From: Rafael David Tinoco <rafaeldtinoco@xxxxxxxxx>
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@xxxxxxxxxx>
Signed-off-by: Rafael David Tinoco <rafael.tinoco@xxxxxxxxxxx>
Signed-off-by: Lorenzo Fontana <lorenzo.fontana@xxxxxxxxxx>
Signed-off-by: Leonardo Di Donato <leonardo.didonato@xxxxxxxxxx>
---
.../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
+ 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**
+
+::
+
+ [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");
+
+ obj = bpf_object__open_file("one.bpf.o", &opts);
+
+ ...
--
2.25.1
