From: Tejun Heo <tj@kernel.org> mainline inclusion from mainline-v6.12-rc1 commit 59cfdf3f3349019fbfc986a285afcc3873d155f4 category: feature bugzilla: https://gitee.com/openeuler/kernel/issues/IDC9YK Reference: https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commi... -------------------------------- ARRAY_ELEM_PTR() is an access macro used to help the BPF verifier not confused by offseted memory acceeses by yiedling a valid pointer or NULL in a way that's clear to the verifier. As such, the canonical usage involves checking NULL return from the macro. Note that in many cases, the NULL condition can never happen - they're there just to hint the verifier. In a bpf_loop in scx_central.bpf.c::central_dispatch(), the NULL check was incorrect in that there was another dereference of the pointer in addition to the NULL checked access. This worked as the pointer can never be NULL and the verifier could tell it would never be NULL in this case. However, this still looks wrong and trips smatch: ./tools/sched_ext/scx_central.bpf.c:205 ____central_dispatch() error: we previously assumed 'gimme' could be null (see line 201) ./tools/sched_ext/scx_central.bpf.c 195 196 if (!scx_bpf_dispatch_nr_slots()) 197 break; 198 199 /* central's gimme is never set */ 200 gimme = ARRAY_ELEM_PTR(cpu_gimme_task, cpu, nr_cpu_ids); 201 if (gimme && !*gimme) ^^^^^ If gimme is NULL 202 continue; 203 204 if (dispatch_to_cpu(cpu)) --> 205 *gimme = false; Fix the NULL check so that there are no derefs if NULL. This doesn't change actual behavior. Signed-off-by: Tejun Heo <tj@kernel.org> Reported-by: Dan Carpenter <dan.carpenter@linaro.org> Link: http://lkml.kernel.org/r/<955e1c3c-ace2-4a1d-b246-15b8196038a3@stanley.mountain> Signed-off-by: Zicheng Qu <quzicheng@huawei.com> --- tools/sched_ext/scx_central.bpf.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/tools/sched_ext/scx_central.bpf.c b/tools/sched_ext/scx_central.bpf.c index 1d8fd570eaa7..8dd8eb73b6b8 100644 --- a/tools/sched_ext/scx_central.bpf.c +++ b/tools/sched_ext/scx_central.bpf.c @@ -198,7 +198,7 @@ void BPF_STRUCT_OPS(central_dispatch, s32 cpu, struct task_struct *prev) /* central's gimme is never set */ gimme = ARRAY_ELEM_PTR(cpu_gimme_task, cpu, nr_cpu_ids); - if (gimme && !*gimme) + if (!gimme || !*gimme) continue; if (dispatch_to_cpu(cpu)) -- 2.34.1