[PATCH 3.2 052/102] be careful with nd->inode in path_init() and follow_dotdot_rcu()

[Ben Hutchings]

3.2.64-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

commit 4023bfc9f351a7994fb6a7d515476c320f94a574 upstream.

in the former we simply check if dentry is still valid after picking
its ->d_inode; in the latter we fetch ->d_inode in the same places
where we fetch dentry and its ->d_seq, under the same checks.

Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/namei.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

--- a/fs/namei.c
+++ b/fs/namei.c
@@ -913,6 +913,7 @@ static bool __follow_mount_rcu(struct na
 
 static int follow_dotdot_rcu(struct nameidata *nd)
 {
+	struct inode *inode = nd->inode;
 	if (!nd->root.mnt)
 		set_root_rcu(nd);
 
@@ -926,6 +927,7 @@ static int follow_dotdot_rcu(struct name
 			struct dentry *parent = old->d_parent;
 			unsigned seq;
 
+			inode = parent->d_inode;
 			seq = read_seqcount_begin(&parent->d_seq);
 			if (read_seqcount_retry(&old->d_seq, nd->seq))
 				goto failed;
@@ -935,6 +937,7 @@ static int follow_dotdot_rcu(struct name
 		}
 		if (!follow_up_rcu(&nd->path))
 			break;
+		inode = nd->path.dentry->d_inode;
 		nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
 	}
 	while (d_mountpoint(nd->path.dentry)) {
@@ -944,9 +947,10 @@ static int follow_dotdot_rcu(struct name
 			break;
 		nd->path.mnt = mounted;
 		nd->path.dentry = mounted->mnt_root;
+		inode = nd->path.dentry->d_inode;
 		nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
 	}
-	nd->inode = nd->path.dentry->d_inode;
+	nd->inode = inode;
 	return 0;
 
 failed:
@@ -1556,7 +1560,14 @@ static int path_init(int dfd, const char
 	}
 
 	nd->inode = nd->path.dentry->d_inode;
-	return 0;
+	if (!(flags & LOOKUP_RCU))
+		return 0;
+	if (likely(!read_seqcount_retry(&nd->path.dentry->d_seq, nd->seq)))
+		return 0;
+	if (!(nd->flags & LOOKUP_ROOT))
+		nd->root.mnt = NULL;
+	rcu_read_unlock();
+	return -ECHILD;
 
 fput_fail:
 	fput_light(file, fput_needed);

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