[PATCH v5 5/8] kernel: tracepoints: add support for relative references

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

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        | 50 +++++++++++---------
 2 files changed, 42 insertions(+), 27 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..05649fef106c 100644
--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -327,6 +327,28 @@ int tracepoint_probe_unregister(struct tracepoint *tp, 
void *probe, void *data)
 }
 EXPORT_SYMBOL_GPL(tracepoint_probe_unregister);
 
+static void for_each_tracepoint_range(struct tracepoint * const *begin,
+               struct tracepoint * const *end,
+               void (*fct)(struct tracepoint *tp, void *priv),
+               void *priv)
+{
+       if (!begin)
+               return;
+
+       if (IS_ENABLED(CONFIG_HAVE_ARCH_PREL32_RELOCATIONS)) {
+               const int *iter;
+
+               for (iter = (const int *)begin; iter < (const int *)end; iter++)
+                       fct((struct tracepoint *)((unsigned long)iter + *iter),
+                           priv);
+       } else {
+               struct tracepoint * const *iter;
+
+               for (iter = begin; iter < end; iter++)
+                       fct(*iter, priv);
+       }
+}
+
 #ifdef CONFIG_MODULES
 bool trace_module_has_bad_taint(struct module *mod)
 {
@@ -391,15 +413,9 @@ EXPORT_SYMBOL_GPL(unregister_tracepoint_module_notifier);
  * Ensure the tracer unregistered the module's probes before the module
  * teardown is performed. Prevents leaks of probe and data pointers.
  */
-static void tp_module_going_check_quiescent(struct tracepoint * const *begin,
-               struct tracepoint * const *end)
+static void tp_module_going_check_quiescent(struct tracepoint *tp, void *priv)
 {
-       struct tracepoint * const *iter;
-
-       if (!begin)
-               return;
-       for (iter = begin; iter < end; iter++)
-               WARN_ON_ONCE((*iter)->funcs);
+       WARN_ON_ONCE(tp->funcs);
 }
 
 static int tracepoint_module_coming(struct module *mod)
@@ -450,8 +466,9 @@ static void tracepoint_module_going(struct module *mod)
                         * Called the going notifier before checking for
                         * quiescence.
                         */
-                       tp_module_going_check_quiescent(mod->tracepoints_ptrs,
-                               mod->tracepoints_ptrs + mod->num_tracepoints);
+                       for_each_tracepoint_range(mod->tracepoints_ptrs,
+                               mod->tracepoints_ptrs + mod->num_tracepoints,
+                               tp_module_going_check_quiescent, NULL);
                        break;
                }
        }
@@ -503,19 +520,6 @@ 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,
-               void (*fct)(struct tracepoint *tp, void *priv),
-               void *priv)
-{
-       struct tracepoint * const *iter;
-
-       if (!begin)
-               return;
-       for (iter = begin; iter < end; iter++)
-               fct(*iter, priv);
-}
-
 /**
  * for_each_kernel_tracepoint - iteration on all kernel tracepoints
  * @fct: callback
-- 
2.11.0