cifs: build_path_from_dentry() race fix

deal with d_move() races properly; rename_lock read-retry loop,
rcu_read_lock() held while walking to root, d_lock held over
subtraction from namelen and copying the component to stabilize
->d_name.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/cifs/dir.c b/fs/cifs/dir.c
index 81914df47ef1612c1ab742d228503bbf5e598046..fa8c21d913bc5b212191d2c444cb5ce74e1d4058 100644 (file)
--- a/fs/cifs/dir.c
+++ b/fs/cifs/dir.c
@@ -55,6 +55,7 @@ build_path_from_dentry(struct dentry *direntry)
        char dirsep;
        struct cifs_sb_info *cifs_sb = CIFS_SB(direntry->d_sb);
        struct cifs_tcon *tcon = cifs_sb_master_tcon(cifs_sb);
+       unsigned seq;
 
        if (direntry == NULL)
                return NULL;  /* not much we can do if dentry is freed and
@@ -68,22 +69,29 @@ build_path_from_dentry(struct dentry *direntry)
                dfsplen = 0;
 cifs_bp_rename_retry:
        namelen = dfsplen;
+       seq = read_seqbegin(&rename_lock);
+       rcu_read_lock();
        for (temp = direntry; !IS_ROOT(temp);) {
                namelen += (1 + temp->d_name.len);
                temp = temp->d_parent;
                if (temp == NULL) {
                        cERROR(1, "corrupt dentry");
+                       rcu_read_unlock();
                        return NULL;
                }
        }
+       rcu_read_unlock();
 
        full_path = kmalloc(namelen+1, GFP_KERNEL);
        if (full_path == NULL)
                return full_path;
        full_path[namelen] = 0; /* trailing null */
+       rcu_read_lock();
        for (temp = direntry; !IS_ROOT(temp);) {
+               spin_lock(&temp->d_lock);
                namelen -= 1 + temp->d_name.len;
                if (namelen < 0) {
+                       spin_unlock(&temp->d_lock);
                        break;
                } else {
                        full_path[namelen] = dirsep;
@@ -91,14 +99,17 @@ cifs_bp_rename_retry:
                                temp->d_name.len);
                        cFYI(0, "name: %s", full_path + namelen);
                }
+               spin_unlock(&temp->d_lock);
                temp = temp->d_parent;
                if (temp == NULL) {
                        cERROR(1, "corrupt dentry");
+                       rcu_read_unlock();
                        kfree(full_path);
                        return NULL;
                }
        }
-       if (namelen != dfsplen) {
+       rcu_read_unlock();
+       if (namelen != dfsplen || read_seqretry(&rename_lock, seq)) {
                cERROR(1, "did not end path lookup where expected namelen is %d",
                        namelen);
                /* presumably this is only possible if racing with a rename