[PATCH v3 13/24] binfmt_misc: factor out the entry matching

[Christian Brauner <brauner@kernel.org>]

search_binfmt_handler() open-codes both match types in one loop body
with the maskless magic comparison spelled as a manual xor loop that
is just memcmp() in disguise. Move the extension and magic checks
into helpers so the walk reads as policy - skip disabled entries,
match by entry type - and the maskless case actually uses memcmp().

No functional change.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/binfmt_misc.c | 50 +++++++++++++++++++++++++++++---------------------
 1 file changed, 29 insertions(+), 21 deletions(-)

diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index 23a2020d9266..6a56f40f23ae 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -78,6 +78,29 @@ static struct file_system_type bm_fs_type;
  */
 #define MAX_REGISTER_LENGTH 1920
 
+/* Check if @e's magic matches @bprm's buffer, applying the mask if set. */
+static bool entry_matches_magic(const struct binfmt_misc_entry *e,
+				const struct linux_binprm *bprm)
+{
+	const char *s = bprm->buf + e->offset;
+	int i;
+
+	if (!e->mask)
+		return !memcmp(s, e->magic, e->size);
+
+	for (i = 0; i < e->size; i++)
+		if ((s[i] ^ e->magic[i]) & e->mask[i])
+			return false;
+	return true;
+}
+
+/* Check if @e's registered extension matches @ext, NULL if there is none. */
+static bool entry_matches_extension(const struct binfmt_misc_entry *e,
+				    const char *ext)
+{
+	return ext && !strcmp(e->magic, ext);
+}
+
 /**
  * search_binfmt_handler - search for a binary handler for @bprm
  * @misc: handle to binfmt_misc instance
@@ -93,38 +116,23 @@ static struct file_system_type bm_fs_type;
 static struct binfmt_misc_entry *
 search_binfmt_handler(struct binfmt_misc *misc, struct linux_binprm *bprm)
 {
-	char *p = strrchr(bprm->interp, '.');
+	char *dot = strrchr(bprm->interp, '.');
+	const char *ext = dot ? dot + 1 : NULL;
 	struct binfmt_misc_entry *e;
 
 	/* Walk all the registered handlers. */
 	hlist_for_each_entry_rcu(e, &misc->entries, node) {
-		char *s;
-		int j;
-
 		/* Make sure this one is currently enabled. */
 		if (!test_bit(MISC_FMT_ENABLED_BIT, &e->flags))
 			continue;
 
-		/* Do matching based on extension if applicable. */
-		if (!test_bit(MISC_FMT_MAGIC_BIT, &e->flags)) {
-			if (p && !strcmp(e->magic, p + 1))
+		if (test_bit(MISC_FMT_MAGIC_BIT, &e->flags)) {
+			if (entry_matches_magic(e, bprm))
 				return e;
-			continue;
-		}
-
-		/* Do matching based on magic & mask. */
-		s = bprm->buf + e->offset;
-		if (e->mask) {
-			for (j = 0; j < e->size; j++)
-				if ((*s++ ^ e->magic[j]) & e->mask[j])
-					break;
 		} else {
-			for (j = 0; j < e->size; j++)
-				if ((*s++ ^ e->magic[j]))
-					break;
+			if (entry_matches_extension(e, ext))
+				return e;
 		}
-		if (j == e->size)
-			return e;
 	}
 
 	return NULL;

-- 
2.53.0