make it easier to catch those who try to modify ->d_name

Turn d_name into an anon union of const struct qstr d_name with
struct qstr __d_name.  Very few places need to modify it (all
in fs/dcache.c); those are switched to use of ->__d_name.

Note that ->d_name can actually change under you unless you have
the right locking environment; this const just prohibits accidentally
doing stores without being easily spotted.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Author: Al Viro

fs/dcache.c

@@ -1717,13 +1717,13 @@ static struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
 		dname = dentry->d_shortname.string;
 	}	
 
-	dentry->d_name.len = name->len;
-	dentry->d_name.hash = name->hash;
+	dentry->__d_name.len = name->len;
+	dentry->__d_name.hash = name->hash;
 	memcpy(dname, name->name, name->len);
 	dname[name->len] = 0;
 
 	/* Make sure we always see the terminating NUL character */
-	smp_store_release(&dentry->d_name.name, dname); /* ^^^ */
+	smp_store_release(&dentry->__d_name.name, dname); /* ^^^ */
 
 	dentry->d_flags = 0;
 	lockref_init(&dentry->d_lockref);
@@ -2743,25 +2743,25 @@ static void swap_names(struct dentry *dentry, struct dentry *target)
 			/*
 			 * Both external: swap the pointers
 			 */
-			swap(target->d_name.name, dentry->d_name.name);
+			swap(target->__d_name.name, dentry->__d_name.name);
 		} else {
 			/*
 			 * dentry:internal, target:external.  Steal target's
 			 * storage and make target internal.
 			 */
-			dentry->d_name.name = target->d_name.name;
+			dentry->__d_name.name = target->__d_name.name;
 			target->d_shortname = dentry->d_shortname;
-			target->d_name.name = target->d_shortname.string;
+			target->__d_name.name = target->d_shortname.string;
 		}
 	} else {
 		if (unlikely(dname_external(dentry))) {
 			/*
 			 * dentry:external, target:internal.  Give dentry's
 			 * storage to target and make dentry internal
 			 */
-			target->d_name.name = dentry->d_name.name;
+			target->__d_name.name = dentry->__d_name.name;
 			dentry->d_shortname = target->d_shortname;
-			dentry->d_name.name = dentry->d_shortname.string;
+			dentry->__d_name.name = dentry->d_shortname.string;
 		} else {
 			/*
 			 * Both are internal.
@@ -2771,7 +2771,7 @@ static void swap_names(struct dentry *dentry, struct dentry *target)
 				     target->d_shortname.words[i]);
 		}
 	}
-	swap(dentry->d_name.hash_len, target->d_name.hash_len);
+	swap(dentry->__d_name.hash_len, target->__d_name.hash_len);
 }
 
 static void copy_name(struct dentry *dentry, struct dentry *target)
@@ -2781,11 +2781,11 @@ static void copy_name(struct dentry *dentry, struct dentry *target)
 		old_name = external_name(dentry);
 	if (unlikely(dname_external(target))) {
 		atomic_inc(&external_name(target)->count);
-		dentry->d_name = target->d_name;
+		dentry->__d_name = target->__d_name;
 	} else {
 		dentry->d_shortname = target->d_shortname;
-		dentry->d_name.name = dentry->d_shortname.string;
-		dentry->d_name.hash_len = target->d_name.hash_len;
+		dentry->__d_name.name = dentry->d_shortname.string;
+		dentry->__d_name.hash_len = target->__d_name.hash_len;
 	}
 	if (old_name && likely(atomic_dec_and_test(&old_name->count)))
 		kfree_rcu(old_name, head);
@@ -3133,7 +3133,7 @@ void d_mark_tmpfile(struct file *file, struct inode *inode)
 		!d_unlinked(dentry));
 	spin_lock(&dentry->d_parent->d_lock);
 	spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
-	dentry->d_name.len = sprintf(dentry->d_shortname.string, "#%llu",
+	dentry->__d_name.len = sprintf(dentry->d_shortname.string, "#%llu",
 				(unsigned long long)inode->i_ino);
 	spin_unlock(&dentry->d_lock);
 	spin_unlock(&dentry->d_parent->d_lock);

include/linux/dcache.h

@@ -95,7 +95,10 @@ struct dentry {
 	seqcount_spinlock_t d_seq;	/* per dentry seqlock */
 	struct hlist_bl_node d_hash;	/* lookup hash list */
 	struct dentry *d_parent;	/* parent directory */
-	struct qstr d_name;
+	union {
+	struct qstr __d_name;		/* for use ONLY in fs/dcache.c */
+	const struct qstr d_name;
+	};
 	struct inode *d_inode;		/* Where the name belongs to - NULL is
 					 * negative */
 	union shortname_store d_shortname;