netdev - Re: [PATCH bpf-next v2] libbpf: Use dynamically allocated buffer when receiving netlink messages

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Date:   Sun, 13 Feb 2022 16:17:47 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Zhiqian Guan <zhguan@...hat.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2] libbpf: Use dynamically allocated buffer
 when receiving netlink messages

Andrii Nakryiko <andrii.nakryiko@...il.com> writes:

> On Fri, Feb 11, 2022 at 3:49 PM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>>
>> When receiving netlink messages, libbpf was using a statically allocated
>> stack buffer of 4k bytes. This happened to work fine on systems with a 4k
>> page size, but on systems with larger page sizes it can lead to truncated
>> messages. The user-visible impact of this was that libbpf would insist no
>> XDP program was attached to some interfaces because that bit of the netlink
>> message got chopped off.
>>
>> Fix this by switching to a dynamically allocated buffer; we borrow the
>> approach from iproute2 of using recvmsg() with MSG_PEEK|MSG_TRUNC to get
>> the actual size of the pending message before receiving it, adjusting the
>> buffer as necessary. While we're at it, also add retries on interrupted
>> system calls around the recvmsg() call.
>>
>> v2:
>>   - Move peek logic to libbpf_netlink_recv(), don't double free on ENOMEM.
>>
>> Reported-by: Zhiqian Guan <zhguan@...hat.com>
>> Fixes: 8bbb77b7c7a2 ("libbpf: Add various netlink helpers")
>> Acked-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
>> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
>> ---
>
> Applied to bpf-next.

Awesome, thanks!

> One improvement would be to avoid initial malloc of 4096, especially
> if that size is enough for most cases. You could detect this through
> iov.iov_base == buf and not free(iov.iov_base) at the end. Seems
> reliable and simple enough. I'll leave it up to you to follow up, if
> you think it's a good idea.

Hmm, seems distributions tend to default the stack size limit to 8k; so
not sure if blowing half of that on a buffer just to avoid a call to
malloc() in a non-performance-sensitive is ideal to begin with? I think
I'd prefer to just keep the dynamic allocation...

-Toke