RE: do a single memdup_user in sctp_setsockopt

[David Laight <David.Laight@ACULAB.COM>]

From: 'Christoph Hellwig'
> Sent: 23 May 2020 08:19
...
> Alternatively I'll also happily only do a partial conversion for what
> I need for the kernel_setsockopt removal and let you and Dave decided
> what you guys prefer for the rest.

I presume the justification for removing kernel_[sg]etsockopt()
is that you want to get rid of set_fs(KERNEL_DS).
(Which I believe is only a flag to access_ok()?)
In any case I'm not against that at all.

To do that you also need to solve the problem of the BPF 'hook'
in setsockopt() that can also need to pass a kernel buffer through.

I don't see a rush to remove kernel_[sg]etsockopt() until you
have a solution for the BPF hook.
Especially since, until set_fs() is removed, any driver can
(and probably will) just implement their own version.

I think you may end up with the protocols being able to export
either [sg]etsockopt() functions or kernel_[sg]setsockopt()
functions and some paths erroring if the required function is absent.

But even that isn't trivial given the broken nature of one
sctp option - where the returned length has to be invalid!

I'm going to post a V3 of my big patch - I spotted an error.
I'll include a different (smaller) patch in 0/1 that generates
exactly the same object code but is easier to review.

	David

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