netdev - RE: remove kernel_setsockopt and kernel

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Date:   Thu, 21 May 2020 08:01:33 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christoph Hellwig' <hch@....de>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
CC:     Eric Dumazet <edumazet@...gle.com>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        "Vlad Yasevich" <vyasevich@...il.com>,
        Neil Horman <nhorman@...driver.com>,
        "Marcelo Ricardo Leitner" <marcelo.leitner@...il.com>,
        Jon Maloy <jmaloy@...hat.com>,
        Ying Xue <ying.xue@...driver.com>,
        "drbd-dev@...ts.linbit.com" <drbd-dev@...ts.linbit.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "target-devel@...r.kernel.org" <target-devel@...r.kernel.org>,
        "linux-afs@...ts.infradead.org" <linux-afs@...ts.infradead.org>,
        "linux-cifs@...r.kernel.org" <linux-cifs@...r.kernel.org>,
        "cluster-devel@...hat.com" <cluster-devel@...hat.com>,
        "ocfs2-devel@....oracle.com" <ocfs2-devel@....oracle.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>,
        "ceph-devel@...r.kernel.org" <ceph-devel@...r.kernel.org>,
        "rds-devel@....oracle.com" <rds-devel@....oracle.com>,
        "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>
Subject: RE: remove kernel_setsockopt and kernel_getsockopt v2

From: Christoph Hellwig
> Sent: 20 May 2020 20:55
> 
> this series removes the kernel_setsockopt and kernel_getsockopt
> functions, and instead switches their users to small functions that
> implement setting (or in one case getting) a sockopt directly using
> a normal kernel function call with type safety and all the other
> benefits of not having a function call.
> 
> In some cases these functions seem pretty heavy handed as they do
> a lock_sock even for just setting a single variable, but this mirrors
> the real setsockopt implementation unlike a few drivers that just set
> set the fields directly.

How much does this increase the kernel code by?

You are also replicating a lot of code making it more
difficult to maintain.

I don't think the performance of an socket option code
really matters - it is usually done once when a socket
is initialised and the other costs of establishing a
connection will dominate.

Pulling the user copies outside the [gs]etsocksopt switch
statement not only reduces the code size (source and object)
and trivially allows kernel_[sg]sockopt() to me added to
the list of socket calls.

It probably isn't possible to pull the usercopies right
out into the syscall wrapper because of some broken
requests.

I worried about whether getsockopt() should read the entire
user buffer first. SCTP needs the some of it often (including a
sockaddr_storage in one case), TCP needs it once.
However the cost of reading a few words is small, and a big
buffer probably needs setting to avoid leaking kernel
memory if the structure has holes or fields that don't get set.
Reading from userspace solves both issues.

	David

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