vfs: fix dentry RCU to refcounting possibly sleeping dput()
This is the fix that the last two commits indirectly led up to - making
sure that we don't call dput() in a bad context on the dentries we've
looked up in RCU mode after the sequence count validation fails.
This basically expands d_rcu_to_refcount() into the callers, and then
fixes the callers to delay the dput() in the failure case until _after_
we've dropped all locks and are no longer in an RCU-locked region.
The case of 'complete_walk()' was trivial, since its failure case did
the unlock_rcu_walk() directly after the call to d_rcu_to_refcount(),
and as such that is just a pure expansion of the function with a trivial
movement of the resulting dput() to after 'unlock_rcu_walk()'.
In contrast, the unlazy_walk() case was much more complicated, because
not only does convert two different dentries from RCU to be reference
counted, but it used to not call unlock_rcu_walk() at all, and instead
just returned an error and let the caller clean everything up in
"terminate_walk()".
Happily, one of the dentries in question (called "parent" inside
unlazy_walk()) is the dentry of "nd->path", which terminate_walk() wants
a refcount to anyway for the non-RCU case.
So what the new and improved unlazy_walk() does is to first turn that
dentry into a refcounted one, and once that is set up, the error cases
can continue to use the terminate_walk() helper for cleanup, but for the
non-RCU case. Which makes it possible to drop out of RCU mode if we
actually hit the sequence number failure case.
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/namei.c b/fs/namei.c
index cc4bcfa..56e4f4d 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -494,37 +494,6 @@
br_read_unlock(&vfsmount_lock);
}
-/*
- * When we move over from the RCU domain to properly refcounted
- * long-lived dentries, we need to check the sequence numbers
- * we got before lookup very carefully.
- *
- * We cannot blindly increment a dentry refcount - even if it
- * is not locked - if it is zero, because it may have gone
- * through the final d_kill() logic already.
- *
- * So for a zero refcount, we need to get the spinlock (which is
- * safe even for a dead dentry because the de-allocation is
- * RCU-delayed), and check the sequence count under the lock.
- *
- * Once we have checked the sequence count, we know it is live,
- * and since we hold the spinlock it cannot die from under us.
- *
- * In contrast, if the reference count wasn't zero, we can just
- * increment the lockref without having to take the spinlock.
- * Even if the sequence number ends up being stale, we haven't
- * gone through the final dput() and killed the dentry yet.
- */
-static inline int d_rcu_to_refcount(struct dentry *dentry, seqcount_t *validate, unsigned seq)
-{
- if (likely(lockref_get_not_dead(&dentry->d_lockref))) {
- if (!read_seqcount_retry(validate, seq))
- return 0;
- dput(dentry);
- }
- return -ECHILD;
-}
-
/**
* unlazy_walk - try to switch to ref-walk mode.
* @nd: nameidata pathwalk data
@@ -539,16 +508,29 @@
{
struct fs_struct *fs = current->fs;
struct dentry *parent = nd->path.dentry;
- int want_root = 0;
BUG_ON(!(nd->flags & LOOKUP_RCU));
- if (nd->root.mnt && !(nd->flags & LOOKUP_ROOT)) {
- want_root = 1;
- spin_lock(&fs->lock);
- if (nd->root.mnt != fs->root.mnt ||
- nd->root.dentry != fs->root.dentry)
- goto err_root;
- }
+
+ /*
+ * Get a reference to the parent first: we're
+ * going to make "path_put(nd->path)" valid in
+ * non-RCU context for "terminate_walk()".
+ *
+ * If this doesn't work, return immediately with
+ * RCU walking still active (and then we will do
+ * the RCU walk cleanup in terminate_walk()).
+ */
+ if (!lockref_get_not_dead(&parent->d_lockref))
+ return -ECHILD;
+
+ /*
+ * After the mntget(), we terminate_walk() will do
+ * the right thing for non-RCU mode, and all our
+ * subsequent exit cases should unlock_rcu_walk()
+ * before returning.
+ */
+ mntget(nd->path.mnt);
+ nd->flags &= ~LOOKUP_RCU;
/*
* For a negative lookup, the lookup sequence point is the parents
@@ -562,30 +544,39 @@
* be valid if the child sequence number is still valid.
*/
if (!dentry) {
- if (d_rcu_to_refcount(parent, &parent->d_seq, nd->seq) < 0)
- goto err_root;
+ if (read_seqcount_retry(&parent->d_seq, nd->seq))
+ goto out;
BUG_ON(nd->inode != parent->d_inode);
} else {
- if (d_rcu_to_refcount(dentry, &dentry->d_seq, nd->seq) < 0)
- goto err_root;
- if (d_rcu_to_refcount(parent, &dentry->d_seq, nd->seq) < 0)
- goto err_parent;
+ if (!lockref_get_not_dead(&dentry->d_lockref))
+ goto out;
+ if (read_seqcount_retry(&dentry->d_seq, nd->seq))
+ goto drop_dentry;
}
- if (want_root) {
+
+ /*
+ * Sequence counts matched. Now make sure that the root is
+ * still valid and get it if required.
+ */
+ if (nd->root.mnt && !(nd->flags & LOOKUP_ROOT)) {
+ spin_lock(&fs->lock);
+ if (nd->root.mnt != fs->root.mnt || nd->root.dentry != fs->root.dentry)
+ goto unlock_and_drop_dentry;
path_get(&nd->root);
spin_unlock(&fs->lock);
}
- mntget(nd->path.mnt);
unlock_rcu_walk();
- nd->flags &= ~LOOKUP_RCU;
return 0;
-err_parent:
+unlock_and_drop_dentry:
+ spin_unlock(&fs->lock);
+drop_dentry:
+ unlock_rcu_walk();
dput(dentry);
-err_root:
- if (want_root)
- spin_unlock(&fs->lock);
+ return -ECHILD;
+out:
+ unlock_rcu_walk();
return -ECHILD;
}
@@ -614,10 +605,15 @@
if (!(nd->flags & LOOKUP_ROOT))
nd->root.mnt = NULL;
- if (d_rcu_to_refcount(dentry, &dentry->d_seq, nd->seq) < 0) {
+ if (unlikely(!lockref_get_not_dead(&dentry->d_lockref))) {
unlock_rcu_walk();
return -ECHILD;
}
+ if (read_seqcount_retry(&dentry->d_seq, nd->seq)) {
+ unlock_rcu_walk();
+ dput(dentry);
+ return -ECHILD;
+ }
mntget(nd->path.mnt);
unlock_rcu_walk();
}
