Re: [PATCH] A long explanation for a short patch

[Sasha Levin <sasha.levin@oracle.com>]

On 03/13/2014 10:30 PM, john.hubbard@gmail.com wrote:
> From: John Hubbard <jhubbard@nvidia.com>
>
> Hi Sasha and linux-mm,
>
> Prior to commit 309381feaee564281c3d9e90fbca8963bb7428ad, it was
> possible to build MIT-licensed (non-GPL) drivers on Fedora. Fedora is
> semi-unique, in that it sets CONFIG_VM_DEBUG.
>
> Because Fedora sets CONFIG_VM_DEBUG, they end up pulling in
> dump_page(), via VM_BUG_ON_PAGE, via get_page().  As one of the
> authors of NVIDIA's new, open source, "UVM-Lite" kernel module, I
> originally choose to use the kernel's get_page() routine from within
> nvidia_uvm_page_cache.c, because get_page() has always seemed to be
> very clearly intended for use by non-GPL, driver code.
>
> So I'm hoping that making get_page() widely accessible again will not
> be too controversial. We did check with Fedora first, and they
> responded (https://bugzilla.redhat.com/show_bug.cgi?id=1074710#c3)
> that we should try to get upstream changed, before asking Fedora
> to change.  Their reasoning seems beneficial to Linux: leaving
> CONFIG_DEBUG_VM set allows Fedora to help catch mm bugs.

Thanks for pointing it out. I've definitely overlooked it as a
consequence of the patch. My reasoning behind making it _GPL() was
simply that it's a new export, so it's GPL unless there's a really
good excuse to make it non-GPL.

However, dump_page() as well as the regular VM_BUG_ON() are debug
functions that access functionality which isn't essential for
non-GPL modules.

This isn't the first and only case where enabling debug options will
turn code that was previously usable under a non-GPL license into
GPL specific. For example:

  - CONFIG_LOCKDEP* will turn locks GPL-only.
  - CONFIG_DYNAMIC_DEBUG will turn module loading GPL-only.
  - CONFIG_SUNRPC_DEBUG will turn the net RPC code GPL-only.

To keep it short, my opinion is that since it doesn't break any existing
code it should be kept as _GPL(), same way it was done for various other
subsystems.

Also, I think that enabling CONFIG_DEBUG_VM for end-users is a very risky
thing to do. I agree you'll find more bugs, but you'll also hit one of the many
false-positives hidden there as well. I've reported a few of those but
in some cases it's hard to determine whether it's an actual false-positive
or a bug somewhere else. Since the assumption is that end-users won't
have CONFIG_DEBUG_VM, they don't get all the attention they deserve and
end up slipping into releases: http://www.spinics.net/lists/linux-mm/msg70368.html .

Actually, I can think of a few cases where having CONFIG_DEBUG_VM enabled would
qualify a rather simple code to a CVE status.


Thanks,
Sasha


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