static_call: Allow module use without exposing static_call_key

When exporting static_call_key; with EXPORT_STATIC_CALL*(), the module
can use static_call_update() to change the function called.  This is
not desirable in general.

Not exporting static_call_key however also disallows usage of
static_call(), since objtool needs the key to construct the
static_call_site.

Solve this by allowing objtool to create the static_call_site using
the trampoline address when it builds a module and cannot find the
static_call_key symbol. The module loader will then try and map the
trampole back to a key before it constructs the normal sites list.

Doing this requires a trampoline -> key associsation, so add another
magic section that keeps those.

Originally-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20210127231837.ifddpn7rhwdaepiu@treble

3 files changed, 49 insertions, 5 deletions

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index b97c628ad91ff3..3f747de1934d77 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -393,7 +393,10 @@
 	. = ALIGN(8);							\
 	__start_static_call_sites = .;					\
 	KEEP(*(.static_call_sites))					\
-	__stop_static_call_sites = .;
+	__stop_static_call_sites = .;					\
+	__start_static_call_tramp_key = .;				\
+	KEEP(*(.static_call_tramp_key))					\
+	__stop_static_call_tramp_key = .;
 
 /*
  * Allow architectures to handle ro_after_init data on their
diff --git a/include/linux/static_call.h b/include/linux/static_call.h
index d69dd8b976ca0a..85ecc789f4ffdd 100644
--- a/include/linux/static_call.h
+++ b/include/linux/static_call.h
@@ -138,6 +138,12 @@ struct static_call_key {
 	};
 };
 
+/* For finding the key associated with a trampoline */
+struct static_call_tramp_key {
+	s32 tramp;
+	s32 key;
+};
+
 extern void __static_call_update(struct static_call_key *key, void *tramp, void *func);
 extern int static_call_mod_init(struct module *mod);
 extern int static_call_text_reserved(void *start, void *end);
@@ -165,11 +171,18 @@ extern long __static_call_return0(void);
 #define EXPORT_STATIC_CALL(name)					\
 	EXPORT_SYMBOL(STATIC_CALL_KEY(name));				\
 	EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
-
 #define EXPORT_STATIC_CALL_GPL(name)					\
 	EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name));			\
 	EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
 
+/* Leave the key unexported, so modules can't change static call targets: */
+#define EXPORT_STATIC_CALL_TRAMP(name)					\
+	EXPORT_SYMBOL(STATIC_CALL_TRAMP(name));				\
+	ARCH_ADD_TRAMP_KEY(name)
+#define EXPORT_STATIC_CALL_TRAMP_GPL(name)				\
+	EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name));			\
+	ARCH_ADD_TRAMP_KEY(name)
+
 #elif defined(CONFIG_HAVE_STATIC_CALL)
 
 static inline int static_call_init(void) { return 0; }
@@ -216,11 +229,16 @@ static inline long __static_call_return0(void)
 #define EXPORT_STATIC_CALL(name)					\
 	EXPORT_SYMBOL(STATIC_CALL_KEY(name));				\
 	EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
-
 #define EXPORT_STATIC_CALL_GPL(name)					\
 	EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name));			\
 	EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
 
+/* Leave the key unexported, so modules can't change static call targets: */
+#define EXPORT_STATIC_CALL_TRAMP(name)					\
+	EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
+#define EXPORT_STATIC_CALL_TRAMP_GPL(name)				\
+	EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
+
 #else /* Generic implementation */
 
 static inline int static_call_init(void) { return 0; }
diff --git a/include/linux/static_call_types.h b/include/linux/static_call_types.h
index 08f78b1b88b478..ae5662d368b98f 100644
--- a/include/linux/static_call_types.h
+++ b/include/linux/static_call_types.h
@@ -10,6 +10,7 @@
 #define STATIC_CALL_KEY_PREFIX_STR	__stringify(STATIC_CALL_KEY_PREFIX)
 #define STATIC_CALL_KEY_PREFIX_LEN	(sizeof(STATIC_CALL_KEY_PREFIX_STR) - 1)
 #define STATIC_CALL_KEY(name)		__PASTE(STATIC_CALL_KEY_PREFIX, name)
+#define STATIC_CALL_KEY_STR(name)	__stringify(STATIC_CALL_KEY(name))
 
 #define STATIC_CALL_TRAMP_PREFIX	__SCT__
 #define STATIC_CALL_TRAMP_PREFIX_STR	__stringify(STATIC_CALL_TRAMP_PREFIX)
@@ -39,17 +40,39 @@ struct static_call_site {
 
 #ifdef CONFIG_HAVE_STATIC_CALL
 
+#define __raw_static_call(name)	(&STATIC_CALL_TRAMP(name))
+
+#ifdef CONFIG_HAVE_STATIC_CALL_INLINE
+
 /*
  * __ADDRESSABLE() is used to ensure the key symbol doesn't get stripped from
  * the symbol table so that objtool can reference it when it generates the
  * .static_call_sites section.
  */
+#define __STATIC_CALL_ADDRESSABLE(name) \
+	__ADDRESSABLE(STATIC_CALL_KEY(name))
+
 #define __static_call(name)						\
 ({									\
-	__ADDRESSABLE(STATIC_CALL_KEY(name));				\
-	&STATIC_CALL_TRAMP(name);					\
+	__STATIC_CALL_ADDRESSABLE(name);				\
+	__raw_static_call(name);					\
 })
 
+#else /* !CONFIG_HAVE_STATIC_CALL_INLINE */
+
+#define __STATIC_CALL_ADDRESSABLE(name)
+#define __static_call(name)	__raw_static_call(name)
+
+#endif /* CONFIG_HAVE_STATIC_CALL_INLINE */
+
+#ifdef MODULE
+#define __STATIC_CALL_MOD_ADDRESSABLE(name)
+#define static_call_mod(name)	__raw_static_call(name)
+#else
+#define __STATIC_CALL_MOD_ADDRESSABLE(name) __STATIC_CALL_ADDRESSABLE(name)
+#define static_call_mod(name)	__static_call(name)
+#endif
+
 #define static_call(name)	__static_call(name)
 
 #else