Subject: Re: [PATCH bpf-next v2] libbpf: Add support for dynamic program attach target

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Thu, 13 Feb 2020 16:32:01 +0100

Eelco Chaudron <[email-masked]> writes:

> 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)
>
> Signed-off-by: Eelco Chaudron <[email-masked]>

Hmm, one question about the attach_prog_fd usage:

> +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);

This implies that no one would end up using fd 0 as a legitimate prog
fd. This already seems to be the case for the existing code, but is that
really a safe assumption? Couldn't a caller that closes fd 0 (for
instance while forking) end up having it reused? Seems like this could
result in weird hard-to-debug bugs?

-Toke