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bpftool: Gen min_core_btf explanation and examples

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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>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220215225856.671072-7-mauricio@kinvolk.io
This commit is contained in:
Rafael David Tinoco 2022-02-15 17:58:55 -05:00 committed by Andrii Nakryiko
parent dc695516b6
commit 1d1ffbf7f0
1 changed files with 90 additions and 0 deletions
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90
tools/bpf/bpftool/Documentation/bpftool-gen.rst
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@ -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 on external
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 +352,70 @@ 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**
::
[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.
::
LIBBPF_OPTS(bpf_object_open_opts, opts, .btf_custom_path = "5.4.0-smaller.btf");
struct bpf_object *obj;
obj = bpf_object__open_file("one.bpf.o", &opts);
...