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Date:   Sat, 16 May 2020 08:36:52 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     David Laight <David.Laight@aculab.com>
Cc:     'David Howells' <dhowells@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
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Subject: Re: [Ocfs2-devel] [PATCH 27/33] sctp: export sctp_setsockopt_bindx
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On Sat, May 16, 2020 at 03:11:40PM +0000, David Laight wrote:
> From: David Howells
> > Sent: 15 May 2020 16:20
> > Christoph Hellwig <hch@lst.de> wrote:
> > 
> > > > The advantage on using kernel_setsockopt here is that sctp module will
> > > > only be loaded if dlm actually creates a SCTP socket.  With this
> > > > change, sctp will be loaded on setups that may not be actually using
> > > > it. It's a quite big module and might expose the system.
> > >
> > > True.  Not that the intent is to kill kernel space callers of setsockopt,
> > > as I plan to remove the set_fs address space override used for it.
> > 
> > For getsockopt, does it make sense to have the core kernel load optval/optlen
> > into a buffer before calling the protocol driver?  Then the driver need not
> > see the userspace pointer at all.
> > 
> > Similar could be done for setsockopt - allocate a buffer of the size requested
> > by the user inside the kernel and pass it into the driver, then copy the data
> > back afterwards.
> 
> Yes, it also simplifies all the compat code.
> And there is a BPF test in setsockopt that also wants to
> pass on a kernel buffer.
> 
> I'm willing to sit and write the patch.
> Quoting from a post I made later on Friday.
> 
> Basically:
> 
> This patch sequence (to be written) does the following:
> 
> Patch 1: Change __sys_setsockopt() to allocate a kernel buffer,
>          copy the data into it then call set_fs(KERNEL_DS).
>          An on-stack buffer (say 64 bytes) will be used for
>          small transfers.
> 
> Patch 2: The same for __sys_getsockopt().
> 
> Patch 3: Compat setsockopt.
> 
> Patch 4: Compat getsockopt.
> 
> Patch 5: Remove the user copies from the global socket options code.
> 
> Patches 6 to n-1; Remove the user copies from the per-protocol code.
> 
> Patch n: Remove the set_fs(KERNEL_DS) from the entry points.
> 
> This should be bisectable.

I appreciate your dedication to not publishing the source code to
your kernel module, but Christoph's patch series is actually better.
It's typesafe rather than passing void pointers around.