Re: [PATCH v1 net] netlink: Use copy_to_user() for optval in netlink_getsockopt().

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From:   Jakub Kicinski <kuba@kernel.org>
Date:   Tue, 18 Apr 2023 20:33:18 -0700
> 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?

Ah, I missed that point, yes.  I hope big endian users always
initialise the value to avoid the tricky behaviour.