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Date:   Wed, 13 Feb 2019 18:46:22 +0100
From:   Phil Sutter <phil@nwl.cc>
To:     Eric Dumazet <edumazet@google.com>
Cc:     David Ahern <dsahern@gmail.com>,
        Stephen Hemminger <stephen@networkplumber.org>,
        netdev <netdev@vger.kernel.org>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        Hangbin Liu <liuhangbin@gmail.com>
Subject: Re: [PATCH iproute2] lib/libnetlink: ensure a minimum of 32KB for
 the buffer used in rtnl_recvmsg()
Message-ID: <20190213174622.GD19329@orbyte.nwl.cc>
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        Eric Dumazet <edumazet@google.com>, David Ahern <dsahern@gmail.com>,
        Stephen Hemminger <stephen@networkplumber.org>,
        netdev <netdev@vger.kernel.org>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        Hangbin Liu <liuhangbin@gmail.com>
References: <20190213015841.140383-1-edumazet@google.com>
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Hi Eric,

On Tue, Feb 12, 2019 at 05:58:41PM -0800, Eric Dumazet wrote:
> In the past, we tried to increase the buffer size up to 32 KB in order
> to reduce number of syscalls per dump.
> 
> Commit 2d34851cd341 ("lib/libnetlink: re malloc buff if size is not enough")
> brought the size back to 4KB because the kernel can not know the application
> is ready to receive bigger requests.
> 
> See kernel commits 9063e21fb026 ("netlink: autosize skb lengthes") and
> d35c99ff77ec ("netlink: do not enter direct reclaim from netlink_dump()")
> for more details.

Wouldn't it be better if the kernel recognized MSG_TRUNC and allocated a
buffer large enough to hold the full message in that case? I have no
idea how hard that would be to implement, but calling recvmsg() with
MSG_TRUNC set and not getting the full message length in return is not
quite what one expects after reading recvmsg(2).

Cheers, Phil