Re: OpenAFS module libafs.ko uses GPL-only symbol '__put_devmap_managed_page'

Re: OpenAFS module libafs.ko uses GPL-only symbol '__put_devmap_managed_page'

[Dan Williams]

[ adding Andrew, Christoph, and linux-mm ]

On Wed, Jun 13, 2018 at 12:33 PM, Joe Gorse <jhgorse@gmail.com> wrote:
> Greetings,
>
> Please CC answers & comments to this email. Thanks! =)
>
> Our build is breaking as of
> commit e7638488434415aa478e78435cac8f0365737638
> Author: Dan Williams <dan.j.williams@intel.com>
> Date: Wed May 16 11:46:08 2018 -0700
>
> mm: introduce MEMORY_DEVICE_FS_DAX and CONFIG_DEV_PAGEMAP_OPS
> ... snip ...
> https://patchwork.kernel.org/patch/10412459/
>
> We do not directly use the GPL-only symbol '__put_devmap_managed_page'. It
> appears to be picked up from static-inlining in put_page(), which we need.
>
> How shall we proceed? Would it be reasonable to request the change of the
> GPL-only exports for this commit?
>
> Cheers,
> Joe Gorse
>
> P.S. The build failure, for the morbidly curious:
>> FATAL: modpost: GPL-incompatible module libafs.ko uses GPL-only symbol
>> '__put_devmap_managed_page'
>> scripts/Makefile.modpost:92: recipe for target '__modpost' failed
>> make[6]: *** [__modpost] Error 1

I think the right answer here is to make __put_devmap_managed_page()
EXPORT_SYMBOL(), since features like devm_memremap_pages() want to
change the behavior of all users of put_page(). It again holds that
devm_memremap_pages() needs to become EXPORT_SYMBOL_GPL() because it,
not put_page(), is the interface that is leaking control of core
kernel state/infrastructure to its users.

Re: OpenAFS module libafs.ko uses GPL-only symbol '__put_devmap_managed_page'

[John Hubbard]

On 06/13/2018 12:51 PM, Dan Williams wrote:
> [ adding Andrew, Christoph, and linux-mm ]
> 
> On Wed, Jun 13, 2018 at 12:33 PM, Joe Gorse <jhgorse@gmail.com> wrote:
>> Greetings,
>>
>> Please CC answers & comments to this email. Thanks! =)
>>
>> Our build is breaking as of
>> commit e7638488434415aa478e78435cac8f0365737638
>> Author: Dan Williams <dan.j.williams@intel.com>
>> Date: Wed May 16 11:46:08 2018 -0700
>>
>> mm: introduce MEMORY_DEVICE_FS_DAX and CONFIG_DEV_PAGEMAP_OPS
>> ... snip ...
>> https://patchwork.kernel.org/patch/10412459/
>>
>> We do not directly use the GPL-only symbol '__put_devmap_managed_page'. It
>> appears to be picked up from static-inlining in put_page(), which we need.
>>
>> How shall we proceed? Would it be reasonable to request the change of the
>> GPL-only exports for this commit?
>>
>> Cheers,
>> Joe Gorse
>>
>> P.S. The build failure, for the morbidly curious:
>>> FATAL: modpost: GPL-incompatible module libafs.ko uses GPL-only symbol
>>> '__put_devmap_managed_page'
>>> scripts/Makefile.modpost:92: recipe for target '__modpost' failed
>>> make[6]: *** [__modpost] Error 1
> 
> I think the right answer here is to make __put_devmap_managed_page()
> EXPORT_SYMBOL(), since features like devm_memremap_pages() want to
> change the behavior of all users of put_page(). It again holds that
> devm_memremap_pages() needs to become EXPORT_SYMBOL_GPL() because it,
> not put_page(), is the interface that is leaking control of core
> kernel state/infrastructure to its users.
> 

Hi Dan and all,

It looks like put_page() also picks up one more GPL symbol: 
devmap_managed_key.

put_page
    put_devmap_managed_page
        devmap_managed_key

    __put_devmap_managed_page


So if the goal is to restore put_page() to be effectively EXPORT_SYMBOL
again, then I think there would also need to be either a non-inlined 
wrapper for devmap_managed_key (awkward for a static key), or else make
it EXPORT_SYMBOL, or maybe something else that's less obvious to me at the
moment.


thanks,
-- 
John Hubbard
NVIDIA

Re: OpenAFS module libafs.ko uses GPL-only symbol '__put_devmap_managed_page'

[Dan Williams]

On Fri, Jun 15, 2018 at 9:43 PM, John Hubbard <jhubbard@nvidia.com> wrote:
> On 06/13/2018 12:51 PM, Dan Williams wrote:
>> [ adding Andrew, Christoph, and linux-mm ]
>>
>> On Wed, Jun 13, 2018 at 12:33 PM, Joe Gorse <jhgorse@gmail.com> wrote:
>>> Greetings,
>>>
>>> Please CC answers & comments to this email. Thanks! =)
>>>
>>> Our build is breaking as of
>>> commit e7638488434415aa478e78435cac8f0365737638
>>> Author: Dan Williams <dan.j.williams@intel.com>
>>> Date: Wed May 16 11:46:08 2018 -0700
>>>
>>> mm: introduce MEMORY_DEVICE_FS_DAX and CONFIG_DEV_PAGEMAP_OPS
>>> ... snip ...
>>> https://patchwork.kernel.org/patch/10412459/
>>>
>>> We do not directly use the GPL-only symbol '__put_devmap_managed_page'. It
>>> appears to be picked up from static-inlining in put_page(), which we need.
>>>
>>> How shall we proceed? Would it be reasonable to request the change of the
>>> GPL-only exports for this commit?
>>>
>>> Cheers,
>>> Joe Gorse
>>>
>>> P.S. The build failure, for the morbidly curious:
>>>> FATAL: modpost: GPL-incompatible module libafs.ko uses GPL-only symbol
>>>> '__put_devmap_managed_page'
>>>> scripts/Makefile.modpost:92: recipe for target '__modpost' failed
>>>> make[6]: *** [__modpost] Error 1
>>
>> I think the right answer here is to make __put_devmap_managed_page()
>> EXPORT_SYMBOL(), since features like devm_memremap_pages() want to
>> change the behavior of all users of put_page(). It again holds that
>> devm_memremap_pages() needs to become EXPORT_SYMBOL_GPL() because it,
>> not put_page(), is the interface that is leaking control of core
>> kernel state/infrastructure to its users.
>>
>
> Hi Dan and all,
>
> It looks like put_page() also picks up one more GPL symbol:
> devmap_managed_key.
>
> put_page
>     put_devmap_managed_page
>         devmap_managed_key
>
>     __put_devmap_managed_page
>
>
> So if the goal is to restore put_page() to be effectively EXPORT_SYMBOL
> again, then I think there would also need to be either a non-inlined
> wrapper for devmap_managed_key (awkward for a static key), or else make
> it EXPORT_SYMBOL, or maybe something else that's less obvious to me at the
> moment.

Right, certainly flipping the key is a kernel internal detail since it
is giving the dev_pagemap owner purview over all kernel page events,
but put_page() users are silent consumers. And you're right there's
currently no good way I see to export the 'producer' and 'consumer'
side of the key with different export types.

Re: OpenAFS module libafs.ko uses GPL-only symbol '__put_devmap_managed_page'

[John Hubbard]

On 06/15/2018 10:22 PM, Dan Williams wrote:
> On Fri, Jun 15, 2018 at 9:43 PM, John Hubbard <jhubbard@nvidia.com> wrote:
>> On 06/13/2018 12:51 PM, Dan Williams wrote:
>>> [ adding Andrew, Christoph, and linux-mm ]
>>>
>>> On Wed, Jun 13, 2018 at 12:33 PM, Joe Gorse <jhgorse@gmail.com> wrote:
[snip]
>>>>
>>>> P.S. The build failure, for the morbidly curious:
>>>>> FATAL: modpost: GPL-incompatible module libafs.ko uses GPL-only symbol
>>>>> '__put_devmap_managed_page'
>>>>> scripts/Makefile.modpost:92: recipe for target '__modpost' failed
>>>>> make[6]: *** [__modpost] Error 1
>>>
>>> I think the right answer here is to make __put_devmap_managed_page()
>>> EXPORT_SYMBOL(), since features like devm_memremap_pages() want to
>>> change the behavior of all users of put_page(). It again holds that
>>> devm_memremap_pages() needs to become EXPORT_SYMBOL_GPL() because it,
>>> not put_page(), is the interface that is leaking control of core
>>> kernel state/infrastructure to its users.
>>>
>>
>> Hi Dan and all,
>>
>> It looks like put_page() also picks up one more GPL symbol:
>> devmap_managed_key.
>>
>> put_page
>>     put_devmap_managed_page
>>         devmap_managed_key
>>
>>     __put_devmap_managed_page
>>
>>
>> So if the goal is to restore put_page() to be effectively EXPORT_SYMBOL
>> again, then I think there would also need to be either a non-inlined
>> wrapper for devmap_managed_key (awkward for a static key), or else make
>> it EXPORT_SYMBOL, or maybe something else that's less obvious to me at the
>> moment.
> 
> Right, certainly flipping the key is a kernel internal detail since it
> is giving the dev_pagemap owner purview over all kernel page events,
> but put_page() users are silent consumers. And you're right there's
> currently no good way I see to export the 'producer' and 'consumer'
> side of the key with different export types.
> 

It's hard to imagine how anyone could end up using devmap_managed_key
in an out-of-tree driver, given that other related symbols are already
locked down with EXPORT_SYMBOL_GPL. So one easy fix might be to just
make it EXPORT_SYMBOL, with a comment that explains the intent, and just
call it a day:

/* This is intended to be EXPORT_SYMBOL_GPL, but actually doing so would
 * cause a problem for put_page().
 */
EXPORT_SYMBOL(devmap_managed_key);

...I'm sort of winging it here... :)