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Subject[PATCH 5/5] kernel: tracepoints: add support for relative references
Date
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@goodmis.org>
Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
include/linux/tracepoint.h | 19 +++++++++++++++----
kernel/tracepoint.c | 19 ++++++++++++++-----
2 files changed, 29 insertions(+), 9 deletions(-)

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index a26ffbe09e71..d02bf1a695e8 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -228,6 +228,19 @@ 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")
+#else
+#define __TRACEPOINT_ENTRY(name) \
+ static struct tracepoint * const __tracepoint_ptr_##name __used \
+ __attribute__((section("__tracepoints_ptrs"))) = \
+ &__tracepoint_##name
+#endif
+
/*
* 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 +250,9 @@ 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..1c6689603764 100644
--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -28,8 +28,8 @@
#include <linux/sched/task.h>
#include <linux/static_key.h>

-extern struct tracepoint * const __start___tracepoints_ptrs[];
-extern struct tracepoint * const __stop___tracepoints_ptrs[];
+extern const unsigned char __start___tracepoints_ptrs[];
+extern const unsigned char __stop___tracepoints_ptrs[];

/* Set to 1 to enable tracepoint debug output */
static const int tracepoint_debug;
@@ -503,17 +503,26 @@ static __init int init_tracepoints(void)
__initcall(init_tracepoints);
#endif /* CONFIG_MODULES */

-static void for_each_tracepoint_range(struct tracepoint * const *begin,
- struct tracepoint * const *end,
+static void for_each_tracepoint_range(const void *begin, const void *end,
void (*fct)(struct tracepoint *tp, void *priv),
void *priv)
{
+#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
+ const signed int *iter;
+
+ if (!begin)
+ return;
+ for (iter = begin; iter < (signed int *)end; iter++) {
+ fct((struct tracepoint *)((unsigned long)iter + *iter), priv);
+ }
+#else
struct tracepoint * const *iter;

if (!begin)
return;
- for (iter = begin; iter < end; iter++)
+ for (iter = begin; iter < (struct tracepoint * const *)end; iter++)
fct(*iter, priv);
+#endif
}

/**
--
2.11.0
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