From: "Eelco Chaudron" <echaudro@redhat.com>
To: "Toke Høiland-Jørgensen" <toke@redhat.com>
Cc: bpf@vger.kernel.org, davem@davemloft.net, netdev@vger.kernel.org,
ast@kernel.org, daniel@iogearbox.net, kafai@fb.com,
songliubraving@fb.com, yhs@fb.com, andriin@fb.com
Subject: Re: [PATCH bpf-next v2] libbpf: Add support for dynamic program attach target
Date: Thu, 13 Feb 2020 18:00:04 +0100 [thread overview]
Message-ID: <47AD4CC2-4D14-419C-87FC-A86F5B7E0974@redhat.com> (raw)
In-Reply-To: <87h7zuh5am.fsf@toke.dk>
On 13 Feb 2020, at 16:32, Toke Høiland-Jørgensen wrote:
> Eelco Chaudron <echaudro@redhat.com> 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 <echaudro@redhat.com>
>
> 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?
Yes, in theory, this can happen but it has nothing to do with this
specific patch. The existing code already assumes that attach_prog_fd ==
0 means attach to a kernel function :(
next prev parent reply other threads:[~2020-02-13 17:00 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-13 15:04 [PATCH bpf-next v2] libbpf: Add support for dynamic program attach target Eelco Chaudron
2020-02-13 15:32 ` Toke Høiland-Jørgensen
2020-02-13 17:00 ` Eelco Chaudron [this message]
2020-02-13 17:13 ` Toke Høiland-Jørgensen
2020-02-13 17:13 ` Toke Høiland-Jørgensen
2020-02-13 17:42 ` Andrii Nakryiko
2020-02-14 7:34 ` Eelco Chaudron
2020-02-14 17:53 ` Andrii Nakryiko
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