selinux: simplify and clean up inode_has_perm()

This is a rather hot function that is called with a potentially NULL
"struct common_audit_data" pointer argument.  And in that case it has to
provide and initialize its own dummy common_audit_data structure.

However, all the _common_ cases already pass it a real audit-data
structure, so that uncommon NULL case not only creates a silly run-time
test, more importantly it causes that function to have a big stack frame
for the dummy variable that isn't even used in the common case!

So get rid of that stupid run-time behavior, and make the (few)
functions that currently call with a NULL pointer just call a new helper
function instead (naturally called inode_has_perm_noapd(), since it has
no adp argument).

This makes the run-time test be a static code generation issue instead,
and allows for a much denser stack since none of the common callers need
the dummy structure.  And a denser stack not only means less stack space
usage, it means better cache behavior.  So we have a win-win-win from
this simplification: less code executed, smaller stack footprint, and
better cache behavior.
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

security/selinux/hooks.c

@@ -1476,7 +1476,6 @@ static int inode_has_perm(const struct cred *cred,
 			  unsigned flags)
 {
 	struct inode_security_struct *isec;
-	struct common_audit_data ad;
 	u32 sid;
 
 	validate_creds(cred);
@@ -1487,13 +1486,19 @@ static int inode_has_perm(const struct cred *cred,
 	sid = cred_sid(cred);
 	isec = inode->i_security;
 
-	if (!adp) {
-		adp = &ad;
+	return avc_has_perm_flags(sid, isec->sid, isec->sclass, perms, adp, flags);
+}
+
+static int inode_has_perm_noadp(const struct cred *cred,
+				struct inode *inode,
+				u32 perms,
+				unsigned flags)
+{
+	struct common_audit_data ad;
+
 	COMMON_AUDIT_DATA_INIT(&ad, INODE);
 	ad.u.inode = inode;
-	}
-
-	return avc_has_perm_flags(sid, isec->sid, isec->sclass, perms, adp, flags);
+	return inode_has_perm(cred, inode, perms, &ad, flags);
 }
 
 /* Same as inode_has_perm, but pass explicit audit data containing
@@ -2122,8 +2127,8 @@ static inline void flush_unauthorized_files(const struct cred *cred,
 						struct tty_file_private, list);
 			file = file_priv->file;
 			inode = file->f_path.dentry->d_inode;
-			if (inode_has_perm(cred, inode,
-					   FILE__READ | FILE__WRITE, NULL, 0)) {
+			if (inode_has_perm_noadp(cred, inode,
+					   FILE__READ | FILE__WRITE, 0)) {
 				drop_tty = 1;
 			}
 		}
@@ -3228,7 +3233,7 @@ static int selinux_dentry_open(struct file *file, const struct cred *cred)
 	 * new inode label or new policy.
 	 * This check is not redundant - do not remove.
 	 */
-	return inode_has_perm(cred, inode, open_file_to_av(file), NULL, 0);
+	return inode_has_perm_noadp(cred, inode, open_file_to_av(file), 0);
 }
 
 /* task security operations */