Re: ppc64le STRICT_MODULE_RWX and livepatch apply_relocate_add() crashes

Joe Lawrence
2021-12-14T13:00:19Z

On 12/14/21 7:44 AM, Christophe Leroy wrote:
>
>
> Le 13/12/2021 à 18:26, Joe Lawrence a écrit :
>> On 12/13/21 11:36 AM, Christophe Leroy wrote:
>>>
>>>
>>> Le 13/12/2021 à 15:47, Joe Lawrence a écrit :
>>>> On 12/13/21 2:42 AM, Christophe Leroy wrote:
>>>>>
>>>>> Hello Joe,
>>>>>
>>>>> I'm implementing LIVEPATCH on PPC32 and I wanted to test with
>>>>> STRICT_MODULE_RWX enabled so I took your branch as suggested, but I'm
>>>>> getting the following errors on build. What shall I do ?
>>>>>
>>>>> CALL scripts/checksyscalls.sh
>>>>> CALL scripts/atomic/check-atomics.sh
>>>>> CHK include/generated/compile.h
>>>>> KLP lib/livepatch/test_klp_convert1.ko
>>>>> klp-convert: section .rela.klp.module_relocs.test_klp_convert_mod length
>>>>> beyond nr_entries
>>>>>
>>>>> klp-convert: Unable to load user-provided sympos
>>>>> make[2]: *** [scripts/Makefile.modfinal:79:
>>>>> lib/livepatch/test_klp_convert1.ko] Error 255
>>>>> KLP lib/livepatch/test_klp_convert2.ko
>>>>> klp-convert: section .rela.klp.module_relocs.test_klp_convert_mod length
>>>>> beyond nr_entries
>>>>>
>>>>> klp-convert: Unable to load user-provided sympos
>>>>> make[2]: *** [scripts/Makefile.modfinal:79:
>>>>> lib/livepatch/test_klp_convert2.ko] Error 255
>>>>> KLP lib/livepatch/test_klp_convert_sections.ko
>>>>> klp-convert: section .rela.klp.module_relocs.test_klp_convert_mod length
>>>>> beyond nr_entries
>>>>>
>>>>> klp-convert: Unable to load user-provided sympos
>>>>> make[2]: *** [scripts/Makefile.modfinal:79:
>>>>> lib/livepatch/test_klp_convert_sections.ko] Error 255
>>>>> make[2]: Target '__modfinal' not remade because of errors.
>>>>> make[1]: *** [scripts/Makefile.modpost:145: __modpost] Error 2
>>>>> make: *** [Makefile:1770: modules] Error 2
>>>>>
>>>>
>>>> Hi Christophe,
>>>>
>>>> Interesting failure mode. That's klp-convert complaining that it found
>>>> more relocations in a .klp.module_relocs.<objname> section than
>>>> expected, i.e. nr_entries = sec->size / sizeof(struct klp_module_reloc).
>>>>
>>>> A few possibilities: the ELF sec->size was incorrectly set/read by
>>>> build/libelf (I doubt that). Or maybe the layout/size of struct
>>>> klp_module_reloc is not consistent between kernel and userspace (I'm
>>>> more suspicious of this).
>>>>
>>>> Can you post a copy of the build's symbols.klp and
>>>> lib/livepatch/test_klp_convert1.tmp.ko somewhere? I should be able to
>>>> start debug with those files.
>>>>
>>>
>>> I sent you both files off list.
>>>
>>> It looks like klp-convert doesn't use the correct size. It finds a
>>> struct of size 12 hence 3 entries for a section of size 40.
>>>
>>> On PPC32 the struct has size 8 (void * is 4 and int is 4).
>>>
>>> But I'm cross-building from x86_64 where the struct is 8 + 4 = 12.
>>>
>>> Can it be the reason ?
>>>
>>
>> I'm pretty sure that is it. I haven't had much runtime with klp-convert
>> and cross-building (I've only found one big/little endian bug with
>> x86_64->s390x) and was going to ask you how you were testing :)
>>
>> Do you know if there are other kernel build tools that deal with similar
>> situations? This seems like a tricky job for the userspace build tool
>> to determine non-native target struct layout.
>>
>> In the meantime, hacking in:
>>
>> struct klp_module_reloc {
>> - void *sym;
>> + uint32_t sym;
>> unsigned int sympos;
>> } __packed;
>>
>> gets me generating an output .ko file, but the readelf output doesn't
>> look right.
>>
>> I'll add this to the patchset TODO list, but may not get to it for a
>> while -- is there any chance the above hack works or could you test a
>> local non-cross build?
>>
>
> No I have no way to do a non-cross build. My target is an embedded board
> with slow CPU and little memory.
>
> I tested with your hack, I get:
>
> root@vgoip:~# insmod /lib/modules/test_klp_convert1.ko
> insmod: can't insert '/lib/modules/test_klp_convert1.ko': unknown symbol
> in module, or unknown parameter
> root@vgoip:~# insmod /lib/modules/test_klp_livepatch.ko
> insmod: can't insert '/lib/modules/test_klp_livepatch.ko': unknown
> symbol in module, or unknown parameter
>
>
> I agree with you readelf shows something went wrong with relocations.
>

Thanks for trying that. Can you point me to the cross-compiler suite
that you are using for build and readelf? Kernel .config would be handy
too and I can try to reproduce locally for debugging.

Thanks,

--
Joe