netdev - Re: [PATCH bpf-next 07/11] libbpf: fix BTF data layout checks and allow empty BTF

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Date:   Tue, 3 Nov 2020 05:44:13 +0000
From:   Song Liu <songliubraving@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
CC:     Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next 07/11] libbpf: fix BTF data layout checks and
 allow empty BTF



> On Nov 2, 2020, at 9:18 PM, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
> 
> On Mon, Nov 2, 2020 at 4:51 PM Song Liu <songliubraving@...com> wrote:
>> 
>> 
>> 
>>> On Oct 28, 2020, at 5:58 PM, Andrii Nakryiko <andrii@...nel.org> wrote:
>>> 
>>> Make data section layout checks stricter, disallowing overlap of types and
>>> strings data.
>>> 
>>> Additionally, allow BTFs with no type data. There is nothing inherently wrong
>>> with having BTF with no types (put potentially with some strings). This could
>>> be a situation with kernel module BTFs, if module doesn't introduce any new
>>> type information.
>>> 
>>> Also fix invalid offset alignment check for btf->hdr->type_off.
>>> 
>>> Fixes: 8a138aed4a80 ("bpf: btf: Add BTF support to libbpf")
>>> Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
>>> ---
>>> tools/lib/bpf/btf.c | 16 ++++++----------
>>> 1 file changed, 6 insertions(+), 10 deletions(-)
>>> 
>>> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
>>> index 20c64a8441a8..9b0ef71a03d0 100644
>>> --- a/tools/lib/bpf/btf.c
>>> +++ b/tools/lib/bpf/btf.c
>>> @@ -245,22 +245,18 @@ static int btf_parse_hdr(struct btf *btf)
>>>              return -EINVAL;
>>>      }
>>> 
>>> -     if (meta_left < hdr->type_off) {
>>> -             pr_debug("Invalid BTF type section offset:%u\n", hdr->type_off);
>>> +     if (meta_left < hdr->str_off + hdr->str_len) {
>>> +             pr_debug("Invalid BTF total size:%u\n", btf->raw_size);
>>>              return -EINVAL;
>>>      }
>> 
>> Can we make this one as
>>        if (meta_left != hdr->str_off + hdr->str_len) {
> 
> That would be not forward-compatible. I.e., old libbpf loading new BTF
> with extra stuff after the string section. Kernel is necessarily more
> strict, but I'd like to keep libbpf more permissive with this.

Yeah, this makes sense. Let's keep both checks AS-IS. 

Thanks,
Song