Use access mode instead of open flags to determine needed permissions

Way back when (in commit 834f2a4, aka
"VFS: Allow the filesystem to return a full file pointer on open intent"
to be exact), Trond changed the open logic to keep track of the original
flags to a file open, in order to pass down the the intent of a dentry
lookup to the low-level filesystem.

However, when doing that reorganization, it changed the meaning of
namei_flags, and thus inadvertently changed the test of access mode for
directories (and RO filesystem) to use the wrong flag.  So fix those
test back to use access mode ("acc_mode") rather than the open flag
("flag").

Issue noticed by Bill Roman at Datalight.

Reported-and-tested-by: Bill Roman <bill.roman@datalight.com>
Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/namei.c

@@ -1605,7 +1605,7 @@ int may_open(struct nameidata *nd, int acc_mode, int flag)
 	if (S_ISLNK(inode->i_mode))
 		return -ELOOP;
 
-	if (S_ISDIR(inode->i_mode) && (flag & FMODE_WRITE))
+	if (S_ISDIR(inode->i_mode) && (acc_mode & MAY_WRITE))
 		return -EISDIR;
 
 	/*
@@ -1620,7 +1620,7 @@ int may_open(struct nameidata *nd, int acc_mode, int flag)
 			return -EACCES;
 
 		flag &= ~O_TRUNC;
-	} else if (IS_RDONLY(inode) && (flag & FMODE_WRITE))
+	} else if (IS_RDONLY(inode) && (acc_mode & MAY_WRITE))
 		return -EROFS;
 
 	error = vfs_permission(nd, acc_mode);