Re: [PATCH bpf-next 5/5] bpftool: Support dumping a map with btf_vmlinux_value_type_id

[Martin Lau <kafai@fb.com>]

On Tue, Jan 14, 2020 at 05:49:00PM -0800, Andrii Nakryiko wrote:
> On Tue, Jan 14, 2020 at 2:46 PM Martin KaFai Lau <kafai@fb.com> wrote:
> >
> > This patch makes bpftool support dumping a map's value properly
> > when the map's value type is a type of the running kernel's btf.
> > (i.e. map_info.btf_vmlinux_value_type_id is set instead of
> > map_info.btf_value_type_id).  The first usecase is for the
> > BPF_MAP_TYPE_STRUCT_OPS.
> >
> > Signed-off-by: Martin KaFai Lau <kafai@fb.com>
> > ---
> >  tools/bpf/bpftool/map.c | 43 +++++++++++++++++++++++++++++++----------
> >  1 file changed, 33 insertions(+), 10 deletions(-)
> >
> > diff --git a/tools/bpf/bpftool/map.c b/tools/bpf/bpftool/map.c
> > index 4c5b15d736b6..d25f3b2355ad 100644
> > --- a/tools/bpf/bpftool/map.c
> > +++ b/tools/bpf/bpftool/map.c
> > @@ -20,6 +20,7 @@
> >  #include "btf.h"
> >  #include "json_writer.h"
> >  #include "main.h"
> > +#include "libbpf_internal.h"
> >
> >  const char * const map_type_name[] = {
> >         [BPF_MAP_TYPE_UNSPEC]                   = "unspec",
> > @@ -252,6 +253,7 @@ static int do_dump_btf(const struct btf_dumper *d,
> >                        struct bpf_map_info *map_info, void *key,
> >                        void *value)
> >  {
> > +       __u32 value_id;
> >         int ret;
> >
> >         /* start of key-value pair */
> > @@ -265,9 +267,12 @@ static int do_dump_btf(const struct btf_dumper *d,
> >                         goto err_end_obj;
> >         }
> >
> > +       value_id = map_info->btf_vmlinux_value_type_id ?
> > +               : map_info->btf_value_type_id;
> > +
> >         if (!map_is_per_cpu(map_info->type)) {
> >                 jsonw_name(d->jw, "value");
> > -               ret = btf_dumper_type(d, map_info->btf_value_type_id, value);
> > +               ret = btf_dumper_type(d, value_id, value);
> >         } else {
> >                 unsigned int i, n, step;
> >
> > @@ -279,8 +284,7 @@ static int do_dump_btf(const struct btf_dumper *d,
> >                         jsonw_start_object(d->jw);
> >                         jsonw_int_field(d->jw, "cpu", i);
> >                         jsonw_name(d->jw, "value");
> > -                       ret = btf_dumper_type(d, map_info->btf_value_type_id,
> > -                                             value + i * step);
> > +                       ret = btf_dumper_type(d, value_id, value + i * step);
> >                         jsonw_end_object(d->jw);
> >                         if (ret)
> >                                 break;
> > @@ -932,6 +936,27 @@ static int maps_have_btf(int *fds, int nb_fds)
> >         return 1;
> >  }
> >
> > +static struct btf *get_map_kv_btf(const struct bpf_map_info *info)
> > +{
> > +       struct btf *btf = NULL;
> > +
> > +       if (info->btf_vmlinux_value_type_id) {
> > +               btf = bpf_find_kernel_btf();
> 
> If there are multiple maps we are dumping, it might become quite
> costly to re-read and re-parse kernel BTF all the time. Can we lazily
> load it, when required,
It is loaded lazily.

> and cache instead?
Cache it in bpftool/map.c? Sure.

> 
> > +               if (IS_ERR(btf))
> > +                       p_err("failed to get kernel btf");
> > +       } else if (info->btf_value_type_id) {
> > +               int err;
> > +
> > +               err = btf__get_from_id(info->btf_id, &btf);
> > +               if (err || !btf) {
> > +                       p_err("failed to get btf");
> > +                       btf = err ? ERR_PTR(err) : ERR_PTR(-ESRCH);
> > +               }
> > +       }
> > +
> > +       return btf;
> > +}
> > +
> >  static int
> >  map_dump(int fd, struct bpf_map_info *info, json_writer_t *wtr,
> >          bool show_header)
> > @@ -952,13 +977,11 @@ map_dump(int fd, struct bpf_map_info *info, json_writer_t *wtr,
> >         prev_key = NULL;
> >
> >         if (wtr) {
> > -               if (info->btf_id) {
> > -                       err = btf__get_from_id(info->btf_id, &btf);
> > -                       if (err || !btf) {
> > -                               err = err ? : -ESRCH;
> > -                               p_err("failed to get btf");
> > -                               goto exit_free;
> > -                       }
> > +               btf = get_map_kv_btf(info);
> > +               if (IS_ERR(btf)) {
> > +                       err = PTR_ERR(btf);
> > +                       btf = NULL;
> > +                       goto exit_free;
> >                 }
> >
> >                 if (show_header) {
> > --
> > 2.17.1
> >