Re: ROX allocations broke livepatch for modules since 6.13-rc1

[Petr Pavlu <petr.pavlu@suse.com>]

On 12/30/24 01:09, Marek Maślanka wrote:
> Hi Mike and others,
> 
> I discovered that the patch "[v7,4/8] module: prepare to handle ROX allocations
> for text" has disrupted livepatch functionality. Specifically, this occurs when
> livepatch is prepared to patch a kernel module and when the livepatch module
> contains a "special" relocation section named
> ".klp.rela.<MODULE_NAME>.<SECTION_NAME>" to access local symbols.

Thank you for the report. It is possible for me to reproduce the issue
on my system. An annoying part is to create the
.klp.rela.<objname>.<secname> data, for which I eventually used one
floating variant of klp-convert [1]. To hit the problem, <objname> must
point to an object that is different from vmlinux. Such relocations are
processed by the livepatch code later than regular module relocations,
as you pointed out after mod->rw_copy is already reset.

I think the bug should be addressed in principle by Mike's recently
posted rework of the feature [2] but unfortunately, its current version
makes my system also unbootable [3].

My reproducer:

$ cat test_klp_livepatch.c
#include <linux/kernel.h>
#include <linux/livepatch.h>
#include <linux/module.h>
#include <linux/seq_file.h>
extern __attribute__((weak)) void *xfs_btree_new_root asm("\".klp.sym.rela.xfs.xfs.xfs_btree_new_root.0\"");
static int livepatch_cmdline_proc_show(struct seq_file *m, void *v) {
        seq_printf(m, "live patched: xfs_btree_new_root=%p\n", &xfs_btree_new_root); return 0; }
static struct klp_func funcs[] = { { .old_name = "cmdline_proc_show", .new_func = livepatch_cmdline_proc_show }, { } };
static struct klp_func xfs_funcs[] = { { } };
static struct klp_object objs[] = { { .funcs = funcs, }, { .name = "xfs", .funcs = xfs_funcs, }, { } };
static struct klp_patch patch = { .mod = THIS_MODULE, .objs = objs, };
static int test_klp_livepatch_init(void) { return klp_enable_patch(&patch); }
static void test_klp_livepatch_exit(void) { }
module_init(test_klp_livepatch_init);
module_exit(test_klp_livepatch_exit);
MODULE_LICENSE("GPL");
MODULE_INFO(livepatch, "Y");
MODULE_AUTHOR("Petr Pavlu <petr.pavlu@suse.com>");
MODULE_DESCRIPTION("Livepatch test: livepatch module");

$ make ...
$ klp-convert test_klp_livepatch.ko test_klp_livepatch.conv.ko  # [1]
# modprobe -a xfs ./test_klp_livepatch.conv.ko

[1] https://lore.kernel.org/live-patching/20240827123052.9002-3-lhruska@suse.cz/
[2] https://lore.kernel.org/linux-modules/20241227072825.1288491-1-rppt@kernel.org/
[3] https://lore.kernel.org/linux-modules/86eba318-464b-4b9b-a79e-64039b17be34@lucifer.local/

-- 
Thanks,
Petr