Re: [PATCH bpf-next v6 3/9] bpf: Support private stack for struct ops programs

[Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>]

On Wed, Oct 23, 2024 at 2:00 PM Tejun Heo <tj@xxxxxxxxxx> wrote:
>
> Hello,
>
> On Tue, Oct 22, 2024 at 01:19:58PM -0700, Alexei Starovoitov wrote:
> > > The __nullable argument tagging request was originally from sched_ext but I also
> > > don't see its usage in-tree for now.
> >
> > ok. Let's sync up with Tejun whether they have plans to use it.
>
> Yeah, in sched_ext_ops.dispatch(s32 cpu, struct task_struct *prev), @prev
> can be NULL and right now if a BPF scheduler derefs without checking for
> NULL, it can trigger kernel crash, I think, so it needs __nullable tagging.

I see. The following should do it:
diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 3cd7c50a51c5..82bef41d7eae 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -5492,7 +5492,7 @@ static int bpf_scx_validate(void *kdata)
 static s32 select_cpu_stub(struct task_struct *p, s32 prev_cpu, u64
wake_flags) { return -EINVAL; }
 static void enqueue_stub(struct task_struct *p, u64 enq_flags) {}
 static void dequeue_stub(struct task_struct *p, u64 enq_flags) {}
-static void dispatch_stub(s32 prev_cpu, struct task_struct *p) {}
+static void dispatch_stub(s32 prev_cpu, struct task_struct *p__nullable) {}