On Wed, Aug 23, 2023 at 10:53 AM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Wed, Aug 23, 2023 at 10:14 AM Andrii Nakryiko
> <andrii.nakryiko@xxxxxxxxx> wrote:
> >
> > > Long term we need to think how to extend bpf ISA with alloca.
> >
> > To be frank, I'm not following how alloca is relevant here. We don't
> > have anything dynamically sized on the stack.
> >
> > Unless you envision protocol where we have a separate function to get
> > size of iter struct, then alloca enough space, then pass that to
> > bpf_iter_xxx_new()? Not sure whether this is statically verifiable,
> > but given it's long-term, we can put it on backburner for now.
>
> With alloca bpf_for_each() macro can allocate whatever stack necessary.
>
> In other words:
>
> struct bpf_iter_task_vma *it;
>
> it = bpf_alloca(bpf_core_type_size(struct bpf_iter_task_vma));
> bpf_for_each2(task_vma, it, ...) { .. }
ah, I see, not a dedicated kfunc, just CO-RE relocation. Makes sense.
>
> While struct bpf_iter_task_vma can come from vmlinux.h
>
> size of kern data struct is CO-RE-able, so no worries about increase
> in size due to maple tree or lockdep on/off.
> And no concern of failing allocation at run-time.
> (the verifier would reject big stack alloc at load time).
yep, makes sense, the size will be statically known to the verifier. I
was overcomplicating this in my mind with extra kfunc.
