Re: [PATCH bpf-next v2 2/2] bpftool: Support dumping kfunc prototypes from BTF

[Andrii Nakryiko]

On Sun, Feb 4, 2024 at 1:07 PM Daniel Xu <dxu@xxxxxxxxx> wrote:
>
> This patch enables dumping kfunc prototypes from bpftool. This is useful
> b/c with this patch, end users will no longer have to manually define
> kfunc prototypes. For the kernel tree, this also means we can drop
> kfunc prototypes from:
>
>         tools/testing/selftests/bpf/bpf_kfuncs.h
>         tools/testing/selftests/bpf/bpf_experimental.h
>
> Example usage:
>
>         $ make PAHOLE=/home/dxu/dev/pahole/build/pahole -j30 vmlinux
>
>         $ ./tools/bpf/bpftool/bpftool btf dump file ./vmlinux format c | rg "__ksym;" | head -3
>         extern void cgroup_rstat_updated(struct cgroup *cgrp, int cpu) __weak __ksym;
>         extern void cgroup_rstat_flush(struct cgroup *cgrp) __weak __ksym;
>         extern struct bpf_key *bpf_lookup_user_key(u32 serial, u64 flags) __weak __ksym;
>
> Note that this patch is only effective after the enabling pahole [0]
> change is merged and the resulting feature enabled with
> --btf_features=decl_tag_kfuncs.
>
> [0]: https://lore.kernel.org/bpf/cover.1707071969.git.dxu@xxxxxxxxx/
>
> Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> ---
>  tools/bpf/bpftool/btf.c | 45 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 45 insertions(+)
>
> diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
> index 91fcb75babe3..0fd78a476286 100644
> --- a/tools/bpf/bpftool/btf.c
> +++ b/tools/bpf/bpftool/btf.c
> @@ -20,6 +20,8 @@
>  #include "json_writer.h"
>  #include "main.h"
>
> +#define KFUNC_DECL_TAG         "bpf_kfunc"
> +
>  static const char * const btf_kind_str[NR_BTF_KINDS] = {
>         [BTF_KIND_UNKN]         = "UNKNOWN",
>         [BTF_KIND_INT]          = "INT",
> @@ -454,6 +456,39 @@ static int dump_btf_raw(const struct btf *btf,
>         return 0;
>  }
>
> +static int dump_btf_kfuncs(struct btf_dump *d, const struct btf *btf)
> +{
> +       DECLARE_LIBBPF_OPTS(btf_dump_emit_type_decl_opts, opts);

nit: use shorter LIBBPF_OPTS, DECLARE_LIBBPF_OPTS is a "deprecated"
macro name I hid, but didn't remove

> +       int cnt = btf__type_cnt(btf);
> +       int i;
> +
> +       for (i = 1; i < cnt; i++) {
> +               const struct btf_type *t = btf__type_by_id(btf, i);
> +               const struct btf_type *kft;
> +               const char *name;
> +               int err;
> +
> +               if (!btf_is_decl_tag(t))
> +                       continue;
> +
> +               name = btf__name_by_offset(btf, t->name_off);
> +               if (strncmp(name, KFUNC_DECL_TAG, sizeof(KFUNC_DECL_TAG)))
> +                       continue;

should we do a bit more sanity checking here? Check that component_idx
= -1 (entire func) and pointee type is FUNC?

> +
> +               printf("extern ");
> +
> +               kft = btf__type_by_id(btf, t->type);

nit: reuse t?

> +               opts.field_name = btf__name_by_offset(btf, kft->name_off);
> +               err = btf_dump__emit_type_decl(d, kft->type, &opts);
> +               if (err)
> +                       return err;
> +
> +               printf(" __weak __ksym;\n\n");

why extra endline?

though I'd ensure two empty lines before the first kfunc declaration
to visually separate it from other type. Maybe even add a comment like
`/* BPF kfuncs */` or something like that?

> +       }
> +
> +       return 0;
> +}
> +
>  static void __printf(2, 0) btf_dump_printf(void *ctx,
>                                            const char *fmt, va_list args)
>  {
> @@ -476,6 +511,12 @@ static int dump_btf_c(const struct btf *btf,
>         printf("#ifndef BPF_NO_PRESERVE_ACCESS_INDEX\n");
>         printf("#pragma clang attribute push (__attribute__((preserve_access_index)), apply_to = record)\n");
>         printf("#endif\n\n");
> +       printf("#ifndef __ksym\n");
> +       printf("#define __ksym __attribute__((section(\".ksyms\")))\n");
> +       printf("#endif\n\n");
> +       printf("#ifndef __weak\n");
> +       printf("#define __weak __attribute__((weak))\n");
> +       printf("#endif\n\n");
>
>         if (root_type_cnt) {
>                 for (i = 0; i < root_type_cnt; i++) {
> @@ -491,6 +532,10 @@ static int dump_btf_c(const struct btf *btf,
>                         if (err)
>                                 goto done;
>                 }
> +
> +               err = dump_btf_kfuncs(d, btf);
> +               if (err)
> +                       goto done;
>         }
>
>         printf("#ifndef BPF_NO_PRESERVE_ACCESS_INDEX\n");
> --
> 2.42.1
>