netdev - Re: [PATCH 19/26] net/ipv6: switch ipv6_flowlabel_opt to sockptr

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Message-ID: <20200727130029.GA26393@lst.de>
Date:   Mon, 27 Jul 2020 15:00:29 +0200
From:   Christoph Hellwig <hch@....de>
To:     Ido Schimmel <idosch@...sch.org>
Cc:     Christoph Hellwig <hch@....de>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Eric Dumazet <edumazet@...gle.com>,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        linux-sctp@...r.kernel.org, linux-hams@...r.kernel.org,
        linux-bluetooth@...r.kernel.org, bridge@...ts.linux-foundation.org,
        linux-can@...r.kernel.org, dccp@...r.kernel.org,
        linux-decnet-user@...ts.sourceforge.net,
        linux-wpan@...r.kernel.org, linux-s390@...r.kernel.org,
        mptcp@...ts.01.org, lvs-devel@...r.kernel.org,
        rds-devel@....oracle.com, linux-afs@...ts.infradead.org,
        tipc-discussion@...ts.sourceforge.net, linux-x25@...r.kernel.org
Subject: Re: [PATCH 19/26] net/ipv6: switch ipv6_flowlabel_opt to sockptr_t

On Mon, Jul 27, 2020 at 03:15:05PM +0300, Ido Schimmel wrote:
> I see a regression with IPv6 flowlabel that I bisected to this patch.
> When passing '-F 0' to 'ping' the flow label should be random, yet it's
> the same every time after this patch.

Can you send a reproducer?

> 
> It seems that the pointer is never advanced after the call to
> sockptr_advance() because it is passed by value and not by reference.
> Even if you were to pass it by reference I think you would later need to
> call sockptr_decrease() or something similar. Otherwise it is very
> error-prone.
> 
> Maybe adding an offset to copy_to_sockptr() and copy_from_sockptr() is
> better?

We could do that, although I wouldn't add it to the existing functions
to avoid the churns and instead add copy_to_sockptr_offset or something
like that.