[PATCH v2] sched/core: Address classes via __begin_sched_classes

[Kees Cook]

GCC 12 is very sensitive about array checking, and views all negative
array accesses as unsafe (a not unreasonable position). Redefine
sched_class_highest in terms of its location from __begin_sched_classes,
and redefine sched_class_lowest to the actual lowest sched class instead
of one lower (and adjust the for loop so it still works). Silences
this warning:

In file included from kernel/sched/core.c:81:
kernel/sched/core.c: In function ‘set_rq_online.part.0’:
kernel/sched/sched.h:2197:52: error: array subscript -1 is outside array bounds of ‘struct sched_class[44343134792571037]’
 [-Werror=array-bounds]
 2197 | #define sched_class_lowest  (__begin_sched_classes - 1)
      |                                                    ^
kernel/sched/sched.h:2200:41: note: in definition of macro ‘for_class_range’
 2200 |         for (class = (_from); class != (_to); class--)
      |                                         ^~~
kernel/sched/sched.h:2203:53: note: in expansion of macro ‘sched_class_lowest’
 2203 |for_class_range(class, sched_class_highest, sched_class_lowest)
      |                                            ^~~~~~~~~~~~~~~~~~
kernel/sched/core.c:9115:17: note: in expansion of macro ‘for_each_class’
 9115 |                 for_each_class(class) {
      |                 ^~~~~~~~~~~~~~
kernel/sched/sched.h:2193:27: note: at offset -208 into object ‘__begin_sched_classes’ of size [0, 9223372036854775807]
 2193 | extern struct sched_class __begin_sched_classes[];
      |                           ^~~~~~~~~~~~~~~~~~~~~

These can't just be object pointers because GCC still sees it as an
address of a single struct.

Reported-by: Christophe de Dinechin <dinechin@xxxxxxxxxx>
Link: https://lore.kernel.org/lkml/20220414150855.2407137-2-dinechin@xxxxxxxxxx/
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Ben Segall <bsegall@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
v1: https://lore.kernel.org/lkml/20220516194241.3064242-1-keescook@xxxxxxxxxxxx
v2: use the arrays with proper indexes and adjusted loop logic
---
 kernel/sched/sched.h | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 8dccb34eb190..09304c01e5ea 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2193,11 +2193,13 @@ const struct sched_class name##_sched_class \
 extern struct sched_class __begin_sched_classes[];
 extern struct sched_class __end_sched_classes[];
 
-#define sched_class_highest (__end_sched_classes - 1)
-#define sched_class_lowest  (__begin_sched_classes - 1)
+#define sched_class_highest (&__begin_sched_classes[__end_sched_classes     \
+						    - __begin_sched_classes \
+						    - 1])
+#define sched_class_lowest  (&__begin_sched_classes[0])
 
 #define for_class_range(class, _from, _to) \
-	for (class = (_from); class != (_to); class--)
+	for (class = (_from); class >= (_to); class--)
 
 #define for_each_class(class) \
 	for_class_range(class, sched_class_highest, sched_class_lowest)
-- 
2.32.0