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Date:   Thu, 21 May 2020 08:01:33 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christoph Hellwig' <>,
        "David S. Miller" <>,
        Jakub Kicinski <>
CC:     Eric Dumazet <>,
        Alexey Kuznetsov <>,
        Hideaki YOSHIFUJI <>,
        "Vlad Yasevich" <>,
        Neil Horman <>,
        "Marcelo Ricardo Leitner" <>,
        Jon Maloy <>,
        Ying Xue <>,
        "" <>,
        "" <>,
        "" <>,
        "" <>,
        "" <>,
        "" <>,
        "" <>,
        "" <>,
        "" <>,
        "" <>,
        "" <>,
        "" <>,
        "" <>,
        "" <>
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

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.


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