netdev - Re: [PATCH v2 bpf-next 05/12] libbpf: auto-detect btf_id of raw

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Message-ID: <0dbf83e8-10ec-cc17-c575-949639a7f018@fb.com>
Date:   Sat, 12 Oct 2019 00:40:13 +0000
From:   Alexei Starovoitov <ast@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <ast@...nel.org>
CC:     "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        "x86@...nel.org" <x86@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 05/12] libbpf: auto-detect btf_id of
 raw_tracepoint

On 10/11/19 11:07 AM, Andrii Nakryiko wrote:
> On Wed, Oct 9, 2019 at 9:17 PM Alexei Starovoitov <ast@...nel.org> wrote:
>>
>> For raw tracepoint program types libbpf will try to find
>> btf_id of raw tracepoint in vmlinux's BTF.
>> It's a responsiblity of bpf program author to annotate the program
>> with SEC("raw_tracepoint/name") where "name" is a valid raw tracepoint.
>> If "name" is indeed a valid raw tracepoint then in-kernel BTF
>> will have "btf_trace_##name" typedef that points to function
>> prototype of that raw tracepoint. BTF description captures
>> exact argument the kernel C code is passing into raw tracepoint.
>> The kernel verifier will check the types while loading bpf program.
>>
>> libbpf keeps BTF type id in expected_attach_type, but since
>> kernel ignores this attribute for tracing programs copy it
>> into attach_btf_id attribute before loading.
>>
>> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
>> ---
>>   tools/lib/bpf/bpf.c    |  3 +++
>>   tools/lib/bpf/libbpf.c | 17 +++++++++++++++++
>>   2 files changed, 20 insertions(+)
>>
>> diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
>> index cbb933532981..79046067720f 100644
>> --- a/tools/lib/bpf/bpf.c
>> +++ b/tools/lib/bpf/bpf.c
>> @@ -228,6 +228,9 @@ int bpf_load_program_xattr(const struct bpf_load_program_attr *load_attr,
>>          memset(&attr, 0, sizeof(attr));
>>          attr.prog_type = load_attr->prog_type;
>>          attr.expected_attach_type = load_attr->expected_attach_type;
>> +       if (attr.prog_type == BPF_PROG_TYPE_RAW_TRACEPOINT)
>> +               /* expected_attach_type is ignored for tracing progs */
>> +               attr.attach_btf_id = attr.expected_attach_type;
>>          attr.insn_cnt = (__u32)load_attr->insns_cnt;
>>          attr.insns = ptr_to_u64(load_attr->insns);
>>          attr.license = ptr_to_u64(load_attr->license);
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index a02cdedc4e3f..8bf30a67428c 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
>> @@ -4586,6 +4586,23 @@ int libbpf_prog_type_by_name(const char *name, enum bpf_prog_type *prog_type,
>>                          continue;
>>                  *prog_type = section_names[i].prog_type;
>>                  *expected_attach_type = section_names[i].expected_attach_type;
>> +               if (*prog_type == BPF_PROG_TYPE_RAW_TRACEPOINT) {
>> +                       struct btf *btf = bpf_core_find_kernel_btf();
>> +                       char raw_tp_btf_name[128] = "btf_trace_";
>> +                       char *dst = raw_tp_btf_name + sizeof("btf_trace_") - 1;
>> +                       int ret;
>> +
>> +                       if (IS_ERR(btf))
>> +                               /* lack of kernel BTF is not a failure */
>> +                               return 0;
>> +                       /* prepend "btf_trace_" prefix per kernel convention */
>> +                       strncat(dst, name + section_names[i].len,
>> +                               sizeof(raw_tp_btf_name) - (dst - raw_tp_btf_name));
>> +                       ret = btf__find_by_name(btf, raw_tp_btf_name);
>> +                       if (ret > 0)
>> +                               *expected_attach_type = ret;
> 
> Well, actually, I realized after I gave Acked-by, so not yet :)
> 
> This needs kernel feature probe of whether kernel supports specifying
> attach_btf_id, otherwise on older kernels we'll stop successfully
> loading valid program.

The code above won't find anything on older kernels.
The patch 1 of the series has to be there for proper btf to be
generated by pahole.
Before that happens expected_attach_type will stay zero
and corresponding copy in attach_btf_id will be zero as well.
I see no issues being compatible with older kernels.

> But even if kernel supports attach_btf_id, I think users still need to
> opt in into specifying attach_btf_id by libbpf. Think about existing
> raw_tp programs that are using bpf_probe_read() because they were not
> created with this kernel feature in mind. They will suddenly stop
> working without any of user's fault.

This one is excellent catch.
loop1.c should have caught it, since it has
SEC("raw_tracepoint/kfree_skb")
{
   int nested_loops(volatile struct pt_regs* ctx)
    .. = PT_REGS_RC(ctx);

and verifier would have rejected it.
But the way the test is written it's not using libbpf's autodetect
of program type, so everything is passing.