Re: [PATCH v2 bpf-next 15/16] bpftool: Use syscall/loader program in "prog load" and "gen skeleton" command.

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Apr 22, 2021 at 5:27 PM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> 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 */

nit: this comment should have stayed at the original place

> +       for (i = 0, len = 0; i < file_sz; i++) {
> +               int w = obj_data[i] ? 4 : 2;
> +

[...]

> +       bpf_object__for_each_map(map, obj) {
> +               const char * ident;
> +
> +               ident = get_map_ident(map);
> +               if (!ident)
> +                       continue;
> +
> +               if (!bpf_map__is_internal(map) ||
> +                   !(bpf_map__def(map)->map_flags & BPF_F_MMAPABLE))
> +                       continue;
> +
> +               printf("\tskel->%1$s =\n"
> +                      "\t\tmmap(NULL, %2$zd, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_FIXED,\n"
> +                      "\t\t\tskel->maps.%1$s.map_fd, 0);\n",
> +                      ident, bpf_map_mmap_sz(map));

use codegen()?

> +       }
> +       codegen("\
> +               \n\
> +                       return 0;                                           \n\
> +               }                                                           \n\
> +                                                                           \n\
> +               static inline struct %1$s *                                 \n\

[...]

>  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)

please use {} for such a long if/else, even if it's, technically, a
single-statement if

> +               codegen("\
> +               \n\
> +               /* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */   \n\
> +               /* THIS FILE IS AUTOGENERATED! */                           \n\
> +               #ifndef %2$s                                                \n\
> +               #define %2$s                                                \n\
> +                                                                           \n\
> +               #include <stdlib.h>                                         \n\
> +               #include <bpf/bpf.h>                                        \n\
> +               #include <bpf/skel_internal.h>                              \n\
> +                                                                           \n\
> +               struct %1$s {                                               \n\
> +                       struct bpf_loader_ctx ctx;                          \n\
> +               ",
> +               obj_name, header_guard
> +               );
> +       else
> +               codegen("\
>                 \n\
>                 /* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */   \n\
>                                                                             \n\

[...]