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Date: Mon, 16 May 2022 20:00:23 -0700
From: Kees Cook <keescook@...omium.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Kees Cook <keescook@...omium.org>,
Christophe de Dinechin <dinechin@...hat.com>,
Ingo Molnar <mingo@...hat.com>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [PATCH v3] sched/core: Address classes via __begin_sched_classes
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. This also means the for_class_range() must be redefined to
be inclusive, which means its 1 caller must be adjusted to have its
"lowest" argument bumped up one position. 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[];
| ^~~~~~~~~~~~~~~~~~~~~
The introduce and use of sched_class_higher() could just be a bare "+ 1",
but this code's backwards walking and non-inclusive for loop was weird
enough, it seemed back to explicitly describe the manipulation
happening.
These can't just be object pointers because GCC still sees it as an
address of a single struct.
The resulting instruction output is identical to before except that one
less register is used in set_rq_online(), where an immediate can now
be used, resulting in a small instruction count savings:
│ set_rq_online():
│ - push %r12
│ push %rbp
│ push %rbx
│ mov 0x9a0(%rdi),%rax
│ mov 0xa10(%rdi),%edx
│ lock bts %rdx,0x20(%rax)
│ movabs $0x0,%rbx
│ R_X86_64_64 __end_sched_classes-0xd0
│ - movabs $0x0,%r12
│ - R_X86_64_64 __begin_sched_classes-0xd0
│ movl $0x1,0xa14(%rdi)
│ - cmp %r12,%rbx
│ - je 31ea <set_rq_online.part.0+0x5a>
│ - mov %rdi,%rbp
│ + cmp $0x0,%rbx
│ + R_X86_64_32S __begin_sched_classes
│ + jb 31e6 <set_rq_online.part.0+0x56>
│ + mov %rdi,%rbp
│ mov 0x70(%rbx),%rax
│ test %rax,%rax
│ - je 31de <set_rq_online.part.0+0x4e>
│ + je 31d6 <set_rq_online.part.0+0x46>
│ mov %rbp,%rdi
│ - call 31de <set_rq_online.part.0+0x4e>
│ + call 31d6 <set_rq_online.part.0+0x46>
│ R_X86_64_PLT32 __x86_indirect_thunk_rax-0x4
│ sub $0xd0,%rbx
│ - cmp %r12,%rbx
│ - jne 31cd <set_rq_online.part.0+0x3d>
│ + cmp $0x0,%rbx
│ + R_X86_64_32S __begin_sched_classes
│ + jae 31c5 <set_rq_online.part.0+0x35>
│ pop %rbx
│ pop %rbp
│ - pop %r12
│ retq
Reported-by: Christophe de Dinechin <dinechin@...hat.com>
Link: https://lore.kernel.org/lkml/20220414150855.2407137-2-dinechin@redhat.com/
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Ben Segall <bsegall@...gle.com>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
v1: https://lore.kernel.org/lkml/20220516194241.3064242-1-keescook@chromium.org
v2: https://lore.kernel.org/lkml/20220517000630.3383144-1-keescook@chromium.org
v3:
- Add missing increment to the one for_class_range() user
- Provide instruction sequence change analysis in commit log
---
kernel/sched/core.c | 3 ++-
kernel/sched/sched.h | 11 ++++++++---
2 files changed, 10 insertions(+), 4 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d58c0389eb23..f2bcc7f15381 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5665,7 +5665,8 @@ static void put_prev_task_balance(struct rq *rq, struct task_struct *prev,
* We can terminate the balance pass as soon as we know there is
* a runnable task of @class priority or higher.
*/
- for_class_range(class, prev->sched_class, &idle_sched_class) {
+ for_class_range(class, prev->sched_class,
+ sched_class_higher(&idle_sched_class)) {
if (class->balance(rq, prev, rf))
break;
}
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 8dccb34eb190..c757bd26b01a 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2193,11 +2193,16 @@ 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_higher(class) ((class) + 1)
+#define sched_class_highest (&__begin_sched_classes[__end_sched_classes \
+ - __begin_sched_classes \
+ - 1])
+#define sched_class_lowest (&__begin_sched_classes[0])
+
+/* For each class, inclusive from _from down to _to. */
#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
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