Re: [PATCH v2 bpf-next 5/7] bpf: support BPF ksym variables in kernel modules

[Hao Luo <haoluo@google.com>]

Acked-by: Hao Luo <haoluo@google.com>, with a suggestion on adding a comment.

On Fri, Jan 8, 2021 at 2:09 PM Andrii Nakryiko <andrii@kernel.org> wrote:
>
> Add support for directly accessing kernel module variables from BPF programs
> using special ldimm64 instructions. This functionality builds upon vmlinux
> ksym support, but extends ldimm64 with src_reg=BPF_PSEUDO_BTF_ID to allow
> specifying kernel module BTF's FD in insn[1].imm field.
>
> During BPF program load time, verifier will resolve FD to BTF object and will
> take reference on BTF object itself and, for module BTFs, corresponding module
> as well, to make sure it won't be unloaded from under running BPF program. The
> mechanism used is similar to how bpf_prog keeps track of used bpf_maps.
>
> One interesting change is also in how per-CPU variable is determined. The
> logic is to find .data..percpu data section in provided BTF, but both vmlinux
> and module each have their own .data..percpu entries in BTF. So for module's
> case, the search for DATASEC record needs to look at only module's added BTF
> types. This is implemented with custom search function.
>
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> ---
>  include/linux/bpf.h          |  10 +++
>  include/linux/bpf_verifier.h |   3 +
>  include/linux/btf.h          |   3 +
>  kernel/bpf/btf.c             |  31 +++++++-
>  kernel/bpf/core.c            |  23 ++++++
>  kernel/bpf/verifier.c        | 149 ++++++++++++++++++++++++++++-------
>  6 files changed, 189 insertions(+), 30 deletions(-)

[...]

> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 17270b8404f1..af94c6871ab8 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -9703,6 +9703,31 @@ static int do_check(struct bpf_verifier_env *env)
>         return 0;
>  }
>
> +static int find_btf_percpu_datasec(struct btf *btf)
> +{
> +       const struct btf_type *t;
> +       const char *tname;
> +       int i, n;
> +

It would be good to add a short comment here explaining the reason why
the search for DATASEC in the module case needs to skip entries.

> +       n = btf_nr_types(btf);
> +       if (btf_is_module(btf))
> +               i = btf_nr_types(btf_vmlinux);
> +       else
> +               i = 1;
> +
> +       for(; i < n; i++) {
> +               t = btf_type_by_id(btf, i);
> +               if (BTF_INFO_KIND(t->info) != BTF_KIND_DATASEC)
> +                       continue;
> +
> +               tname = btf_name_by_offset(btf, t->name_off);
> +               if (!strcmp(tname, ".data..percpu"))
> +                       return i;
> +       }
> +
> +       return -ENOENT;
> +}
[...]
> 2.24.1
>