Re: [PATCH bpf-next v2 23/25] libbpf: Add support for private BSS map section

Andrii Nakryiko
2022-10-18T04:04:16Z

On Wed, Oct 12, 2022 at 11:24 PM Kumar Kartikeya Dwivedi
<memxor@gmail.com> wrote:
>
> From: Dave Marchevsky <davemarchevsky@fb.com>
>
> Currently libbpf does not allow declaration of a struct bpf_spin_lock in
> global scope. Attempting to do so results in "failed to re-mmap" error,
> as .bss arraymap containing spinlock is not allowed to be mmap'd.
>
> This patch adds support for a .bss.private section. The maps contained
> in this section will not be mmaped into userspace by libbpf, nor will
> they be exposed via bpftool-generated skeleton.
>
> Intent here is to allow more natural programming pattern for
> global-scope spinlocks which will be used by rbtree locking mechanism in
> further patches in this series.
>
> Notes:
>
> * Initially I called the section .bss.no_mmap, but the broader
> 'private' term better indicates that skeleton shouldn't expose these
> maps at all, IMO.
>
> * bpftool/gen.c's is_internal_mmapable_map function checks whether the
> map flags have BPF_F_MMAPABLE, so no bpftool changes were necessary
> to remove .bss.private maps from skeleton
>
> Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---

Please see [0] for what I think is a better way forward specifically
for the libbpf-side part.

[0] https://patchwork.kernel.org/project/netdevbpf/list/?series=686066&state=*

> tools/lib/bpf/libbpf.c | 65 ++++++++++++++++++++++++++++--------------
> 1 file changed, 44 insertions(+), 21 deletions(-)
> …