Re: [PATCH bpf-next v7 6/7] bpftool: gen min_core_btf explanation and examples

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Feb 15, 2022 at 2:59 PM Mauricio Vásquez <mauricio@kinvolk.io> wrote:
>
> From: Rafael David Tinoco <rafaeldtinoco@gmail.com>
>
> Add "min_core_btf" feature explanation and one example of how to use it
> to bpftool-gen man page.
>
> Signed-off-by: Mauricio Vásquez <mauricio@kinvolk.io>
> Signed-off-by: Rafael David Tinoco <rafael.tinoco@aquasec.com>
> Signed-off-by: Lorenzo Fontana <lorenzo.fontana@elastic.co>
> Signed-off-by: Leonardo Di Donato <leonardo.didonato@elastic.co>
> ---
>  .../bpf/bpftool/Documentation/bpftool-gen.rst | 91 +++++++++++++++++++
>  1 file changed, 91 insertions(+)
>
> diff --git a/tools/bpf/bpftool/Documentation/bpftool-gen.rst b/tools/bpf/bpftool/Documentation/bpftool-gen.rst
> index bc276388f432..4bf8e6447718 100644
> --- a/tools/bpf/bpftool/Documentation/bpftool-gen.rst
> +++ b/tools/bpf/bpftool/Documentation/bpftool-gen.rst
> @@ -25,6 +25,7 @@ GEN COMMANDS
>
>  |      **bpftool** **gen object** *OUTPUT_FILE* *INPUT_FILE* [*INPUT_FILE*...]
>  |      **bpftool** **gen skeleton** *FILE* [**name** *OBJECT_NAME*]
> +|      **bpftool** **gen min_core_btf** *INPUT* *OUTPUT* *OBJECT* [*OBJECT*...]
>  |      **bpftool** **gen help**
>
>  DESCRIPTION
> @@ -149,6 +150,26 @@ DESCRIPTION
>                   (non-read-only) data from userspace, with same simplicity
>                   as for BPF side.
>
> +       **bpftool** **gen min_core_btf** *INPUT* *OUTPUT* *OBJECT* [*OBJECT*...]
> +                 Generate a minimum BTF file as *OUTPUT*, derived from a given
> +                 *INPUT* BTF file, containing all needed BTF types so one, or
> +                 more, given eBPF objects CO-RE relocations may be satisfied.
> +
> +                 When kernels aren't compiled with CONFIG_DEBUG_INFO_BTF,
> +                 libbpf, when loading an eBPF object, has to rely in external

typo: in -> on

> +                 BTF files to be able to calculate CO-RE relocations.
> +
> +                 Usually, an external BTF file is built from existing kernel
> +                 DWARF data using pahole. It contains all the types used by
> +                 its respective kernel image and, because of that, is big.
> +
> +                 The min_core_btf feature builds smaller BTF files, customized
> +                 to one or multiple eBPF objects, so they can be distributed
> +                 together with an eBPF CO-RE based application, turning the
> +                 application portable to different kernel versions.
> +
> +                 Check examples bellow for more information how to use it.
> +
>         **bpftool gen help**
>                   Print short help message.
>
> @@ -215,7 +236,9 @@ This is example BPF application with two BPF programs and a mix of BPF maps
>  and global variables. Source code is split across two source code files.
>
>  **$ clang -target bpf -g example1.bpf.c -o example1.bpf.o**
> +
>  **$ clang -target bpf -g example2.bpf.c -o example2.bpf.o**
> +
>  **$ bpftool gen object example.bpf.o example1.bpf.o example2.bpf.o**
>
>  This set of commands compiles *example1.bpf.c* and *example2.bpf.c*
> @@ -329,3 +352,71 @@ BPF ELF object file *example.bpf.o*.
>    my_static_var: 7
>
>  This is a stripped-out version of skeleton generated for above example code.
> +
> +min_core_btf
> +------------
> +
> +**$ bpftool btf dump file ./5.4.0-example.btf format raw**

I dropped ./ everywhere, they are not needed and create bad impression
that they do matter

> +
> +::
> +
> +  [1] INT 'long unsigned int' size=8 bits_offset=0 nr_bits=64 encoding=(none)
> +  [2] CONST '(anon)' type_id=1
> +  [3] VOLATILE '(anon)' type_id=1
> +  [4] ARRAY '(anon)' type_id=1 index_type_id=21 nr_elems=2
> +  [5] PTR '(anon)' type_id=8
> +  [6] CONST '(anon)' type_id=5
> +  [7] INT 'char' size=1 bits_offset=0 nr_bits=8 encoding=(none)
> +  [8] CONST '(anon)' type_id=7
> +  [9] INT 'unsigned int' size=4 bits_offset=0 nr_bits=32 encoding=(none)
> +  <long output>
> +
> +**$ bpftool btf dump file ./one.bpf.o format raw**
> +
> +::
> +
> +  [1] PTR '(anon)' type_id=2
> +  [2] STRUCT 'trace_event_raw_sys_enter' size=64 vlen=4
> +        'ent' type_id=3 bits_offset=0
> +        'id' type_id=7 bits_offset=64
> +        'args' type_id=9 bits_offset=128
> +        '__data' type_id=12 bits_offset=512
> +  [3] STRUCT 'trace_entry' size=8 vlen=4
> +        'type' type_id=4 bits_offset=0
> +        'flags' type_id=5 bits_offset=16
> +        'preempt_count' type_id=5 bits_offset=24
> +  <long output>
> +
> +**$ bpftool gen min_core_btf ./5.4.0-example.btf ./5.4.0-smaller.btf ./one.bpf.o**
> +
> +**$ bpftool btf dump file ./5.4.0-smaller.btf format raw**
> +
> +::
> +
> +  [1] TYPEDEF 'pid_t' type_id=6
> +  [2] STRUCT 'trace_event_raw_sys_enter' size=64 vlen=1
> +        'args' type_id=4 bits_offset=128
> +  [3] STRUCT 'task_struct' size=9216 vlen=2
> +        'pid' type_id=1 bits_offset=17920
> +        'real_parent' type_id=7 bits_offset=18048
> +  [4] ARRAY '(anon)' type_id=5 index_type_id=8 nr_elems=6
> +  [5] INT 'long unsigned int' size=8 bits_offset=0 nr_bits=64 encoding=(none)
> +  [6] TYPEDEF '__kernel_pid_t' type_id=8
> +  [7] PTR '(anon)' type_id=3
> +  [8] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
> +  <end>
> +
> +Now, the "5.4.0-smaller.btf" file may be used by libbpf as an external BTF file
> +when loading the "one.bpf.o" object into the "5.4.0-example" kernel. Note that
> +the generated BTF file won't allow other eBPF objects to be loaded, just the
> +ones given to min_core_btf.
> +
> +::
> +
> +  struct bpf_object *obj = NULL;
> +
> +  DECLARE_LIBBPF_OPTS(bpf_object_open_opts, opts, .btf_custom_path = "5.4.0-smaller.btf");

nit: still prefer LIBBPF_OPTS here, adjusted this piece of code a bit



> +
> +  obj = bpf_object__open_file("one.bpf.o", &opts);
> +
> +  ...
> --
> 2.25.1
>