static_call: Handle tail-calls

GCC can turn our static_call(name)(args...) into a tail call, in which
case we get a JMP.d32 into the trampoline (which then does a further
tail-call).

Teach objtool to recognise and mark these in .static_call_sites and
adjust the code patching to deal with this.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20200818135805.101186767@infradead.org

Author: Peter Zijlstra

arch/x86/kernel/static_call.c

@@ -41,15 +41,30 @@ static void __static_call_transform(void *insn, enum insn_type type, void *func)
 	text_poke_bp(insn, code, size, NULL);
 }
 
-void arch_static_call_transform(void *site, void *tramp, void *func)
+static inline enum insn_type __sc_insn(bool null, bool tail)
+{
+	/*
+	 * Encode the following table without branches:
+	 *
+	 *	tail	null	insn
+	 *	-----+-------+------
+	 *	  0  |   0   |  CALL
+	 *	  0  |   1   |  NOP
+	 *	  1  |   0   |  JMP
+	 *	  1  |   1   |  RET
+	 */
+	return 2*tail + null;
+}
+
+void arch_static_call_transform(void *site, void *tramp, void *func, bool tail)
 {
 	mutex_lock(&text_mutex);
 
 	if (tramp)
-		__static_call_transform(tramp, func ? JMP : RET, func);
+		__static_call_transform(tramp, __sc_insn(!func, true), func);
 
 	if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE) && site)
-		__static_call_transform(site, func ? CALL : NOP, func);
+		__static_call_transform(site, __sc_insn(!func, tail), func);
 
 	mutex_unlock(&text_mutex);
 }

include/linux/static_call.h

@@ -103,7 +103,7 @@
 /*
  * Either @site or @tramp can be NULL.
  */
-extern void arch_static_call_transform(void *site, void *tramp, void *func);
+extern void arch_static_call_transform(void *site, void *tramp, void *func, bool tail);
 
 #define STATIC_CALL_TRAMP_ADDR(name) &STATIC_CALL_TRAMP(name)
 
@@ -206,7 +206,7 @@ void __static_call_update(struct static_call_key *key, void *tramp, void *func)
 {
 	cpus_read_lock();
 	WRITE_ONCE(key->func, func);
-	arch_static_call_transform(NULL, tramp, func);
+	arch_static_call_transform(NULL, tramp, func, false);
 	cpus_read_unlock();
 }
 

include/linux/static_call_types.h

@@ -16,6 +16,13 @@
 #define STATIC_CALL_TRAMP(name)		__PASTE(STATIC_CALL_TRAMP_PREFIX, name)
 #define STATIC_CALL_TRAMP_STR(name)	__stringify(STATIC_CALL_TRAMP(name))
 
+/*
+ * Flags in the low bits of static_call_site::key.
+ */
+#define STATIC_CALL_SITE_TAIL 1UL	/* tail call */
+#define STATIC_CALL_SITE_INIT 2UL	/* init section */
+#define STATIC_CALL_SITE_FLAGS 3UL
+
 /*
  * The static call site table needs to be created by external tooling (objtool
  * or a compiler plugin).

kernel/static_call.c

@@ -15,8 +15,6 @@ extern struct static_call_site __start_static_call_sites[],
 
 static bool static_call_initialized;
 
-#define STATIC_CALL_INIT 1UL
-
 /* mutex to protect key modules/sites */
 static DEFINE_MUTEX(static_call_mutex);
 
@@ -39,18 +37,23 @@ static inline void *static_call_addr(struct static_call_site *site)
 static inline struct static_call_key *static_call_key(const struct static_call_site *site)
 {
 	return (struct static_call_key *)
-		(((long)site->key + (long)&site->key) & ~STATIC_CALL_INIT);
+		(((long)site->key + (long)&site->key) & ~STATIC_CALL_SITE_FLAGS);
 }
 
 /* These assume the key is word-aligned. */
 static inline bool static_call_is_init(struct static_call_site *site)
 {
-	return ((long)site->key + (long)&site->key) & STATIC_CALL_INIT;
+	return ((long)site->key + (long)&site->key) & STATIC_CALL_SITE_INIT;
+}
+
+static inline bool static_call_is_tail(struct static_call_site *site)
+{
+	return ((long)site->key + (long)&site->key) & STATIC_CALL_SITE_TAIL;
 }
 
 static inline void static_call_set_init(struct static_call_site *site)
 {
-	site->key = ((long)static_call_key(site) | STATIC_CALL_INIT) -
+	site->key = ((long)static_call_key(site) | STATIC_CALL_SITE_INIT) -
 		    (long)&site->key;
 }
 
@@ -104,7 +107,7 @@ void __static_call_update(struct static_call_key *key, void *tramp, void *func)
 
 	key->func = func;
 
-	arch_static_call_transform(NULL, tramp, func);
+	arch_static_call_transform(NULL, tramp, func, false);
 
 	/*
 	 * If uninitialized, we'll not update the callsites, but they still
@@ -154,7 +157,8 @@ void __static_call_update(struct static_call_key *key, void *tramp, void *func)
 				continue;
 			}
 
-			arch_static_call_transform(site_addr, NULL, func);
+			arch_static_call_transform(site_addr, NULL, func,
+				static_call_is_tail(site));
 		}
 	}
 
@@ -198,7 +202,8 @@ static int __static_call_init(struct module *mod,
 			key->mods = site_mod;
 		}
 
-		arch_static_call_transform(site_addr, NULL, key->func);
+		arch_static_call_transform(site_addr, NULL, key->func,
+				static_call_is_tail(site));
 	}
 
 	return 0;

tools/include/linux/static_call_types.h

@@ -16,6 +16,13 @@
 #define STATIC_CALL_TRAMP(name)		__PASTE(STATIC_CALL_TRAMP_PREFIX, name)
 #define STATIC_CALL_TRAMP_STR(name)	__stringify(STATIC_CALL_TRAMP(name))
 
+/*
+ * Flags in the low bits of static_call_site::key.
+ */
+#define STATIC_CALL_SITE_TAIL 1UL	/* tail call */
+#define STATIC_CALL_SITE_INIT 2UL	/* init section */
+#define STATIC_CALL_SITE_FLAGS 3UL
+
 /*
  * The static call site table needs to be created by external tooling (objtool
  * or a compiler plugin).

tools/objtool/check.c

@@ -516,7 +516,7 @@ static int create_static_call_sections(struct objtool_file *file)
 		}
 		memset(reloc, 0, sizeof(*reloc));
 		reloc->sym = key_sym;
-		reloc->addend = 0;
+		reloc->addend = is_sibling_call(insn) ? STATIC_CALL_SITE_TAIL : 0;
 		reloc->type = R_X86_64_PC32;
 		reloc->offset = idx * sizeof(struct static_call_site) + 4;
 		reloc->sec = reloc_sec;
@@ -747,6 +747,10 @@ static int add_jump_destinations(struct objtool_file *file)
 		} else {
 			/* external sibling call */
 			insn->call_dest = reloc->sym;
+			if (insn->call_dest->static_call_tramp) {
+				list_add_tail(&insn->static_call_node,
+					      &file->static_call_list);
+			}
 			continue;
 		}
 
@@ -798,6 +802,10 @@ static int add_jump_destinations(struct objtool_file *file)
 
 				/* internal sibling call */
 				insn->call_dest = insn->jump_dest->func;
+				if (insn->call_dest->static_call_tramp) {
+					list_add_tail(&insn->static_call_node,
+						      &file->static_call_list);
+				}
 			}
 		}
 	}
@@ -1684,6 +1692,10 @@ static int decode_sections(struct objtool_file *file)
 	if (ret)
 		return ret;
 
+	ret = read_static_call_tramps(file);
+	if (ret)
+		return ret;
+
 	ret = add_jump_destinations(file);
 	if (ret)
 		return ret;
@@ -1716,10 +1728,6 @@ static int decode_sections(struct objtool_file *file)
 	if (ret)
 		return ret;
 
-	ret = read_static_call_tramps(file);
-	if (ret)
-		return ret;
-
 	return 0;
 }