[PATCH for 4.19] tracepoint: Fix: tracepoint array element size mismatch

[Mathieu Desnoyers]

commit 46e0c9be206f ("kernel: tracepoints: add support for relative
references") changes the layout of the __tracepoint_ptrs section on
architectures supporting relative references. However, it does so
without turning struct tracepoint * const into const int elsewhere in
the tracepoint code, which has the following side-effect:

Setting mod->num_tracepoints is done in by module.c:

    mod->tracepoints_ptrs = section_objs(info, "__tracepoints_ptrs",
                                         sizeof(*mod->tracepoints_ptrs),
                                         &mod->num_tracepoints);

Basically, since sizeof(*mod->tracepoints_ptrs) is a pointer size
(rather than sizeof(int)), num_tracepoints is erroneously set to half the
size it should be on 64-bit arch. So a module with an odd number of
tracepoints misses the last tracepoint due to effect of integer
division.

So in the module going notifier:

        for_each_tracepoint_range(mod->tracepoints_ptrs,
                mod->tracepoints_ptrs + mod->num_tracepoints,
                tp_module_going_check_quiescent, NULL);

the expression (mod->tracepoints_ptrs + mod->num_tracepoints) actually
evaluates to something within the bounds of the array, but miss the
last tracepoint if the number of tracepoints is odd on 64-bit arch.

Fix this by introducing a new typedef: tracepoint_ptr_t, which
is either "const int" on architectures that have PREL32 relocations,
or "struct tracepoint * const" on architectures that does not have
this feature.

Also provide a new tracepoint_ptr_defer() static inline to
encapsulate deferencing this type rather than duplicate code and
ugly idefs within the for_each_tracepoint_range() implementation.

This issue appears in 4.19-rc kernels, and should ideally be fixed
before the end of the rc cycle.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Acked-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
Link: http://lkml.kernel.org/r/20180704083651.24360-7-ard.biesheuvel@xxxxxxxxxx
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: James Morris <james.morris@xxxxxxxxxxxxx>
Cc: James Morris <jmorris@xxxxxxxxx>
Cc: Jessica Yu <jeyu@xxxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Nicolas Pitre <nico@xxxxxxxxxx>
Cc: Paul Mackerras <paulus@xxxxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxxx>
Cc: Russell King <linux@xxxxxxxxxxxxxxx>
Cc: "Serge E. Hallyn" <serge@xxxxxxxxxx>
Cc: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
Cc: Thomas Garnier <thgarnie@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 include/linux/module.h          |  3 ++-
 include/linux/tracepoint-defs.h |  6 ++++++
 include/linux/tracepoint.h      | 36 +++++++++++++++++++++------------
 kernel/tracepoint.c             | 24 ++++++++--------------
 4 files changed, 39 insertions(+), 30 deletions(-)

diff --git a/include/linux/module.h b/include/linux/module.h
index f807f15bebbe..e19ae08c7fb8 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -20,6 +20,7 @@
 #include <linux/export.h>
 #include <linux/rbtree_latch.h>
 #include <linux/error-injection.h>
+#include <linux/tracepoint-defs.h>
 
 #include <linux/percpu.h>
 #include <asm/module.h>
@@ -430,7 +431,7 @@ struct module {
 
 #ifdef CONFIG_TRACEPOINTS
 	unsigned int num_tracepoints;
-	struct tracepoint * const *tracepoints_ptrs;
+	tracepoint_ptr_t *tracepoints_ptrs;
 #endif
 #ifdef HAVE_JUMP_LABEL
 	struct jump_entry *jump_entries;
diff --git a/include/linux/tracepoint-defs.h b/include/linux/tracepoint-defs.h
index 22c5a46e9693..49ba9cde7e4b 100644
--- a/include/linux/tracepoint-defs.h
+++ b/include/linux/tracepoint-defs.h
@@ -35,6 +35,12 @@ struct tracepoint {
 	struct tracepoint_func __rcu *funcs;
 };
 
+#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
+typedef const int tracepoint_ptr_t;
+#else
+typedef struct tracepoint * const tracepoint_ptr_t;
+#endif
+
 struct bpf_raw_event_map {
 	struct tracepoint	*tp;
 	void			*bpf_func;
diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 041f7e56a289..538ba1a58f5b 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -99,6 +99,29 @@ extern void syscall_unregfunc(void);
 #define TRACE_DEFINE_ENUM(x)
 #define TRACE_DEFINE_SIZEOF(x)
 
+#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
+static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
+{
+	return offset_to_ptr(p);
+}
+
+#define __TRACEPOINT_ENTRY(name)					\
+	asm("	.section \"__tracepoints_ptrs\", \"a\"		\n"	\
+	    "	.balign 4					\n"	\
+	    "	.long 	__tracepoint_" #name " - .		\n"	\
+	    "	.previous					\n")
+#else
+static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
+{
+	return *p;
+}
+
+#define __TRACEPOINT_ENTRY(name)					 \
+	static tracepoint_ptr_t __tracepoint_ptr_##name __used		 \
+	__attribute__((section("__tracepoints_ptrs"))) =		 \
+		&__tracepoint_##name
+#endif
+
 #endif /* _LINUX_TRACEPOINT_H */
 
 /*
@@ -253,19 +276,6 @@ 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
diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index bf2c06ef9afc..a3be42304485 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 tracepoint_ptr_t __start___tracepoints_ptrs[];
+extern tracepoint_ptr_t __stop___tracepoints_ptrs[];
 
 DEFINE_SRCU(tracepoint_srcu);
 EXPORT_SYMBOL_GPL(tracepoint_srcu);
@@ -371,25 +371,17 @@ 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,
+static void for_each_tracepoint_range(
+		tracepoint_ptr_t *begin, tracepoint_ptr_t *end,
 		void (*fct)(struct tracepoint *tp, void *priv),
 		void *priv)
 {
+	tracepoint_ptr_t *iter;
+
 	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(offset_to_ptr(iter), priv);
-	} else {
-		struct tracepoint * const *iter;
-
-		for (iter = begin; iter < end; iter++)
-			fct(*iter, priv);
-	}
+	for (iter = begin; iter < end; iter++)
+		fct(tracepoint_ptr_deref(iter), priv);
 }
 
 #ifdef CONFIG_MODULES
-- 
2.17.1