netdev - Re: [PATCH net-next v2] netlink: split up copies in the ack construction

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Date:   Wed, 16 Nov 2022 17:05:26 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Kees Cook <keescook@...omium.org>,
        David Ahern <dsahern@...nel.org>, davem@...emloft.net,
        netdev@...r.kernel.org, edumazet@...gle.com, pabeni@...hat.com,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH net-next v2] netlink: split up copies in the ack
 construction

On Wed, 16 Nov 2022 18:55:36 -0600 Gustavo A. R. Silva wrote:
> > @@ -56,7 +55,6 @@ struct nlmsghdr {
> >   	__u16		nlmsg_flags;
> >   	__u32		nlmsg_seq;
> >   	__u32		nlmsg_pid;
> > -	__u8		nlmsg_data[];
> >   };  
> 
> This seems to be a sensible change. In general, it's not a good idea
> to have variable length objects (flex-array members) in structures used
> as headers, and that we know will ultimately be followed by more objects
> when embedded inside other structures.

Meaning we should go back to zero-length arrays instead?
Will this not bring back out-of-bound warnings that Kees 
has been fixing?

Is there something in the standard that makes flexible array
at the end of an embedded struct a problem?
Or it's just unlikely compiler people will budge?

AFAICT this is just one of 3 such structs which iproute2 build hits.