jump_label,module: Don't alloc static_key_mod for __ro_after_init keys

When a static_key is marked ro_after_init, its state will never change
(after init), therefore jump_label_update() will never need to iterate
the entries, and thus module load won't actually need to track this --
avoiding the static_key::next write.

Therefore, mark these keys such that jump_label_add_module() might
recognise them and avoid the modification.

Use the special state: 'static_key_linked(key) && !static_key_mod(key)'
to denote such keys.

jump_label_add_module() does not exist under CONFIG_JUMP_LABEL=n, so the
newly-introduced jump_label_init_ro() can be defined as a nop for that
configuration.

[ mingo: Renamed jump_label_ro() to jump_label_init_ro() ]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/r/20240313180106.2917308-2-vschneid@redhat.com

include/asm-generic/sections.h

@@ -180,6 +180,11 @@ static inline bool is_kernel_rodata(unsigned long addr)
 	       addr < (unsigned long)__end_rodata;
 }
 
+static inline bool is_kernel_ro_after_init(unsigned long addr)
+{
+	return addr >= (unsigned long)__start_ro_after_init &&
+	       addr < (unsigned long)__end_ro_after_init;
+}
 /**
  * is_kernel_inittext - checks if the pointer address is located in the
  *                      .init.text section

include/linux/jump_label.h

@@ -216,6 +216,7 @@ extern struct jump_entry __start___jump_table[];
 extern struct jump_entry __stop___jump_table[];
 
 extern void jump_label_init(void);
+extern void jump_label_init_ro(void);
 extern void jump_label_lock(void);
 extern void jump_label_unlock(void);
 extern void arch_jump_label_transform(struct jump_entry *entry,
@@ -265,6 +266,8 @@ static __always_inline void jump_label_init(void)
 	static_key_initialized = true;
 }
 
+static __always_inline void jump_label_init_ro(void) { }
+
 static __always_inline bool static_key_false(struct static_key *key)
 {
 	if (unlikely_notrace(static_key_count(key) > 0))

init/main.c

@@ -1408,6 +1408,7 @@ static void mark_readonly(void)
 		 * insecure pages which are W+X.
 		 */
 		flush_module_init_free_work();
+		jump_label_init_ro();
 		mark_rodata_ro();
 		debug_checkwx();
 		rodata_test();

kernel/jump_label.c

@@ -530,6 +530,45 @@ void __init jump_label_init(void)
 	cpus_read_unlock();
 }
 
+static inline bool static_key_sealed(struct static_key *key)
+{
+	return (key->type & JUMP_TYPE_LINKED) && !(key->type & ~JUMP_TYPE_MASK);
+}
+
+static inline void static_key_seal(struct static_key *key)
+{
+	unsigned long type = key->type & JUMP_TYPE_TRUE;
+	key->type = JUMP_TYPE_LINKED | type;
+}
+
+void jump_label_init_ro(void)
+{
+	struct jump_entry *iter_start = __start___jump_table;
+	struct jump_entry *iter_stop = __stop___jump_table;
+	struct jump_entry *iter;
+
+	if (WARN_ON_ONCE(!static_key_initialized))
+		return;
+
+	cpus_read_lock();
+	jump_label_lock();
+
+	for (iter = iter_start; iter < iter_stop; iter++) {
+		struct static_key *iterk = jump_entry_key(iter);
+
+		if (!is_kernel_ro_after_init((unsigned long)iterk))
+			continue;
+
+		if (static_key_sealed(iterk))
+			continue;
+
+		static_key_seal(iterk);
+	}
+
+	jump_label_unlock();
+	cpus_read_unlock();
+}
+
 #ifdef CONFIG_MODULES
 
 enum jump_label_type jump_label_init_type(struct jump_entry *entry)
@@ -650,6 +689,15 @@ static int jump_label_add_module(struct module *mod)
 			static_key_set_entries(key, iter);
 			continue;
 		}
+
+		/*
+		 * If the key was sealed at init, then there's no need to keep a
+		 * reference to its module entries - just patch them now and be
+		 * done with it.
+		 */
+		if (static_key_sealed(key))
+			goto do_poke;
+
 		jlm = kzalloc(sizeof(struct static_key_mod), GFP_KERNEL);
 		if (!jlm)
 			return -ENOMEM;
@@ -675,6 +723,7 @@ static int jump_label_add_module(struct module *mod)
 		static_key_set_linked(key);
 
 		/* Only update if we've changed from our initial state */
+do_poke:
 		if (jump_label_type(iter) != jump_label_init_type(iter))
 			__jump_label_update(key, iter, iter_stop, true);
 	}
@@ -699,6 +748,10 @@ static void jump_label_del_module(struct module *mod)
 		if (within_module((unsigned long)key, mod))
 			continue;
 
+		/* No @jlm allocated because key was sealed at init. */
+		if (static_key_sealed(key))
+			continue;
+
 		/* No memory during module load */
 		if (WARN_ON(!static_key_linked(key)))
 			continue;