netdev - Re: [PATCH v1 net] netlink: Use copy_to_user() for optval in netlink

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Date:   Tue, 18 Apr 2023 20:33:18 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Kuniyuki Iwashima <kuniyu@...zon.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Patrick McHardy <kaber@...sh.net>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Christophe Ricard <christophe-h.ricard@...com>,
        Johannes Berg <johannes.berg@...el.com>,
        David Ahern <dsahern@...il.com>,
        Kuniyuki Iwashima <kuni1840@...il.com>,
        <netdev@...r.kernel.org>, Brad Spencer <bspencer@...ckberry.com>
Subject: Re: [PATCH v1 net] netlink: Use copy_to_user() for optval in
 netlink_getsockopt().

On Tue, 18 Apr 2023 17:42:46 -0700 Kuniyuki Iwashima wrote:
> Brad Spencer provided a detailed report that when calling getsockopt()
> for AF_NETLINK, some SOL_NETLINK options set only 1 byte even though such
> options require more than int as length.
> 
> The options return a flag value that fits into 1 byte, but such behaviour
> confuses users who do not strictly check the value as char.
> 
> Currently, netlink_getsockopt() uses put_user() to copy data to optlen and
> optval, but put_user() casts the data based on the pointer, char *optval.
> So, only 1 byte is set to optval.
> 
> To avoid this behaviour, we need to use copy_to_user() or cast optval for
> put_user().
> 
> Now getsockopt() accepts char as optval as the flags are only 1 byte.

I think it's worth doing, but it will change the return value on big
endian, right?