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

[Andrii Nakryiko]

On Sun, Jan 28, 2024 at 5:35 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) __ksym;
>         extern void cgroup_rstat_flush(struct cgroup * cgrp) __ksym;
>         extern struct bpf_key * bpf_lookup_user_key(u32 serial, u64 flags) __ksym;
>

All kfuncs have to be declared __weak, otherwise it will be impossible
to feature-detect them in BPF code

> Note that this patch is only effective after enabling pahole [0]
> and kernel [1] changes are merged.
>
> [0]: https://lore.kernel.org/bpf/0f25134ec999e368478c4ca993b3b729c2a033831706491733.git.dxu@xxxxxxxxx/
> [1]: https://lore.kernel.org/bpf/cover.1706491398.git.dxu@xxxxxxxxx/
>
> Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> ---
>  tools/bpf/bpftool/btf.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
>
> diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
> index 91fcb75babe3..9ab26ed12733 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 +456,28 @@ static int dump_btf_raw(const struct btf *btf,
>         return 0;
>  }
>
> +static void dump_btf_kfuncs(const struct btf *btf)
> +{
> +       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);
> +               char kfunc_sig[1024];
> +               const char *name;
> +
> +               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;
> +
> +               btf_dumper_type_only(btf, t->type, kfunc_sig, sizeof(kfunc_sig));
> +               printf("extern %s __ksym;\n\n", kfunc_sig);
> +       }
> +}
> +
>  static void __printf(2, 0) btf_dump_printf(void *ctx,
>                                            const char *fmt, va_list args)
>  {
> @@ -476,6 +500,9 @@ 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");
>
>         if (root_type_cnt) {
>                 for (i = 0; i < root_type_cnt; i++) {
> @@ -491,6 +518,8 @@ static int dump_btf_c(const struct btf *btf,
>                         if (err)
>                                 goto done;
>                 }
> +
> +               dump_btf_kfuncs(btf);
>         }
>
>         printf("#ifndef BPF_NO_PRESERVE_ACCESS_INDEX\n");
> --
> 2.42.1
>