On Saturday 21 October 2017 06:29 AM, Balbir Singh wrote:

On Fri, 2017-10-20 at 14:07 +0200, Torsten Duwe wrote:

quoted

On Wed, Oct 18, 2017 at 11:47:35AM +0530, Kamalesh Babulal wrote:

quoted

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

quoted

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.