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, ...) { .. }
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).
