[RFC PATCH v2 10/20] jump_label,module: Don't alloc static_key_mod for __ro_after_init keys

Valentin Schneider
2023-07-20T16:34:38Z

From: Peter Zijlstra <peterz@infradead.org>

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.

Link: http://lore.kernel.org/r/20230705204142.GB2813335@hirez.programming.kicks-ass.net
NOT-Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
---
@Peter: I've barely touched this patch, it's just been writing a comment
and fixing benign compilation issues, so credit's all yours really!
---
include/asm-generic/sections.h | 5 ++++
include/linux/jump_label.h | 1 +
init/main.c | 1 +
kernel/jump_label.c | 49 ++++++++++++++++++++++++++++++++++
4 files changed, 56 insertions(+)

diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
index db13bb620f527..c768de6f19a9a 100644
--- a/include/asm-generic/sections.h
+++ b/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
diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
index f0a949b7c9733..88ef9e776af8d 100644
--- a/include/linux/jump_label.h
+++ b/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_ro(void);
extern void jump_label_lock(void);
extern void jump_label_unlock(void);
extern void arch_jump_label_transform(struct jump_entry *entry,
diff --git a/init/main.c b/init/main.c
index ad920fac325c3..cb5304ca18f4d 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1403,6 +1403,7 @@ static void mark_readonly(void)
* insecure pages which are W+X.
*/
rcu_barrier();
+ jump_label_ro();
mark_rodata_ro();
rodata_test();
} else
diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index d9c822bbffb8d..661ef74dee9b7 100644
--- a/kernel/jump_label.c
+++ b/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_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
+ * 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);
}
--
2.31.1