bpf: add build script for bpf programs

Add a build script to compile bpf source code. A program in restricted
C is compiled into an object file. Object file is converted to BPF
skeleton [0] header file.
If build with custom meson build rule, the target header will reside in
build/ directory (not in source tree), e.g the path for socket_bind:
`build/src/core/bpf/socket_bind/socket-bind.skel.h`

Script runs the phases:
* clang to generate *.o from restricted C
* llvm-strip to remove useless DWARF info
* bpf skeleton generation with bpftool
These phases are logged to stderr for debug purposes.

To include BTF debug information, -g option is passed to clang.

[0] https://lwn.net/Articles/806911/

tools/build-bpf-skel.py

@@ -0,0 +1,123 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: LGPL-2.1+
+
+import argparse
+import logging
+import pathlib
+import subprocess
+import sys
+
+def clang_arch_flag(arch):
+    return '-D__{}__'.format(arch)
+
+
+def target_triplet():
+    gcc_exec = 'gcc'
+
+    try:
+        return subprocess.check_output([gcc_exec, '-dumpmachine'],
+                universal_newlines=True).strip()
+    except subprocess.CalledProcessError as e:
+        logging.error('Failed to get target triplet: {}'.format(e))
+    except FileNotFoundError:
+        logging.error('gcc not installed')
+    return None
+
+
+def clang_compile(clang_exec, clang_flags, src_c, out_file, target_arch,
+        target_triplet):
+    clang_args = [clang_exec, *clang_flags, target_arch, '-I.']
+
+    if target_triplet:
+        clang_args += [
+            '-isystem',
+            '/usr/include/{}'.format(target_triplet)]
+
+    clang_args += [
+        '-idirafter',
+        '/usr/local/include',
+        '-idirafter',
+        '/usr/include']
+
+    clang_args += [src_c, '-o', out_file]
+
+    logging.debug('{}'.format(' '.join(clang_args)))
+    subprocess.check_call(clang_args)
+
+
+def llvm_strip(llvm_strip_exec, in_file):
+    llvm_strip_args = [llvm_strip_exec, '-g', in_file]
+
+    logging.debug('Stripping useless DWARF info:')
+    logging.debug('{}'.format(' '.join(llvm_strip_args)))
+
+    subprocess.check_call(llvm_strip_args)
+
+
+def gen_bpf_skeleton(bpftool_exec, in_file, out_fd):
+    bpftool_args = [bpftool_exec, 'g', 's', in_file]
+
+    logging.debug('Generating BPF skeleton:')
+    logging.debug('{}'.format(' '.join(bpftool_args)))
+    subprocess.check_call(bpftool_args, stdout=out_fd)
+
+
+def bpf_build(args):
+    clang_flags = [
+            '-Wno-compare-distinct-pointer-types',
+            '-O2',
+            '-target',
+            'bpf',
+            '-g',
+            '-c',
+    ]
+
+    clang_out_path = pathlib.Path(args.bpf_src_c).with_suffix('.o')
+    with clang_out_path.open(mode='w') as clang_out, \
+            open(args.bpf_skel_h, mode='w') as bpf_skel_h:
+        clang_compile(clang_exec=args.clang_exec,
+                clang_flags=clang_flags,
+                src_c=args.bpf_src_c,
+                out_file=clang_out.name,
+                target_arch=clang_arch_flag(args.arch),
+                target_triplet=target_triplet())
+
+        llvm_strip(llvm_strip_exec=args.llvm_strip_exec, in_file=clang_out.name)
+
+        gen_bpf_skeleton(bpftool_exec=args.bpftool_exec,
+                in_file=clang_out.name,
+                out_fd=bpf_skel_h)
+
+if __name__ == '__main__':
+    parser = argparse.ArgumentParser()
+
+    parser.add_argument(
+            'bpf_src_c',
+            help='Path to *.c source of BPF program in systemd source tree \
+                    relative to the work directory')
+
+    parser.add_argument(
+            'bpf_skel_h',
+            help='Path to C header file')
+
+    parser.add_argument(
+            '--clang_exec',
+            help='Path to clang exec')
+
+    parser.add_argument(
+            '--llvm_strip_exec',
+            help='Path to llvm-strip exec')
+
+    parser.add_argument(
+            '--bpftool_exec',
+            help='Path to bpftool exec')
+
+    parser.add_argument(
+            '--arch',
+            help='Target CPU architecture',
+            default='x86_64')
+
+    args = parser.parse_args();
+
+    logging.basicConfig(stream=sys.stderr, level=logging.WARNING)
+    bpf_build(args)