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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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