linux-kernel - Re: sched: some non-GPL symbols becoming GPL-only with CONFIG_PREEMPT

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Message-ID: <2606453.mvXUDI8C0e@natalenko.name>
Date:   Sun, 28 Nov 2021 10:46:13 +0100
From:   Oleksandr Natalenko <oleksandr@...alenko.name>
To:     linux-kernel@...r.kernel.org, Angelo Haller <lkml@...nni.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Will Deacon <will@...nel.org>, linux-rt-users@...r.kernel.org
Subject: Re: sched: some non-GPL symbols becoming GPL-only with CONFIG_PREEMPT_RT enabled

Hello.

On neděle 28. listopadu 2021 1:07:49 CET Angelo Haller wrote:
> Greetings. I hope I picked the right mailing list, as this issue might
> be one that affects various subsystems and components:
> 
> When compiling kernel 5.15 (and 5.16-rc2) with `CONFIG_PREEMPT_RT`
> enabled, some of the symbols being exported as `EXPORT_SYMBOL` suddenly
> become `EXPORT_SYMBOL_GPL` through transitive effects.
> 
> In particular the symbols `migrate_enable` and `migrate_disable` are
> currently marked as `EXPORT_SYMBOL_GPL` - yet are called from multiple
> functions that are marked as `EXPORT_SYMBOL`.
> 
> Here an (incomplete?) listing of call sites I came across:
> 
> kernel/locking/spinlock_rt.c - rt_spin_unlock()
> kernel/locking/spinlock_rt.c - rt_read_unlock()
> kernel/locking/spinlock_rt.c - rt_write_unlock()
> mm/highmem.c - kunmap_local_indexed()
> 
> The issue I'm facing in particular is kmap_atomic() calling
> `migrate_disable` and therefore suddenly becoming GPL-only. This breaks
> the out-of-tree CDDL licensed module ZFS and has been reported before
> already [0]. The conversation seemingly going nowhere - or patches at
> least not being applied upstream. Downstream issue for reference [1].

What about going the other way around and let ZFS be re-licensed under GPL?

> As the original implementation of `migrate_enable` and `migrate_disable`
> is apparently by Peter Zijlstra [2]. Peter would you be willing to
> re-license both symbols `migrate_enable` and `migrate_disable` as
> `EXPORT_SYMBOL`?
> 
> The bigger issue I'm seeing though is that there is currently no
> automated test to uncover exported symbols changing their license
> depending on build configuration? I am not intimately familiar with the
> API guarantees the kernel gives, but this seems like a violation. There
> might be other symbols with similar licensing problems.
> 
> I can open a bugzilla ticket too - if that is preferred.
> 
> Angelo
> 
> 
> [0]
> https://lore.kernel.org/linux-rt-users/20201208212841.694b3022@orivej.orivej
> .org/T/ [1] https://github.com/openzfs/zfs/issues/11097
> [2]
> https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/diff/p
> atches/0009-sched-Add-migrate_disable.patch?h=v5.9-rc8-rt14-patches&id=9a89b
> fdb3bc77aecdd0ff8cc69b595541c7b50c4


-- 
Oleksandr Natalenko (post-factum)