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

[Tejun Heo <tj@kernel.org>]

On Wed, Oct 23, 2024 at 04:07:49PM -0700, Alexei Starovoitov wrote:
> On Wed, Oct 23, 2024 at 2:00 PM Tejun Heo <tj@kernel.org> 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) {}

This is a lot neater than the existing workaround:

  http://lkml.kernel.org/r/Zxma-ZFPKYZDqCGu@slm.duckdns.org

Thanks!

-- 
tejun