RE: remove kernel_setsockopt and kernel_getsockopt v2

[David Laight]

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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