integrity: Annotate struct ima_rule_opt_list with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct ima_rule_opt_list.
Additionally, since the element count member must be set before accessing
the annotated flexible array member, move its initialization earlier.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Mimi Zohar <zohar@linux.ibm.com>
Cc: Dmitry Kasatkin <dmitry.kasatkin@gmail.com>
Cc: Paul Moore <paul@paul-moore.com>
Cc: James Morris <jmorris@namei.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: linux-integrity@vger.kernel.org
Cc: linux-security-module@vger.kernel.org
Acked-by: Mimi Zohar <zohar@linux.ibm.com>
Reviewed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Acked-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/r/20230817210327.never.598-kees@kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

Author: kees

security/integrity/ima/ima_policy.c

@@ -68,7 +68,7 @@ enum policy_rule_list { IMA_DEFAULT_POLICY = 1, IMA_CUSTOM_POLICY };
 
 struct ima_rule_opt_list {
 	size_t count;
-	char *items[];
+	char *items[] __counted_by(count);
 };
 
 /*
@@ -342,6 +342,7 @@ static struct ima_rule_opt_list *ima_alloc_rule_opt_list(const substring_t *src)
 		kfree(src_copy);
 		return ERR_PTR(-ENOMEM);
 	}
+	opt_list->count = count;
 
 	/*
 	 * strsep() has already replaced all instances of '|' with '\0',
@@ -357,7 +358,6 @@ static struct ima_rule_opt_list *ima_alloc_rule_opt_list(const substring_t *src)
 		opt_list->items[i] = cur;
 		cur = strchr(cur, '\0') + 1;
 	}
-	opt_list->count = count;
 
 	return opt_list;
 }