[PATCH v2 1/4] ceph: always clone dentry name when building MDS request

[Jeff Layton <jlayton@xxxxxxxxxx>]

Ben reported tripping the BUG_ON in create_request_message during some
performance testing. Analysis of the vmcore showed that the length of
the r_dentry->d_name string changed after we allocated the buffer, but
before we encoded it.

build_dentry_path returns pointers to d_name in the common case of
non-snapped dentries, but this optimization isn't safe. Instead, make a
copy of the string in this case. That is less efficient, but safe.

Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Ben England <bengland@xxxxxxxxxx>
Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
Reviewed-by: "Yan, Zheng" <zyan@xxxxxxxxxx>
---
 fs/ceph/mds_client.c | 40 ++++++++++++++++++++++++++++++++--------
 1 file changed, 32 insertions(+), 8 deletions(-)

diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
index e09feadd2ae1..4cfefe118128 100644
--- a/fs/ceph/mds_client.c
+++ b/fs/ceph/mds_client.c
@@ -2159,9 +2159,35 @@ char *ceph_mdsc_build_path(struct dentry *dentry, int *plen, u64 *base,
 	return path;
 }
 
+/* Duplicate the dentry->d_name.name safely */
+static int clone_dentry_name(struct dentry *dentry, const char **ppath,
+			     int *ppathlen)
+{
+	u32 len;
+	char *name;
+retry:
+	len = READ_ONCE(dentry->d_name.len);
+	name = kmalloc(len + 1, GFP_NOFS);
+	if (!name)
+		return -ENOMEM;
+
+	spin_lock(&dentry->d_lock);
+	if (dentry->d_name.len != len) {
+		spin_unlock(&dentry->d_lock);
+		kfree(name);
+		goto retry;
+	}
+	memcpy(name, dentry->d_name.name, len);
+	spin_unlock(&dentry->d_lock);
+
+	name[len] = '\0';
+	*ppath = name;
+	*ppathlen = len;
+	return 0;
+}
+
 static int build_dentry_path(struct dentry *dentry, struct inode *dir,
-			     const char **ppath, int *ppathlen, u64 *pino,
-			     int *pfreepath)
+			     const char **ppath, int *ppathlen, u64 *pino)
 {
 	char *path;
 
@@ -2171,16 +2197,13 @@ static int build_dentry_path(struct dentry *dentry, struct inode *dir,
 	if (dir && ceph_snap(dir) == CEPH_NOSNAP) {
 		*pino = ceph_ino(dir);
 		rcu_read_unlock();
-		*ppath = dentry->d_name.name;
-		*ppathlen = dentry->d_name.len;
-		return 0;
+		return clone_dentry_name(dentry, ppath, ppathlen);
 	}
 	rcu_read_unlock();
 	path = ceph_mdsc_build_path(dentry, ppathlen, pino, 1);
 	if (IS_ERR(path))
 		return PTR_ERR(path);
 	*ppath = path;
-	*pfreepath = 1;
 	return 0;
 }
 
@@ -2222,8 +2245,9 @@ static int set_request_path_attr(struct inode *rinode, struct dentry *rdentry,
 		dout(" inode %p %llx.%llx\n", rinode, ceph_ino(rinode),
 		     ceph_snap(rinode));
 	} else if (rdentry) {
-		r = build_dentry_path(rdentry, rdiri, ppath, pathlen, ino,
-					freepath);
+		r = build_dentry_path(rdentry, rdiri, ppath, pathlen, ino);
+		if (!r)
+			*freepath = 1;
 		dout(" dentry %p %llx/%.*s\n", rdentry, *ino, *pathlen,
 		     *ppath);
 	} else if (rpath || rino) {
-- 
2.20.1