Re: [RFC PATCH 2/9] bpf/arena: Add BPF_F_ARENA_MAP_ALWAYS for direct kernel access

[Alexei Starovoitov]

On Mon May 11, 2026 at 7:43 PM PDT, Kumar Kartikeya Dwivedi wrote:
>
> If not, the best course to me seems to be to make the flag behavior
> default, and just rely on ASan (and Rust in the future) to prevent any
> memory safety issues, and drop the stream based feedback on fault,
> etc.

Agree that this needs to be new default without new uapi flags.
How about we tweak the idea further.
Let all arena pages be unmapped initially. bpf progs will fault
on them and will be reported via bpf_streams.
But we also prepare one "scratch page". Let's use this name,
since "garbage page" reads too dirty.
When kernel faults we populate pte with that scratch page
and let the kernel code retry.
To implement it the page_fault_oops() can have a callback
into bpf/arena helper similar to kfence_handle_page_fault.
If fault address is in arena, do kfence_unprotect()-like.