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

2 files changed, 40 insertions, 4 deletions

diff --git a/tools/include/linux/static_call_types.h b/tools/include/linux/static_call_types.h
index 08f78b1b88b4..ae5662d368b9 100644
--- a/tools/include/linux/static_call_types.h
+++ b/tools/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
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 4bd30315eb62..f2e5e5ce1a05 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -502,8 +502,21 @@ static int create_static_call_sections(struct objtool_file *file)
 
 		key_sym = find_symbol_by_name(file->elf, tmp);
 		if (!key_sym) {
-			WARN("static_call: can't find static_call_key symbol: %s", tmp);
-			return -1;
+			if (!module) {
+				WARN("static_call: can't find static_call_key symbol: %s", tmp);
+				return -1;
+			}
+
+			/*
+			 * For modules(), the key might not be exported, which
+			 * means the module can make static calls but isn't
+			 * allowed to change them.
+			 *
+			 * In that case we temporarily set the key to be the
+			 * trampoline address.  This is fixed up in
+			 * static_call_add_module().
+			 */
+			key_sym = insn->call_dest;
 		}
 		free(key_name);