bpf, libbpf: Use valid btf in bpf_program__set_attach_target

bpf_program__set_attach_target(prog, fd, ...) will always fail when
fd = 0 (attach to a kernel symbol) because obj->btf_vmlinux is NULL
and there is no way to set it (at the moment btf_vmlinux is meant
to be temporary storage for use in bpf_object__load_xattr()).

Fix this by using libbpf_find_vmlinux_btf_id().

At some point we may want to opportunistically cache btf_vmlinux
so it can be reused with multiple programs.

Signed-off-by: Luigi Rizzo <lrizzo@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Petar Penkov <ppenkov@google.com>
Link: https://lore.kernel.org/bpf/20201005224528.389097-1-lrizzo@google.com

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 9f90d1a..faec389 100644 (file)
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -10451,9 +10451,8 @@ int bpf_program__set_attach_target(struct bpf_program *prog,
                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);
+               btf_id = libbpf_find_vmlinux_btf_id(attach_func_name,
+                                                   prog->expected_attach_type);
 
        if (btf_id < 0)
                return btf_id;