From: Alexei Starovoitov <ast@kernel.org>

Add -L flag to bpftool to use libbpf gen_trace facility and syscall/loader program
for skeleton generation and program loading.

"bpftool gen skeleton -L" command will generate a "light skeleton" or "loader skeleton"
that is similar to existing skeleton, but has one major difference:
$ bpftool gen skeleton lsm.o > lsm.skel.h
$ bpftool gen skeleton -L lsm.o > lsm.lskel.h
$ diff lsm.skel.h lsm.lskel.h

@@ -5,34 +4,34 @@
 #define __LSM_SKEL_H__

 #include <stdlib.h>
-#include <bpf/libbpf.h>
+#include <bpf/bpf.h>

The light skeleton does not use majority of libbpf infrastructure.
It doesn't need libelf. It doesn't parse .o file.
It only needs few sys_bpf wrappers. All of them are in bpf/bpf.h file.
In future libbpf/bpf.c can be inlined into bpf.h, so not even libbpf.a would be
needed to work with light skeleton.

"bpftool prog load -L file.o" command is introduced for debugging of syscall/loader
program generation. Just like the same command without -L it will try to load
the programs from file.o into the kernel. It won't even try to pin them.

"bpftool prog load -L -d file.o" command will provide additional debug messages
on how syscall/loader program was generated.
Also the execution of syscall/loader program will use bpf_trace_printk() for
each step of loading BTF, creating maps, and loading programs.
The user can do "cat /.../trace_pipe" for further debug.

An example of fexit_sleep.lskel.h generated from progs/fexit_sleep.c:
struct fexit_sleep {
        struct bpf_loader_ctx ctx;
        struct {
                struct bpf_map_desc bss;
        } maps;
        struct {
                struct bpf_prog_desc nanosleep_fentry;
                struct bpf_prog_desc nanosleep_fexit;
        } progs;
        struct {
                int nanosleep_fentry_fd;
                int nanosleep_fexit_fd;
        } links;
        struct fexit_sleep__bss {
                int pid;
                int fentry_cnt;
                int fexit_cnt;
        } *bss;
};

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---
 tools/bpf/bpftool/Makefile        |   2 +-
 tools/bpf/bpftool/gen.c           | 313 +++++++++++++++++++++++++++---
 tools/bpf/bpftool/main.c          |   7 +-
 tools/bpf/bpftool/main.h          |   1 +
 tools/bpf/bpftool/prog.c          |  80 ++++++++
 tools/bpf/bpftool/xlated_dumper.c |   3 +
 6 files changed, 382 insertions(+), 24 deletions(-)

@@ -268,6 +269,254 @@ static void codegen(const char *template, ...)
        free(s);
 }

+static void print_hex(const char *obj_data, int file_sz)
+{
+       int i, len;
+
+       /* embed contents of BPF object file */

 static int do_skeleton(int argc, char **argv)
 {
        char header_guard[MAX_OBJ_NAME_LEN + sizeof("__SKEL_H__")];

@@ -277,7 +526,7 @@ static int do_skeleton(int argc, char **argv)
        struct bpf_object *obj = NULL;
        const char *file, *ident;
        struct bpf_program *prog;
-       int fd, len, err = -1;
+       int fd, err = -1;
        struct bpf_map *map;
        struct btf *btf;
        struct stat st;

@@ -359,7 +608,25 @@ static int do_skeleton(int argc, char **argv)
        }

        get_header_guard(header_guard, obj_name);
-       codegen("\
+       if (use_loader)