tracepoints: Fix section alignment using pointer array

Make the tracepoints more robust, making them solid enough to handle compiler
changes by not relying on anything based on compiler-specific behavior with
respect to structure alignment. Implement an approach proposed by David Miller:
use an array of const pointers to refer to the individual structures, and export
this pointer array through the linker script rather than the structures per se.
It will consume 32 extra bytes per tracepoint (24 for structure padding and 8
for the pointers), but are less likely to break due to compiler changes.

History:

commit 7e066fb8 tracepoints: add DECLARE_TRACE() and DEFINE_TRACE()
added the aligned(32) type and variable attribute to the tracepoint structures
to deal with gcc happily aligning statically defined structures on 32-byte
multiples.

One attempt was to use a 8-byte alignment for tracepoint structures by applying
both the variable and type attribute to tracepoint structures definitions and
declarations. It worked fine with gcc 4.5.1, but broke with gcc 4.4.4 and 4.4.5.

The reason is that the "aligned" attribute only specify the _minimum_ alignment
for a structure, leaving both the compiler and the linker free to align on
larger multiples. Because tracepoint.c expects the structures to be placed as an
array within each section, up-alignment cause NULL-pointer exceptions due to the
extra unexpected padding.

(this patch applies on top of -tip)

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: David S. Miller <davem@davemloft.net>
LKML-Reference: <20110126222622.GA10794@Krystal>
CC: Frederic Weisbecker <fweisbec@gmail.com>
CC: Ingo Molnar <mingo@elte.hu>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 8 insertions, 8 deletions

diff --git a/kernel/module.c b/kernel/module.c
index 34e00b708fa..efa290ea94b 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2460,9 +2460,9 @@ static void find_module_sections(struct module *mod, struct load_info *info)
 #endif
 
 #ifdef CONFIG_TRACEPOINTS
-	mod->tracepoints = section_objs(info, "__tracepoints",
-					sizeof(*mod->tracepoints),
-					&mod->num_tracepoints);
+	mod->tracepoints_ptrs = section_objs(info, "__tracepoints_ptrs",
+					     sizeof(*mod->tracepoints_ptrs),
+					     &mod->num_tracepoints);
 #endif
 #ifdef HAVE_JUMP_LABEL
 	mod->jump_entries = section_objs(info, "__jump_table",
@@ -3393,7 +3393,7 @@ void module_layout(struct module *mod,
 		   struct modversion_info *ver,
 		   struct kernel_param *kp,
 		   struct kernel_symbol *ks,
-		   struct tracepoint *tp)
+		   struct tracepoint * const *tp)
 {
 }
 EXPORT_SYMBOL(module_layout);
@@ -3407,8 +3407,8 @@ void module_update_tracepoints(void)
 	mutex_lock(&module_mutex);
 	list_for_each_entry(mod, &modules, list)
 		if (!mod->taints)
-			tracepoint_update_probe_range(mod->tracepoints,
-				mod->tracepoints + mod->num_tracepoints);
+			tracepoint_update_probe_range(mod->tracepoints_ptrs,
+				mod->tracepoints_ptrs + mod->num_tracepoints);
 	mutex_unlock(&module_mutex);
 }
 
@@ -3432,8 +3432,8 @@ int module_get_iter_tracepoints(struct tracepoint_iter *iter)
 			else if (iter_mod > iter->module)
 				iter->tracepoint = NULL;
 			found = tracepoint_get_iter_range(&iter->tracepoint,
-				iter_mod->tracepoints,
-				iter_mod->tracepoints
+				iter_mod->tracepoints_ptrs,
+				iter_mod->tracepoints_ptrs
 					+ iter_mod->num_tracepoints);
 			if (found) {
 				iter->module = iter_mod;