Re: [PATCH bpf-next v2] bpftool: Add support for custom BTF path in prog load/loadall

["Jiayuan Chen" <jiayuan.chen@linux.dev>]

May 15, 2025 at 17:17, "Quentin Monnet" <qmo@kernel.org> wrote:

> 
> 2025-05-15 14:50 UTC+0800 ~ Jiayuan Chen <jiayuan.chen@linux.dev>
> 
> > 
> > This patch exposes the btf_custom_path feature to bpftool, allowing users
> > 
> >  to specify a custom BTF file when loading BPF programs using prog load or
> > 
> >  prog loadall commands.
> > 
> >  
> > 
> >  The argument 'btf_custom_path' in libbpf is used for those kernes that
> > 
> 
> Typo: "kernes"
> 
> > 
> > don't have CONFIG_DEBUG_INFO_BTF enabled but still want to perform CO-RE
> > 
> >  relocations.
> > 
> >  
> > 
> >  Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> > 
> >  ---
> > 
> >  tools/bpf/bpftool/Documentation/bpftool-prog.rst | 7 ++++++-
> > 
> >  tools/bpf/bpftool/bash-completion/bpftool | 2 +-
> > 
> >  tools/bpf/bpftool/prog.c | 12 +++++++++++-
> > 
> >  3 files changed, 18 insertions(+), 3 deletions(-)
> > 
> >  
> > 
> >  diff --git a/tools/bpf/bpftool/Documentation/bpftool-prog.rst b/tools/bpf/bpftool/Documentation/bpftool-prog.rst
> > 
> >  index d6304e01afe0..e60a829ab8d0 100644
> > 
> >  --- a/tools/bpf/bpftool/Documentation/bpftool-prog.rst
> > 
> >  +++ b/tools/bpf/bpftool/Documentation/bpftool-prog.rst
> > 
> >  @@ -127,7 +127,7 @@ bpftool prog pin *PROG* *FILE*
> > 
> >  Note: *FILE* must be located in *bpffs* mount. It must not contain a dot
> > 
> >  character ('.'), which is reserved for future extensions of *bpffs*.
> > 
> >  
> > 
> >  -bpftool prog { load | loadall } *OBJ* *PATH* [type *TYPE*] [map { idx *IDX* | name *NAME* } *MAP*] [{ offload_dev | xdpmeta_dev } *NAME*] [pinmaps *MAP_DIR*] [autoattach]
> > 
> >  +bpftool prog { load | loadall } *OBJ* *PATH* [type *TYPE*] [map { idx *IDX* | name *NAME* } *MAP*] [{ offload_dev | xdpmeta_dev } *NAME*] [pinmaps *MAP_DIR*] [autoattach] [kernel_btf *BTF_DIR*]
> > 
> >  Load bpf program(s) from binary *OBJ* and pin as *PATH*. **bpftool prog
> > 
> >  load** pins only the first program from the *OBJ* as *PATH*. **bpftool prog
> > 
> >  loadall** pins all programs from the *OBJ* under *PATH* directory. **type**
> > 
> >  @@ -153,6 +153,11 @@ bpftool prog { load | loadall } *OBJ* *PATH* [type *TYPE*] [map { idx *IDX* | na
> > 
> >  program does not support autoattach, bpftool falls back to regular pinning
> > 
> >  for that program instead.
> > 
> >  
> > 
> >  + The **kernel_btf** option allows specifying an external BTF file to replace
> > 
> >  + the system's own vmlinux BTF file for CO-RE relocations. NOTE that any
> > 
> >  + other feature (e.g., fentry/fexit programs, struct_ops, etc) will require
> > 
> 
> Nit: No need for both "e.g." and "etc", they're redundant.
> 
> > 
> > + actual kernel BTF like /sys/kernel/btf/vmlinux.
> > 
> >  +
> > 
> 
> Can we rephrase the second part of the paragraph a little bit please?
> 
> “Any other feature” could be clearer, how about:
> 
>  Note that any other feature relying on BTF (such as fentry/fexit
> 
>  programs, struct_ops) requires the BTF file for the actual
> 
>  kernel running on the host, often exposed at
> 
>  /sys/kernel/btf/vmlinux.
> 
> > 
> > Note: *PATH* must be located in *bpffs* mount. It must not contain a dot
> > 
> >  character ('.'), which is reserved for future extensions of *bpffs*.
> > 
> >  
> > 
> >  diff --git a/tools/bpf/bpftool/bash-completion/bpftool b/tools/bpf/bpftool/bash-completion/bpftool
> > 
> >  index 1ce409a6cbd9..609938c287b7 100644
> > 
> >  --- a/tools/bpf/bpftool/bash-completion/bpftool
> > 
> >  +++ b/tools/bpf/bpftool/bash-completion/bpftool
> > 
> >  @@ -511,7 +511,7 @@ _bpftool()
> > 
> >  ;;
> > 
> >  *)
> > 
> >  COMPREPLY=( $( compgen -W "map" -- "$cur" ) )
> > 
> >  - _bpftool_once_attr 'type pinmaps autoattach'
> > 
> >  + _bpftool_once_attr 'type pinmaps autoattach kernel_btf'
> > 
> >  _bpftool_one_of_list 'offload_dev xdpmeta_dev'
> > 
> >  return 0
> > 
> >  ;;
> > 
> 
> Correct, but right before this could you also add the following, please:
> 
>  @@ -505,13 +505,13 @@ _bpftool()
> 
>  _bpftool_get_map_names
> 
>  return 0
> 
>  ;;
> 
>  - pinned|pinmaps)
> 
>  + pinned|pinmaps|kernel_btf)
> 
>  _filedir
> 
>  return 0
> 
>  ;;
> 
>  *)
> 
> This will make the completion offer file names after the user has typed
> 
> "kernel_btf".
> 
> > 
> > diff --git a/tools/bpf/bpftool/prog.c b/tools/bpf/bpftool/prog.c
> > 
> >  index f010295350be..3b6a361dd0f8 100644
> > 
> >  --- a/tools/bpf/bpftool/prog.c
> > 
> >  +++ b/tools/bpf/bpftool/prog.c
> > 
> >  @@ -1681,8 +1681,17 @@ static int load_with_options(int argc, char **argv, bool first_prog_only)
> > 
> >  } else if (is_prefix(*argv, "autoattach")) {
> > 
> >  auto_attach = true;
> > 
> >  NEXT_ARG();
> > 
> >  + } else if (is_prefix(*argv, "kernel_btf")) {
> > 
> >  + NEXT_ARG();
> > 
> >  +
> > 
> >  + if (!REQ_ARGS(1))
> > 
> >  + goto err_free_reuse_maps;
> > 
> >  +
> > 
> >  + open_opts.btf_custom_path = GET_ARG();
> > 
> >  } else {
> > 
> >  - p_err("expected no more arguments, 'type', 'map' or 'dev', got: '%s'?",
> > 
> >  + p_err("expected no more arguments, "
> > 
> >  + "'type', 'map', 'dev', 'offload_dev', 'xdpmeta_dev', 'pinmaps', "
> > 
> >  + "'autoattach', or 'kernel_btf', got: '%s'?",
> > 
> 
> Some of them were missing, thanks for this! Can you remove "dev" from
> 
> the list, please? It's been deprecated in favour of "offload_dev", to
> 
> avoid confusion with "xdpmeta_dev".
> 
> pw-bot: cr
> 
> > 
> > *argv);
> > 
> >  goto err_free_reuse_maps;
> > 
> >  }
> > 
> >  @@ -2474,6 +2483,7 @@ static int do_help(int argc, char **argv)
> > 
> >  " [map { idx IDX | name NAME } MAP]\\\n"
> > 
> >  " [pinmaps MAP_DIR]\n"
> > 
> >  " [autoattach]\n"
> > 
> >  + " [kernel_btf BTF_DIR]\n"
> > 
> >  " %1$s %2$s attach PROG ATTACH_TYPE [MAP]\n"
> > 
> >  " %1$s %2$s detach PROG ATTACH_TYPE [MAP]\n"
> > 
> >  " %1$s %2$s run PROG \\\n"
> > 
> 
> Thanks,
> 
> Quentin
>

Thank you, Quentin. Your suggestions are all very valuable; I will make the updates.