Re: [PATCH] mctp: accept MSG_CMSG_COMPAT in recvmsg

[Simon Horman <horms@kernel.org>]

On Sun, Nov 01, 2026 at 12:03:31PM -0800, Davidlohr Bueso wrote:
> 32-bit userspace running on a 64-bit kernel is unable to receive MCTP
> messages on an AF_MCTP socket, even though sending succeeds. There is
> nothing for MCTP to do with it beyond not rejecting it, as af_packet,
> af_key and can/isotp already do, for example.

Hi Davidlohr,

Thanks for your patch.

As prompted elsewhere by a bot, it would be good if the patch description
included some information about how this was found. And also to what
extent the fix has been tested. I know you answered that already but
I'm going to ask for a v2 for other reasons. And it would be good
to include that detail there.

> 
> Fixes: 833ef3b91de6 ("mctp: Populate socket implementation")
> Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>

This seems to be a fix that effects users. Which I believe you have
reproduced. As such I'd lean towards this being for net, CCed to stable.

Subject: [PATCH net v2] ...

Otherwise, it should be for net-next.

Subject: [PATCH net-next v2] ...

> ---
>  net/mctp/af_mctp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/mctp/af_mctp.c b/net/mctp/af_mctp.c
> index 8af5e2b3c8d1..aa22c0860712 100644
> --- a/net/mctp/af_mctp.c
> +++ b/net/mctp/af_mctp.c
> @@ -292,8 +292,7 @@ static int mctp_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,

Strangely this patch has been mangled - the original code is 7 rather than
8 lines long. (s/,8/,7). Because of this it could not be applied by our
CI which then did not run the usual tests.

I'd like to ask you to fix that and send a v2.

>  	u8 type;
>  	int rc;
>  
> -	if (flags & ~(MSG_DONTWAIT | MSG_TRUNC | MSG_PEEK))
> +	if (flags & ~(MSG_DONTWAIT | MSG_TRUNC | MSG_PEEK | MSG_CMSG_COMPAT))
>  		return -EOPNOTSUPP;
>  
>  	skb = skb_recv_datagram(sk, flags, &rc);

-- 
pw-bot: changes-requested