kernfs: Simply by replacing kernfs_deref_open_node() with of_on()
kernfs_node->attr.open is an RCU pointer to kernfs_open_node. However, RCU dereference is currently only used in kernfs_notify(). Everywhere else, either we're holding the lock which protects it or know that the kernfs_open_node is pinned becaused we have a pointer to a kernfs_open_file which is hanging off of it. kernfs_deref_open_node() is used for the latter case - accessing kernfs_open_node from kernfs_open_file. The lifetime and visibility rules are simple and clear here. To someone who can access a kernfs_open_file, its kernfs_open_node is pinned and visible through of->kn->attr.open. Replace kernfs_deref_open_node() which simpler of_on(). The former takes both @kn and @of and RCU deref @kn->attr.open while sanity checking with @of. The latter takes @of and uses protected deref on of->kn->attr.open. As the return value can't be NULL, remove the error handling in the callers too. This shouldn't cause any functional changes. Cc: Imran Khan <imran.f.khan@oracle.com> Tested-by: Chengming Zhou <zhouchengming@bytedance.com> Reviewed-by: Chengming Zhou <zhouchengming@bytedance.com> Signed-off-by: Tejun Heo <tj@kernel.org> Link: https://lore.kernel.org/r/20220828050440.734579-2-tj@kernel.org Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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@ -57,31 +57,17 @@ static inline struct mutex *kernfs_open_file_mutex_lock(struct kernfs_node *kn)
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}
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/**
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* kernfs_deref_open_node - Get kernfs_open_node corresponding to @kn.
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*
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* @of: associated kernfs_open_file instance.
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* @kn: target kernfs_node.
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*
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* Fetch and return ->attr.open of @kn if @of->list is non empty.
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* If @of->list is not empty we can safely assume that @of is on
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* @kn->attr.open->files list and this guarantees that @kn->attr.open
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* will not vanish i.e. dereferencing outside RCU read-side critical
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* section is safe here.
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*
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* The caller needs to make sure that @of->list is not empty.
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* of_on - Return the kernfs_open_node of the specified kernfs_open_file
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* @of: taret kernfs_open_file
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*/
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static struct kernfs_open_node *
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kernfs_deref_open_node(struct kernfs_open_file *of, struct kernfs_node *kn)
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static struct kernfs_open_node *of_on(struct kernfs_open_file *of)
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{
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struct kernfs_open_node *on;
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on = rcu_dereference_check(kn->attr.open, !list_empty(&of->list));
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return on;
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return rcu_dereference_protected(of->kn->attr.open,
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!list_empty(&of->list));
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}
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/**
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* kernfs_deref_open_node_protected - Get kernfs_open_node corresponding to @kn
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* kernfs_deref_open_node_locked - Get kernfs_open_node corresponding to @kn
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*
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* @kn: target kernfs_node.
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*
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@ -96,7 +82,7 @@ kernfs_deref_open_node(struct kernfs_open_file *of, struct kernfs_node *kn)
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* The caller needs to make sure that kernfs_open_file_mutex is held.
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*/
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static struct kernfs_open_node *
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kernfs_deref_open_node_protected(struct kernfs_node *kn)
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kernfs_deref_open_node_locked(struct kernfs_node *kn)
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{
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return rcu_dereference_protected(kn->attr.open,
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lockdep_is_held(kernfs_open_file_mutex_ptr(kn)));
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@ -207,12 +193,8 @@ static void kernfs_seq_stop(struct seq_file *sf, void *v)
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static int kernfs_seq_show(struct seq_file *sf, void *v)
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{
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struct kernfs_open_file *of = sf->private;
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struct kernfs_open_node *on = kernfs_deref_open_node(of, of->kn);
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if (!on)
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return -EINVAL;
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of->event = atomic_read(&on->event);
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of->event = atomic_read(&of_on(of)->event);
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return of->kn->attr.ops->seq_show(sf, v);
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}
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@ -235,7 +217,6 @@ static ssize_t kernfs_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
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struct kernfs_open_file *of = kernfs_of(iocb->ki_filp);
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ssize_t len = min_t(size_t, iov_iter_count(iter), PAGE_SIZE);
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const struct kernfs_ops *ops;
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struct kernfs_open_node *on;
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char *buf;
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buf = of->prealloc_buf;
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@ -257,14 +238,7 @@ static ssize_t kernfs_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
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goto out_free;
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}
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on = kernfs_deref_open_node(of, of->kn);
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if (!on) {
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len = -EINVAL;
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mutex_unlock(&of->mutex);
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goto out_free;
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}
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of->event = atomic_read(&on->event);
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of->event = atomic_read(&of_on(of)->event);
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ops = kernfs_ops(of->kn);
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if (ops->read)
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@ -584,7 +558,7 @@ static int kernfs_get_open_node(struct kernfs_node *kn,
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struct mutex *mutex = NULL;
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mutex = kernfs_open_file_mutex_lock(kn);
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on = kernfs_deref_open_node_protected(kn);
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on = kernfs_deref_open_node_locked(kn);
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if (on) {
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list_add_tail(&of->list, &on->files);
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@ -629,7 +603,7 @@ static void kernfs_unlink_open_file(struct kernfs_node *kn,
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mutex = kernfs_open_file_mutex_lock(kn);
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on = kernfs_deref_open_node_protected(kn);
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on = kernfs_deref_open_node_locked(kn);
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if (!on) {
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mutex_unlock(mutex);
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return;
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@ -839,7 +813,7 @@ void kernfs_drain_open_files(struct kernfs_node *kn)
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return;
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mutex = kernfs_open_file_mutex_lock(kn);
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on = kernfs_deref_open_node_protected(kn);
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on = kernfs_deref_open_node_locked(kn);
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if (!on) {
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mutex_unlock(mutex);
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return;
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@ -874,11 +848,7 @@ void kernfs_drain_open_files(struct kernfs_node *kn)
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*/
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__poll_t kernfs_generic_poll(struct kernfs_open_file *of, poll_table *wait)
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{
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struct kernfs_node *kn = kernfs_dentry_node(of->file->f_path.dentry);
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struct kernfs_open_node *on = kernfs_deref_open_node(of, kn);
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if (!on)
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return EPOLLERR;
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struct kernfs_open_node *on = of_on(of);
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poll_wait(of->file, &on->poll, wait);
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