powerpc/modules: Fix crashes by adding CONFIG_RELOCATABLE to vermagic

[Ard Biesheuvel]

On 11 April 2018 at 16:49, Michael Ellerman
<patch-notifications at ellerman.id.au> wrote:
> On Tue, 2018-04-10 at 01:22:06 UTC, Michael Ellerman wrote:
>> If you build the kernel with CONFIG_RELOCATABLE=n, then install the
>> modules, rebuild the kernel with CONFIG_RELOCATABLE=y and leave the
>> old modules installed, we crash something like:
>>
>>   Unable to handle kernel paging request for data at address 0xd000000018d66cef
>>   Faulting instruction address: 0xc0000000021ddd08
>>   Oops: Kernel access of bad area, sig: 11 [#1]
>>   Modules linked in: x_tables autofs4
>>   CPU: 2 PID: 1 Comm: systemd Not tainted 4.16.0-rc6-gcc_ubuntu_le-g99fec39 #1
>>   ...
>>   NIP check_version.isra.22+0x118/0x170
>>   Call Trace:
>>     __ksymtab_xt_unregister_table+0x58/0xfffffffffffffcb8 [x_tables] (unreliable)
>>     resolve_symbol+0xb4/0x150
>>     load_module+0x10e8/0x29a0
>>     SyS_finit_module+0x110/0x140
>>     system_call+0x58/0x6c
>>
>> This happens because since commit 71810db27c1c ("modversions: treat
>> symbol CRCs as 32 bit quantities"), a relocatable kernel encodes and
>> handles symbol CRCs differently from a non-relocatable kernel.
>>
>> Although it's possible we could try and detect this situation and
>> handle it, it's much more robust to simply make the state of
>> CONFIG_RELOCATABLE part of the module vermagic.
>>
>> Fixes: 71810db27c1c ("modversions: treat symbol CRCs as 32 bit quantities")
>> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
>
> Applied to powerpc fixes.
>
> https://git.kernel.org/powerpc/c/73aca179d78eaa11604ba0783a6d8b
>
> cheers

Thanks for the cc. I guess this only affects powerpc, given that it is
the only arch that switches between CRC immediate values and CRC
offsets depending on the configuration.