VFS: we need to set LOOKUP_JUMPED on mountpoint crossing

Mountpoint crossing is similar to following procfs symlinks - we do
not get ->d_revalidate() called for dentry we have arrived at, with
unpleasant consequences for NFS4.

Simple way to reproduce the problem in mainline:

    cat >/tmp/a.c <<'EOF'
    #include <unistd.h>
    #include <fcntl.h>
    #include <stdio.h>
    main()
    {
            struct flock fl = {.l_type = F_RDLCK, .l_whence = SEEK_SET, .l_len = 1};
            if (fcntl(0, F_SETLK, &fl))
                    perror("setlk");
    }
    EOF
    cc /tmp/a.c -o /tmp/test

then on nfs4:

    mount --bind file1 file2
    /tmp/test < file1		# ok
    /tmp/test < file2		# spews "setlk: No locks available"...

What happens is the missing call of ->d_revalidate() after mountpoint
crossing and that's where NFS4 would issue OPEN request to server.

The fix is simple - treat mountpoint crossing the same way we deal with
following procfs-style symlinks.  I.e.  set LOOKUP_JUMPED...

Cc: stable@kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 15 insertions, 1 deletions

diff --git a/fs/namei.c b/fs/namei.c
index ac6d214da827..5008f01787f5 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -852,7 +852,7 @@ static int follow_managed(struct path *path, unsigned flags)
                 mntput(path->mnt);
         if (ret == -EISDIR)
                 ret = 0;
-        return ret;
+        return ret < 0 ? ret : need_mntput;
 }
 
 int follow_down_one(struct path *path)
@@ -900,6 +900,7 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
                         break;
                 path->mnt = mounted;
                 path->dentry = mounted->mnt_root;
+                nd->flags |= LOOKUP_JUMPED;
                 nd->seq = read_seqcount_begin(&path->dentry->d_seq);
                 /*
                  * Update the inode too. We don't need to re-check the
@@ -1213,6 +1214,8 @@ retry:
                 path_put_conditional(path, nd);
                 return err;
         }
+        if (err)
+                nd->flags |= LOOKUP_JUMPED;
         *inode = path->dentry->d_inode;
         return 0;
 }
@@ -2146,6 +2149,10 @@ static struct file *do_last(struct nameidata *nd, struct path *path,
         }
 
         /* create side of things */
+        /*
+         * This will *only* deal with leaving RCU mode - LOOKUP_JUMPED has been
+         * cleared when we got to the last component we are about to look up
+         */
         error = complete_walk(nd);
         if (error)
                 return ERR_PTR(error);
@@ -2214,6 +2221,9 @@ static struct file *do_last(struct nameidata *nd, struct path *path,
         if (error < 0)
                 goto exit_dput;
 
+        if (error)
+                nd->flags |= LOOKUP_JUMPED;
+
         error = -ENOENT;
         if (!path->dentry->d_inode)
                 goto exit_dput;
@@ -2223,6 +2233,10 @@ static struct file *do_last(struct nameidata *nd, struct path *path,
 
         path_to_nameidata(path, nd);
         nd->inode = path->dentry->d_inode;
+        /* Why this, you ask?  _Now_ we might have grown LOOKUP_JUMPED... */
+        error = complete_walk(nd);
+        if (error)
+                goto exit;
         error = -EISDIR;
         if (S_ISDIR(nd->inode->i_mode))
                 goto exit;