Re: [PATCH] use valid btf in bpf_program__set_attach_target(prog, 0, ...);

[Yonghong Song <yhs@fb.com>]

On 10/5/20 6:41 AM, Luigi Rizzo wrote:
> bpf_program__set_attach_target() will always fail with fd=0 (attach to a
> kernel symbol) because obj->btf_vmlinux is NULL and there is no way to
> set it.
> 
> Fix this by explicitly calling libbpf_find_kernel_btf() in the function.
> 
> Signed-off-by: Luigi Rizzo <lrizzo@google.com>
> ---
>   tools/lib/bpf/libbpf.c | 9 ++++++---
>   1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index a4f55f8a460d..3a63db86666f 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -10352,10 +10352,13 @@ int bpf_program__set_attach_target(struct bpf_program *prog,
>   	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,
> +	else {
> +		struct btf *btf = libbpf_find_kernel_btf();
> +
> +		btf_id = __find_vmlinux_btf_id(btf, attach_func_name,
>   					       prog->expected_attach_type);
> +		btf__free(btf);
> +	}

Doesn't feel this is right fix. In libbpf, we have API function
    libbpf_find_vmlinux_btf_id
just doing the above.

Also, could you point out why btf_vmlinux is NULL? Is this related
to fd = 0? We probably
should fix the problem there since what if other API functions
like bpf_program__set_attach_target() wants btf_vmlinux. We should
not duplicate this logic to other API functions.

>   
>   	if (btf_id < 0)
>   		return btf_id;
>