[PATCH v2 6/6] kernel: tracepoints: add support for relative references

[Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>]

To avoid the need for relocating absolute references to tracepoint
structures at boot time when running relocatable kernels (which may
take a disproportionate amount of space), add the option to emit
these tables as relative references instead.

Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
---
 include/linux/tracepoint.h | 42 ++++++++++++++++++--
 kernel/tracepoint.c        |  7 +---
 2 files changed, 40 insertions(+), 9 deletions(-)

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index a26ffbe09e71..68701821933a 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -228,6 +228,42 @@ extern void syscall_unregfunc(void);
 		return static_key_false(&__tracepoint_##name.key);	\
 	}
 
+#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
+#define __TRACEPOINT_ENTRY(name)					\
+	asm("	.section \"__tracepoints_ptrs\", \"a\"\n"		\
+	    "	.balign 4\n"						\
+	    "	.long " VMLINUX_SYMBOL_STR(__tracepoint_##name) " - .\n"\
+	    "	.previous\n")
+
+struct tracepoint_entry_t {
+	int tp_offset;
+};
+
+static inline
+struct tracepoint *tracepoint_from_entry(const struct tracepoint_entry_t *ent)
+{
+	return (struct tracepoint *)((unsigned long)ent + ent->tp_offset);
+}
+#else
+#define __TRACEPOINT_ENTRY(name)					 \
+	static struct tracepoint * const __tracepoint_ptr_##name __used	 \
+	__attribute__((section("__tracepoints_ptrs"))) =		 \
+		&__tracepoint_##name
+
+struct tracepoint_entry_t {
+	struct tracepoint *tp;
+};
+
+static inline
+struct tracepoint *tracepoint_from_entry(const struct tracepoint_entry_t *ent)
+{
+	return ent->tp;
+}
+#endif
+
+extern struct tracepoint_entry_t const __start___tracepoints_ptrs[];
+extern struct tracepoint_entry_t const __stop___tracepoints_ptrs[];
+
 /*
  * We have no guarantee that gcc and the linker won't up-align the tracepoint
  * structures, so we create an array of pointers that will be used for iteration
@@ -237,11 +273,9 @@ extern void syscall_unregfunc(void);
 	static const char __tpstrtab_##name[]				 \
 	__attribute__((section("__tracepoints_strings"))) = #name;	 \
 	struct tracepoint __tracepoint_##name				 \
-	__attribute__((section("__tracepoints"))) =			 \
+	__attribute__((section("__tracepoints"))) __used =		 \
 		{ __tpstrtab_##name, STATIC_KEY_INIT_FALSE, reg, unreg, NULL };\
-	static struct tracepoint * const __tracepoint_ptr_##name __used	 \
-	__attribute__((section("__tracepoints_ptrs"))) =		 \
-		&__tracepoint_##name;
+	__TRACEPOINT_ENTRY(name);
 
 #define DEFINE_TRACE(name)						\
 	DEFINE_TRACE_FN(name, NULL, NULL);
diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index 685c50ae6300..21bc41454fd6 100644
--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -28,9 +28,6 @@
 #include <linux/sched/task.h>
 #include <linux/static_key.h>
 
-extern struct tracepoint * const __start___tracepoints_ptrs[];
-extern struct tracepoint * const __stop___tracepoints_ptrs[];
-
 /* Set to 1 to enable tracepoint debug output */
 static const int tracepoint_debug;
 
@@ -508,12 +505,12 @@ static void for_each_tracepoint_range(struct tracepoint * const *begin,
 		void (*fct)(struct tracepoint *tp, void *priv),
 		void *priv)
 {
-	struct tracepoint * const *iter;
+	struct tracepoint_entry_t const *iter;
 
 	if (!begin)
 		return;
 	for (iter = begin; iter < end; iter++)
-		fct(*iter, priv);
+		fct(tracepoint_from_entry(iter), priv);
 }
 
 /**
-- 
2.11.0