Re: [PATCH bpf-next] bpftool: add support for split BTF to gen min_core_btf

[Yonghong Song <yonghong.song@linux.dev>]

On 1/19/24 1:18 PM, Bryce Kahle wrote:
> Enables a user to generate minimized kernel module BTF.
>
> If an eBPF program probes a function within a kernel module or uses
> types that come from a kernel module, split BTF is required. The split
> module BTF contains only the BTF types that are unique to the module.
> It will reference the base/vmlinux BTF types and always starts its type
> IDs at X+1 where X is the largest type ID in the base BTF.
>
> Minimization allows a user to ship only the types necessary to do
> relocations for the program(s) in the provided eBPF object file(s). A
> minimized module BTF will still not contain vmlinux BTF types, so you
> should always minimize the vmlinux file first, and then minimize the
> kernel module file.
>
> Example:
>
> bpftool gen min_core_btf vmlinux.btf vm-min.btf prog.bpf.o
> bpftool -B vm-min.btf gen min_core_btf mod.btf mod-min.btf prog.bpf.o
>
> Signed-off-by: Bryce Kahle <bryce.kahle@datadoghq.com>
> ---
>   .../bpf/bpftool/Documentation/bpftool-gen.rst  | 18 +++++++++++++++++-
>   tools/bpf/bpftool/gen.c                        | 17 ++++++++++++-----
>   2 files changed, 29 insertions(+), 6 deletions(-)
>
> diff --git a/tools/bpf/bpftool/Documentation/bpftool-gen.rst
> b/tools/bpf/bpftool/Documentation/bpftool-gen.rst
> index 5006e724d1bc..9c357d339000 100644
> --- a/tools/bpf/bpftool/Documentation/bpftool-gen.rst
> +++ b/tools/bpf/bpftool/Documentation/bpftool-gen.rst
> @@ -16,7 +16,7 @@ SYNOPSIS
>
>    **bpftool** [*OPTIONS*] **gen** *COMMAND*
>
> - *OPTIONS* := { |COMMON_OPTIONS| | { **-L** | **--use-loader** } }
> + *OPTIONS* := { |COMMON_OPTIONS| | { **-B** | **--base-btf** } | {
> **-L** | **--use-loader** } }
>
>    *COMMAND* := { **object** | **skeleton** | **help** }
>
> @@ -202,6 +202,14 @@ OPTIONS
>   =======
>    .. include:: common_options.rst
>
> + -B, --base-btf *FILE*
> +   Pass a base BTF object. Base BTF objects are typically used
> +   with BTF objects for kernel modules. To avoid duplicating
> +   all kernel symbols required by modules, BTF objects for
> +   modules are "split", they are built incrementally on top of
> +   the kernel (vmlinux) BTF object. So the base BTF reference
> +   should usually point to the kernel BTF.
> +
>    -L, --use-loader
>      For skeletons, generate a "light" skeleton (also known as "loader"
>      skeleton). A light skeleton contains a loader eBPF program. It does
> @@ -444,3 +452,11 @@ ones given to min_core_btf.
>     obj = bpf_object__open_file("one.bpf.o", &opts);
>
>     ...
> +
> +Kernel module BTF may also be minimized by using the -B option:
> +
> +**$ bpftool -B 5.4.0-smaller.btf gen min_core_btf 5.4.0-module.btf
> 5.4.0-module-smaller.btf one.bpf.o**
> +
> +A minimized module BTF will still not contain vmlinux BTF types, so you
> +should always minimize the vmlinux file first, and then minimize the
> +kernel module file.
> diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
> index ee3ce2b8000d..634c809a5173 100644
> --- a/tools/bpf/bpftool/gen.c
> +++ b/tools/bpf/bpftool/gen.c
> @@ -1630,6 +1630,7 @@ static int do_help(int argc, char **argv)
>    "       %1$s %2$s help\n"
>    "\n"
>    "       " HELP_SPEC_OPTIONS " |\n"
> + "                    {-B|--base-btf} |\n"
>    "                    {-L|--use-loader} }\n"
>    "",
>    bin_name, "gen");
> @@ -1695,14 +1696,14 @@ btfgen_new_info(const char *targ_btf_path)
>    if (!info)
>    return NULL;
>
> - info->src_btf = btf__parse(targ_btf_path, NULL);
> + info->src_btf = btf__parse_split(targ_btf_path, base_btf);
>    if (!info->src_btf) {
>    err = -errno;

Looks like you have formating issues here. The proper identation is gone.

>    p_err("failed parsing '%s' BTF file: %s", targ_btf_path, strerror(errno));
>    goto err_out;
>    }
>
> - info->marked_btf = btf__parse(targ_btf_path, NULL);
> + info->marked_btf = btf__parse_split(targ_btf_path, base_btf);
>    if (!info->marked_btf) {
>    err = -errno;
>    p_err("failed parsing '%s' BTF file: %s", targ_btf_path, strerror(errno));

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