kernel: tracepoints: add support for relative references

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.

Link: http://lkml.kernel.org/r/20180704083651.24360-7-ard.biesheuvel@linaro.org
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morris <james.morris@microsoft.com>
Cc: James Morris <jmorris@namei.org>
Cc: Jessica Yu <jeyu@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Nicolas Pitre <nico@linaro.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Thomas Garnier <thgarnie@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 15 insertions, 4 deletions

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index d9a084c72541..7f2e16e76ac4 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -249,6 +249,19 @@ 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   __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
@@ -258,11 +271,9 @@ 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);