->mnt_ns) and replace that line in your fix
with
if (is_real_ns(mnt_ns) && !is_anon_ns(mnt_ns))
Objections?
