[PATCH v2] kernel/module_64.c: Add REL24 relocation support of livepatch symbols

[Kamalesh Babulal]

On Saturday 21 October 2017 06:29 AM, Balbir Singh wrote:
> On Fri, 2017-10-20 at 14:07 +0200, Torsten Duwe wrote:
>> On Wed, Oct 18, 2017 at 11:47:35AM +0530, Kamalesh Babulal wrote:
>>>
>>> Consider a trivial patch, supplied to kpatch tool for generating a
>>> livepatch module:
>>>
>>> --- a/fs/proc/meminfo.c
>>> +++ b/fs/proc/meminfo.c
>>> @@ -132,7 +132,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
>>>         seq_printf(m, "VmallocTotal:   %8lu kB\n",
>>>                    (unsigned long)VMALLOC_TOTAL >> 10);
>>>         show_val_kb(m, "VmallocUsed:    ", 0ul);
>>> -       show_val_kb(m, "VmallocChunk:   ", 0ul);
>>> +       show_val_kb(m, "VMALLOCCHUNK:   ", 0ul);
>>>
>>
>> Am I assuming correctly that "kpatch tool" simply recompiles all code the
>> way it would get compiled in a regular kernel build?
>
> kpatch is open source and is available on github. This patch is specific
> to the way kpatch works
>
>> My understanding is
>> that live patching modules need to be carefully prepared, which involves
>> source code reorganisation and recompilation. In that process, you can
>> easily declare show_val_kb() extern, and get the suitable instruction sequence
>> for the call.
>
> Yes, we agree. For the current versions of kpatch, which involve a process of
> applying the patch and building the kernel without and with the patch and doing
> an elf diff (programatically), we cannot deviate from that process as it's
> architecture independent. This patch solves arch specific issues related
> to that process.

Yes, that's the essence of the kpatch tool on building livepatchable
kernel module, by doing an elf diff on the kernel with and without the
patch applied. show_val_kb() is a simple example, consider more complex
patch(s), if they need to be prepared manually as suggested. It beats
the whole purpose of a kpatch tool, which programmatically prepares a
livepatch module with close to no manual preparation required. It's
the architecture limitation, which is addressed in this patch.

This patch is outcome of long discussion at kpatch
https://github.com/dynup/kpatch/pull/650

-- 
cheers,
Kamalesh.