[added to the 3.18 stable tree] atomic_open(): fix the handling of create_error

[Sasha Levin <sasha.levin@oracle.com>]

From: Al Viro <viro@zeniv.linux.org.uk>

This patch has been added to the 3.18 stable tree. If you have any
objections, please let us know.

===============

[ Upstream commit 10c64cea04d3c75c306b3f990586ffb343b63287 ]

* if we have a hashed negative dentry and either CREAT|EXCL on
r/o filesystem, or CREAT|TRUNC on r/o filesystem, or CREAT|EXCL
with failing may_o_create(), we should fail with EROFS or the
error may_o_create() has returned, but not ENOENT.  Which is what
the current code ends up returning.

* if we have CREAT|TRUNC hitting a regular file on a read-only
filesystem, we can't fail with EROFS here.  At the very least,
not until we'd done follow_managed() - we might have a writable
file (or a device, for that matter) bound on top of that one.
Moreover, the code downstream will see that O_TRUNC and attempt
to grab the write access (*after* following possible mount), so
if we really should fail with EROFS, it will happen.  No need
to do that inside atomic_open().

The real logics is much simpler than what the current code is
trying to do - if we decided to go for simple lookup, ended
up with a negative dentry *and* had create_error set, fail with
create_error.  No matter whether we'd got that negative dentry
from lookup_real() or had found it in dcache.

Cc: stable@vger.kernel.org # v3.6+
Acked-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
---
 fs/namei.c | 20 ++++----------------
 1 file changed, 4 insertions(+), 16 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 2a8ba68..e197645 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2797,22 +2797,10 @@ no_open:
 		dentry = lookup_real(dir, dentry, nd->flags);
 		if (IS_ERR(dentry))
 			return PTR_ERR(dentry);
-
-		if (create_error) {
-			int open_flag = op->open_flag;
-
-			error = create_error;
-			if ((open_flag & O_EXCL)) {
-				if (!dentry->d_inode)
-					goto out;
-			} else if (!dentry->d_inode) {
-				goto out;
-			} else if ((open_flag & O_TRUNC) &&
-				   S_ISREG(dentry->d_inode->i_mode)) {
-				goto out;
-			}
-			/* will fail later, go on to get the right error */
-		}
+	}
+	if (create_error && !dentry->d_inode) {
+		error = create_error;
+		goto out;
 	}
 looked_up:
 	path->dentry = dentry;
-- 
2.5.0