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

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Message-ID: <87y22h6klq.fsf@toke.dk>
Date:   Sat, 12 Feb 2022 00:37:21 +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] libbpf: Use dynamically allocated buffer when
 receiving netlink messages

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

> On Fri, Feb 11, 2022 at 11:51 AM 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.
>>
>> Reported-by: Zhiqian Guan <zhguan@...hat.com>
>> Fixes: 8bbb77b7c7a2 ("libbpf: Add various netlink helpers")
>> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
>> ---
>>  tools/lib/bpf/netlink.c | 55 ++++++++++++++++++++++++++++++++++++++---
>>  1 file changed, 52 insertions(+), 3 deletions(-)
>>
>> diff --git a/tools/lib/bpf/netlink.c b/tools/lib/bpf/netlink.c
>> index c39c37f99d5c..9a6e95206bf0 100644
>> --- a/tools/lib/bpf/netlink.c
>> +++ b/tools/lib/bpf/netlink.c
>> @@ -87,22 +87,70 @@ enum {
>>         NL_DONE,
>>  };
>>
>> +static int __libbpf_netlink_recvmsg(int sock, struct msghdr *mhdr, int flags)
>
> let's not use names starting with underscored. Just call it
> "netlink_recvmsg" or something like that.

Alright, will fix.

>> +{
>> +       int len;
>> +
>> +       do {
>> +               len = recvmsg(sock, mhdr, flags);
>
> recvmsg returns ssize_t, is it ok to truncate to int?

In practice, yeah; the kernel is not going to return a single message
that overflows an int, even on 32bit. And with an int return type it's
more natural to return -errno instead of having the caller deal with
that. So unless you have strong objections I'd prefer to keep it this
way...

>> +       } while (len < 0 && (errno == EINTR || errno == EAGAIN));
>> +
>> +       if (len < 0)
>> +               return -errno;
>> +       return len;
>> +}
>> +
>> +static int libbpf_netlink_recvmsg(int sock, struct msghdr *mhdr, char **buf)
>> +{
>> +       struct iovec *iov = mhdr->msg_iov;
>> +       void *nbuf;
>> +       int len;
>> +
>> +       len = __libbpf_netlink_recvmsg(sock, mhdr, MSG_PEEK | MSG_TRUNC);
>> +       if (len < 0)
>> +               return len;
>> +
>> +       if (len < 4096)
>> +               len = 4096;
>> +
>> +       if (len > iov->iov_len) {
>> +               nbuf = realloc(iov->iov_base, len);
>> +               if (!nbuf) {
>> +                       free(iov->iov_base);
>> +                       return -ENOMEM;
>> +               }
>> +               iov->iov_base = nbuf;
>
> this function both sets iov->iov_base *and* returns buf. It's quite a
> convoluted contract. Seems like buf is not necessary (and also NULL
> out iov->iov_base in case of error above?). But it might be cleaner to
> do this MSG_PEEK  + realloc + recvmsg  in libbpf_netlink_recv()
> explicitly. It's only one place.

Hmm, yeah, if I wrap the realloc code in a small helper that works; will
fix.

-Toke