autofs braino fix for do_last()

It's an analogue of commit 7500c38a (fix the braino in "namei:
massage lookup_slow() to be usable by lookup_one_len_unlocked()").
The same problem (->lookup()-returned unhashed negative dentry
just might be an autofs one with ->d_manage() that would wait
until the daemon makes it positive) applies in do_last() - we
need to do follow_managed() first.

Fortunately, remaining callers of follow_managed() are OK - only
autofs has that weirdness (negative dentry that does not mean
an instant -ENOENT)) and autofs never has its negative dentries
hashed, so we can't pick one from a dcache lookup.

->d_manage() is a bloody mess ;-/

Cc: stable@vger.kernel.org # v4.6
Spotted-by: Ian Kent <raven@themaw.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namei.c b/fs/namei.c
index 3d9511e656ab9f90ffbb03c1462e03e1623b79aa..d7c0cac56d890964afd16696a08e0e31f80505a2 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3260,6 +3260,10 @@ static int do_last(struct nameidata *nd,
                got_write = false;
        }
 
+       error = follow_managed(&path, nd);
+       if (unlikely(error < 0))
+               return error;
+
        if (unlikely(d_is_negative(path.dentry))) {
                path_to_nameidata(&path, nd);
                return -ENOENT;
@@ -3275,10 +3279,6 @@ static int do_last(struct nameidata *nd,
                return -EEXIST;
        }
 
-       error = follow_managed(&path, nd);
-       if (unlikely(error < 0))
-               return error;
-
        seq = 0;        /* out of RCU mode, so the value doesn't matter */
        inode = d_backing_inode(path.dentry);
 finish_lookup: