bpftool: Add `gen object` command to perform BPF static linking

Add `bpftool gen object <output-file> <input_file>...` command to statically
link multiple BPF ELF object files into a single output BPF ELF object file.

This patch also updates bash completions and man page. Man page gets a short
section on `gen object` command, but also updates the skeleton example to show
off workflow for BPF application with two .bpf.c files, compiled individually
with Clang, then resulting object files are linked together with `gen object`,
and then final object file is used to generate usable BPF skeleton. This
should help new users understand realistic workflow w.r.t. compiling
mutli-file BPF application.

Signed-off-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Reviewed-by: Quentin Monnet <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/bpf/bpftool/Documentation/bpftool-gen.rst

···
 
 	*OPTIONS* := { { **-j** | **--json** } [{ **-p** | **--pretty** }] }
 
-	*COMMAND* := { **skeleton** | **help** }
+	*COMMAND* := { **object** | **skeleton** | **help** }
 
 GEN COMMANDS
 =============
 
+|	**bpftool** **gen object** *OUTPUT_FILE* *INPUT_FILE* [*INPUT_FILE*...]
 |	**bpftool** **gen skeleton** *FILE* [**name** *OBJECT_NAME*]
 |	**bpftool** **gen help**
 
 DESCRIPTION
 ===========
+	**bpftool gen object** *OUTPUT_FILE* *INPUT_FILE* [*INPUT_FILE*...]
+		  Statically link (combine) together one or more *INPUT_FILE*'s
+		  into a single resulting *OUTPUT_FILE*. All the files involved
+		  are BPF ELF object files.
+
+		  The rules of BPF static linking are mostly the same as for
+		  user-space object files, but in addition to combining data
+		  and instruction sections, .BTF and .BTF.ext (if present in
+		  any of the input files) data are combined together. .BTF
+		  data is deduplicated, so all the common types across
+		  *INPUT_FILE*'s will only be represented once in the resulting
+		  BTF information.
+
+		  BPF static linking allows to partition BPF source code into
+		  individually compiled files that are then linked into
+		  a single resulting BPF object file, which can be used to
+		  generated BPF skeleton (with **gen skeleton** command) or
+		  passed directly into **libbpf** (using **bpf_object__open()**
+		  family of APIs).
+
 	**bpftool gen skeleton** *FILE*
 		  Generate BPF skeleton C header file for a given *FILE*.
 
···
 
 EXAMPLES
 ========
-**$ cat example.c**
+**$ cat example1.bpf.c**
 
 ::
 
   #include <stdbool.h>
   #include <linux/ptrace.h>
   #include <linux/bpf.h>
-  #include "bpf_helpers.h"
+  #include <bpf/bpf_helpers.h>
 
   const volatile int param1 = 42;
   bool global_flag = true;
   struct { int x; } data = {};
-
-  struct {
-  	__uint(type, BPF_MAP_TYPE_HASH);
-  	__uint(max_entries, 128);
-  	__type(key, int);
-  	__type(value, long);
-  } my_map SEC(".maps");
 
   SEC("raw_tp/sys_enter")
   int handle_sys_enter(struct pt_regs *ctx)
···
   	return 0;
   }
 
+**$ cat example2.bpf.c**
+
+::
+
+  #include <linux/ptrace.h>
+  #include <linux/bpf.h>
+  #include <bpf/bpf_helpers.h>
+
+  struct {
+  	__uint(type, BPF_MAP_TYPE_HASH);
+  	__uint(max_entries, 128);
+  	__type(key, int);
+  	__type(value, long);
+  } my_map SEC(".maps");
+
   SEC("raw_tp/sys_exit")
   int handle_sys_exit(struct pt_regs *ctx)
   {
···
   }
 
 This is example BPF application with two BPF programs and a mix of BPF maps
-and global variables.
+and global variables. Source code is split across two source code files.
 
-**$ bpftool gen skeleton example.o**
+**$ 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*
+individually and then statically links respective object files into the final
+BPF ELF object file *example.bpf.o*.
+
+**$ bpftool gen skeleton example.bpf.o name example | tee example.skel.h**
 
 ::
 
···
 
   #endif /* __EXAMPLE_SKEL_H__ */
 
-**$ cat example_user.c**
+**$ cat example.c**
 
 ::
 
···
   	return err;
   }
 
-**# ./example_user**
+**# ./example**
 
 ::
 

tools/bpf/bpftool/bash-completion/bpftool

···
             ;;
         gen)
             case $command in
+                object)
+                    _filedir
+                    return 0
+                    ;;
                 skeleton)
                     case $prev in
                         $command)
···
                     ;;
                 *)
                     [[ $prev == $object ]] && \
-                        COMPREPLY=( $( compgen -W 'skeleton help' -- "$cur" ) )
+                        COMPREPLY=( $( compgen -W 'object skeleton help' -- "$cur" ) )
                     ;;
             esac
             ;;

tools/bpf/bpftool/gen.c

···
 	return err;
 }
 
+static int do_object(int argc, char **argv)
+{
+	struct bpf_linker *linker;
+	const char *output_file, *file;
+	int err = 0;
+
+	if (!REQ_ARGS(2)) {
+		usage();
+		return -1;
+	}
+
+	output_file = GET_ARG();
+
+	linker = bpf_linker__new(output_file, NULL);
+	if (!linker) {
+		p_err("failed to create BPF linker instance");
+		return -1;
+	}
+
+	while (argc) {
+		file = GET_ARG();
+
+		err = bpf_linker__add_file(linker, file);
+		if (err) {
+			p_err("failed to link '%s': %s (%d)", file, strerror(err), err);
+			goto out;
+		}
+	}
+
+	err = bpf_linker__finalize(linker);
+	if (err) {
+		p_err("failed to finalize ELF file: %s (%d)", strerror(err), err);
+		goto out;
+	}
+
+	err = 0;
+out:
+	bpf_linker__free(linker);
+	return err;
+}
+
 static int do_help(int argc, char **argv)
 {
 	if (json_output) {
···
 	}
 
 	fprintf(stderr,
-		"Usage: %1$s %2$s skeleton FILE [name OBJECT_NAME]\n"
+		"Usage: %1$s %2$s object OUTPUT_FILE INPUT_FILE [INPUT_FILE...]\n"
+		"       %1$s %2$s skeleton FILE [name OBJECT_NAME]\n"
 		"       %1$s %2$s help\n"
 		"\n"
 		"       " HELP_SPEC_OPTIONS "\n"
···
 }
 
 static const struct cmd cmds[] = {
+	{ "object",	do_object },
 	{ "skeleton",	do_skeleton },
 	{ "help",	do_help },
 	{ 0 }