[PATCH] fs: call security_d_instantiate in d_obtain_alias V2

[Josef Bacik <josef@redhat.com>]

While trying to track down some NFS problems with BTRFS, I kept noticing I was
getting -EACCESS for no apparent reason.  Eric Paris and printk() helped me
figure out that it was SELinux that was giving me grief, with the following
denial

type=AVC msg=audit(1290013638.413:95): avc:  denied  { 0x800000 } for  pid=1772
comm="nfsd" name="" dev=sda1 ino=256 scontext=system_u:system_r:kernel_t:s0
tcontext=system_u:object_r:unlabeled_t:s0 tclass=file

Turns out this is because in d_obtain_alias if we can't find an alias we create
one and do all the normal instantiation stuff, but we don't do the
security_d_instantiate.

Usually we are protected from getting a hashed dentry that hasn't yet run
security_d_instantiate() by the parent's i_mutex, but obviously this isn't an
option there, so in order to deal with the case that a second thread comes in
and finds our new dentry before we get to run security_d_instantiate(), we go
ahead and call it if we find a dentry already.  Eric assures me that this is ok
as the code checks to see if the dentry has been initialized already so calling
security_d_instantiate() against the same dentry multiple times is ok.  With
this patch I'm no longer getting errant -EACCESS values.

Signed-off-by: Josef Bacik <josef@redhat.com>
---
V1->V2:
-added second security_d_instantiate() call

 fs/dcache.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 23702a9..119d489 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1201,9 +1201,12 @@ struct dentry *d_obtain_alias(struct inode *inode)
 	spin_unlock(&tmp->d_lock);
 
 	spin_unlock(&dcache_lock);
+	security_d_instantiate(tmp, inode);
 	return tmp;
 
  out_iput:
+	if (res && !IS_ERR(res))
+		security_d_instantiate(res, inode);
 	iput(inode);
 	return res;
 }
-- 
1.6.6.1