afs: Fix use-after-loss-of-ref

afs_lookup() has a tracepoint to indicate the outcome of
d_splice_alias(), passing it the inode to retrieve the fid from.
However, the function gave up its ref on that inode when it called
d_splice_alias(), which may have failed and dropped the inode.

Fix this by caching the fid.

Fixes: 80548b03991f ("afs: Add more tracepoints")
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 5 deletions

diff --git a/fs/afs/dir.c b/fs/afs/dir.c
index 497f979018c2..813db1708494 100644
--- a/fs/afs/dir.c
+++ b/fs/afs/dir.c
@@ -908,6 +908,7 @@ static struct dentry *afs_lookup(struct inode *dir, struct dentry *dentry,
 				 unsigned int flags)
 {
 	struct afs_vnode *dvnode = AFS_FS_I(dir);
+	struct afs_fid fid = {};
 	struct inode *inode;
 	struct dentry *d;
 	struct key *key;
@@ -957,15 +958,16 @@ static struct dentry *afs_lookup(struct inode *dir, struct dentry *dentry,
 		dentry->d_fsdata =
 			(void *)(unsigned long)dvnode->status.data_version;
 	}
+
+	if (!IS_ERR_OR_NULL(inode))
+		fid = AFS_FS_I(inode)->fid;
+
 	d = d_splice_alias(inode, dentry);
 	if (!IS_ERR_OR_NULL(d)) {
 		d->d_fsdata = dentry->d_fsdata;
-		trace_afs_lookup(dvnode, &d->d_name,
-				 inode ? AFS_FS_I(inode) : NULL);
+		trace_afs_lookup(dvnode, &d->d_name, &fid);
 	} else {
-		trace_afs_lookup(dvnode, &dentry->d_name,
-				 IS_ERR_OR_NULL(inode) ? NULL
-				 : AFS_FS_I(inode));
+		trace_afs_lookup(dvnode, &dentry->d_name, &fid);
 	}
 	return d;
 }