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Message-ID: <87r0v3jcsf.fsf@cloudflare.com>
Date: Mon, 06 Feb 2023 11:41:56 +0100
From: Jakub Sitnicki <jakub@...udflare.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>, kernel-team@...udflare.com
Subject: Re: [PATCH net] udp: Pass 2 bytes of data with UDP_GRO cmsg to
user-space
On Tue, Jan 31, 2023 at 06:57 PM +01, Eric Dumazet wrote:
> On Tue, Jan 31, 2023 at 6:46 PM Jakub Sitnicki <jakub@...udflare.com> wrote:
>>
>> While UDP_GRO cmsg interface lacks documentation, the selftests added in
>> commit 3327a9c46352 ("selftests: add functionals test for UDP GRO") suggest
>> that the user-space should allocate CMSG_SPACE for an u16 value and
>> interpret the returned bytes as such:
>>
>> static int recv_msg(int fd, char *buf, int len, int *gso_size)
>> {
>> char control[CMSG_SPACE(sizeof(uint16_t))] = {0};
>> ...
>> if (cmsg->cmsg_level == SOL_UDP
>> && cmsg->cmsg_type == UDP_GRO) {
>> gsosizeptr = (uint16_t *) CMSG_DATA(cmsg);
>> *gso_size = *gsosizeptr;
>> break;
>> }
>> ...
>> }
>>
>> Today user-space will receive 4 bytes of data with an UDP_GRO cmsg, because
>> the kernel packs an int into the cmsg data, as we can confirm with strace:
>>
>> recvmsg(8, {msg_name=...,
>> msg_iov=[{iov_base="\0\0..."..., iov_len=96000}],
>> msg_iovlen=1,
>> msg_control=[{cmsg_len=20, <-- sizeof(cmsghdr) + 4
>> cmsg_level=SOL_UDP,
>> cmsg_type=0x68}], <-- UDP_GRO
>> msg_controllen=24,
>> msg_flags=0}, 0) = 11200
>>
>> This means that either UDP_GRO selftests are broken on big endian, or this
>> is a programming error. Assume the latter and pass only the needed 2 bytes
>> of data with the cmsg.
>>
>> Fixing it like that has an added advantage that the cmsg becomes compatible
>> with what is expected by UDP_SEGMENT cmsg. It becomes possible to reuse the
>> cmsg when GSO packets are received on one socket and sent out of another.
>>
>> Fixes: bcd1665e3569 ("udp: add support for UDP_GRO cmsg")
>> Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
>> ---
>> include/linux/udp.h | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/include/linux/udp.h b/include/linux/udp.h
>> index a2892e151644..44bb8d699248 100644
>> --- a/include/linux/udp.h
>> +++ b/include/linux/udp.h
>> @@ -125,7 +125,7 @@ static inline bool udp_get_no_check6_rx(struct sock *sk)
>> static inline void udp_cmsg_recv(struct msghdr *msg, struct sock *sk,
>> struct sk_buff *skb)
>> {
>> - int gso_size;
>> + __u16 gso_size;
>>
>> if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4) {
>> gso_size = skb_shinfo(skb)->gso_size;
>> --
>> 2.39.1
>>
>
> This would break some applications.
>
> I think the test can be fixed instead, this seems less risky.
Thanks for guidance. Will fix the test.
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