bpftool: Gen min_core_btf explanation and examples

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

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 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);
+
+  ...