Re: [PATCH bpf-next v4 2/3] libbpf: Add support for dynamic program attach target

On 18 Feb 2020, at 22:24, Andrii Nakryiko wrote:

On Tue, Feb 18, 2020 at 8:34 AM Jakub Sitnicki [off-list ref] 
wrote:

quoted

Hey Eelco,

On Mon, Feb 17, 2020 at 12:43 PM GMT, Eelco Chaudron wrote:

quoted

Currently when you want to attach a trace program to a bpf program
the section name needs to match the tracepoint/function semantics.

However the addition of the bpf_program__set_attach_target() API
allows you to specify the tracepoint/function dynamically.

The call flow would look something like this:

  xdp_fd = bpf_prog_get_fd_by_id(id);
  trace_obj = bpf_object__open_file("func.o", NULL);
  prog = bpf_object__find_program_by_title(trace_obj,
                                           "fentry/myfunc");
  bpf_program__set_expected_attach_type(prog, BPF_TRACE_FENTRY);
  bpf_program__set_attach_target(prog, xdp_fd,
                                 "xdpfilt_blk_all");
  bpf_object__load(trace_obj)

Acked-by: Toke Høiland-Jørgensen <redacted>
Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
---
 tools/lib/bpf/libbpf.c   |   34 ++++++++++++++++++++++++++++++----
 tools/lib/bpf/libbpf.h   |    4 ++++
 tools/lib/bpf/libbpf.map |    2 ++
 3 files changed, 36 insertions(+), 4 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 514b1a524abb..0c25d78fb5d8 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c

[...]

quoted

@@ -8132,6 +8133,31 @@ void bpf_program__bpil_offs_to_addr(struct 

bpf_prog_info_linear *info_linear)
      }
 }

+int bpf_program__set_attach_target(struct bpf_program *prog,
+                                int attach_prog_fd,
+                                const char *attach_func_name)
+{
+     int btf_id;
+
+     if (!prog || attach_prog_fd < 0 || !attach_func_name)
+             return -EINVAL;
+
+     if (attach_prog_fd)
+             btf_id = libbpf_find_prog_btf_id(attach_func_name,
+                                              attach_prog_fd);
+     else
+             btf_id = __find_vmlinux_btf_id(prog->obj->btf_vmlinux,
+                                            attach_func_name,
+                                            
prog->expected_attach_type);
+
+     if (btf_id <= 0)
+             return btf_id;

Looks like we can get 0 as return value on both error and success
(below)?  Is that intentional?

Neither libbpf_find_prog_btf_id nor __find_vmlinux_btf_id are going to
return 0 on failure. But I do agree that if (btf_id < 0) check would
be better here.

Is see in theory btf__find_by_name_kind() could return 0:

	if (kind == BTF_KIND_UNKN || !strcmp(type_name, "void"))
   		return 0;

But for our case, this will not happen and is invalid, so what about 
just to make sure its future proof?:

   if (btf_id <= 0)
         return btf_id ? btf_id : -ENOENT;

With that minor nit:

Acked-by: Andrii Nakryiko <redacted>

quoted

quoted

+
+     prog->attach_btf_id = btf_id;
+     prog->attach_prog_fd = attach_prog_fd;
+     return 0;
+}
+
 int parse_cpu_mask_str(const char *s, bool **mask, int *mask_sz)
 {
      int err = 0, n, len, start, end = -1;

[...]