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

[Daniel Xu]

Hi Andrii,

On Wed, Feb 07, 2024 at 04:50:15PM -0800, Andrii Nakryiko wrote:
> 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

Ack.

> 
> > +       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?

Makes sense. Will add.

> 
> > +
> > +               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?

Was thinking b/c other entities (structs) are double newline separated
that we should keep it up for kfunc prototypes. No opinion -- I'll
change it.

> 
> 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?

Ack.

> 
> > +       }
> > +
> > +       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
> >

I'll send the next rev after the pahole changes get merged.

Thanks,
Daniel