Re: [PATCH bpf-next v3 3/4] bpftool: btf: Support dumping a single type from file

[Andrii Nakryiko]

On Mon, Dec 9, 2024 at 3:45 PM Daniel Xu <dxu@xxxxxxxxx> wrote:
>
> Some projects, for example xdp-tools [0], prefer to check in a minimized
> vmlinux.h rather than the complete file which can get rather large.
>
> However, when you try to add a minimized version of a complex struct (eg
> struct xfrm_state), things can get quite complex if you're trying to
> manually untangle and deduplicate the dependencies.
>
> This commit teaches bpftool to do a minimized dump of a single type by
> providing an optional root_id argument.
>
> Example usage:
>
>     $ ./bpftool btf dump file ~/dev/linux/vmlinux | rg "STRUCT 'xfrm_state'"
>     [12643] STRUCT 'xfrm_state' size=912 vlen=58
>
>     $ ./bpftool btf dump file ~/dev/linux/vmlinux root_id 12643 format c
>     #ifndef __VMLINUX_H__
>     #define __VMLINUX_H__
>
>     [..]
>
>     struct xfrm_type_offload;
>
>     struct xfrm_sec_ctx;
>
>     struct xfrm_state {
>             possible_net_t xs_net;
>             union {
>                     struct hlist_node gclist;
>                     struct hlist_node bydst;
>             };
>             union {
>                     struct hlist_node dev_gclist;
>                     struct hlist_node bysrc;
>             };
>             struct hlist_node byspi;
>     [..]
>
> [0]: https://github.com/xdp-project/xdp-tools/blob/master/headers/bpf/vmlinux.h
>
> Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> ---
>  .../bpf/bpftool/Documentation/bpftool-btf.rst |  7 +++++--
>  tools/bpf/bpftool/btf.c                       | 21 ++++++++++++++++++-
>  2 files changed, 25 insertions(+), 3 deletions(-)
>
> diff --git a/tools/bpf/bpftool/Documentation/bpftool-btf.rst b/tools/bpf/bpftool/Documentation/bpftool-btf.rst
> index 245569f43035..4899b2c10777 100644
> --- a/tools/bpf/bpftool/Documentation/bpftool-btf.rst
> +++ b/tools/bpf/bpftool/Documentation/bpftool-btf.rst
> @@ -24,7 +24,7 @@ BTF COMMANDS
>  =============
>
>  | **bpftool** **btf** { **show** | **list** } [**id** *BTF_ID*]
> -| **bpftool** **btf dump** *BTF_SRC* [**format** *FORMAT*]
> +| **bpftool** **btf dump** *BTF_SRC* [**format** *FORMAT*] [**root_id** *ROOT_ID*]
>  | **bpftool** **btf help**
>  |
>  | *BTF_SRC* := { **id** *BTF_ID* | **prog** *PROG* | **map** *MAP* [{**key** | **value** | **kv** | **all**}] | **file** *FILE* }
> @@ -43,7 +43,7 @@ bpftool btf { show | list } [id *BTF_ID*]
>      that hold open file descriptors (FDs) against BTF objects. On such kernels
>      bpftool will automatically emit this information as well.
>
> -bpftool btf dump *BTF_SRC* [format *FORMAT*]
> +bpftool btf dump *BTF_SRC* [format *FORMAT*] [root_id *ROOT_ID*]
>      Dump BTF entries from a given *BTF_SRC*.
>
>      When **id** is specified, BTF object with that ID will be loaded and all
> @@ -67,6 +67,9 @@ bpftool btf dump *BTF_SRC* [format *FORMAT*]
>      formatting, the output is sorted by default. Use the **unsorted** option
>      to avoid sorting the output.
>
> +    **root_id** option can be used to filter a dump to a single type and all
> +    its dependent types. It cannot be used with any other types of filtering.
> +
>  bpftool btf help
>      Print short help message.
>
> diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
> index 3e995faf9efa..18b037a1414b 100644
> --- a/tools/bpf/bpftool/btf.c
> +++ b/tools/bpf/bpftool/btf.c
> @@ -993,6 +993,25 @@ static int do_dump(int argc, char **argv)
>                                 goto done;
>                         }
>                         NEXT_ARG();
> +               } else if (is_prefix(*argv, "root_id")) {
> +                       __u32 root_id;
> +                       char *end;
> +
> +                       if (root_type_cnt) {
> +                               p_err("cannot use root_id with other type filtering");

this is a confusing error if the user just wanted to provide two
root_id arguments... Also, why don't we allow multiple root_ids?

I'd bump root_type_ids[] to have something like 16 elements or
something (though we can always do dynamic realloc as well, probably),
and allow multiple types to be specified.

Thoughts?


> +                               err = -EINVAL;
> +                               goto done;
> +                       }
> +
> +                       NEXT_ARG();
> +                       root_id = strtoul(*argv, &end, 0);
> +                       if (*end) {
> +                               err = -1;
> +                               p_err("can't parse %s as root ID", *argv);
> +                               goto done;
> +                       }
> +                       root_type_ids[root_type_cnt++] = root_id;
> +                       NEXT_ARG();
>                 } else if (is_prefix(*argv, "unsorted")) {
>                         sort_dump_c = false;
>                         NEXT_ARG();
> @@ -1403,7 +1422,7 @@ static int do_help(int argc, char **argv)
>
>         fprintf(stderr,
>                 "Usage: %1$s %2$s { show | list } [id BTF_ID]\n"
> -               "       %1$s %2$s dump BTF_SRC [format FORMAT]\n"
> +               "       %1$s %2$s dump BTF_SRC [format FORMAT] [root_id ROOT_ID]\n"
>                 "       %1$s %2$s help\n"
>                 "\n"
>                 "       BTF_SRC := { id BTF_ID | prog PROG | map MAP [{key | value | kv | all}] | file FILE }\n"
> --
> 2.46.0
>