[RFC 0/4] klp-build: simple OOT module support

[Joe Lawrence <joe.lawrence@redhat.com>]

This patchset introduces support for patching basic out-of-tree (OOT)
modules.  The primary motivation is to streamline testing for objtool klp
diff by providing a flexible and stable environment.

While much of the objtool klp diff logic can be validated using standard
in-tree modules (avoiding the overhead of full kernel rebuilds), OOT
support provides significant value for reproducing and fixing specific
kernel code patterns.  Standard in-tree drivers are subject to frequent
refactoring and API updates, making them unreliable for producing the
consistent binary patterns required for stable testing.  While dedicated
test modules could be introduced into the kernel tree to ensure
stability, starting with OOT modules allows for faster iteration before
committing to a permanent in-tree landing spot.

For an example, I have inlined a module below used to verify Josh's
latest commit ("objtool/klp: Rewrite symbol correlation algorithm")
[1].  This specific test case confirms that his patch resolves the
thinLTO ambiguity issue originally reported by Song Liu in February [2].

  [1] https://lore.kernel.org/live-patching/cq5uytz6edj75w53f2eubypvqm66hgh4eag7ec2vgqjefzzqts@lcnvt7fcrtmd/T/#mbfaf71b314b6600424f9c5504c415a2e3a87ade3
  [2] https://lore.kernel.org/live-patching/20260226005436.379303-9-song@kernel.org

To make full use of the example OOT module, I have additional test-
harness code that automates fetching pre-built object files, integrating
with kselftests, etc.  I chose to spin out this basic OOT support
first since it mostly stands alone and may have other helpful contexts.

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==> Kbuild <==
obj-m := klp_test_ambig.o
klp_test_ambig-y := klp_test_ambig_a.o klp_test_ambig_b.o klp_test_ambig_c.o

==> klp_test_ambig_a.c <==
#include <linux/module.h>
#include <linux/kernel.h>
#include "klp_test_ambig.h"

static noinline int __helper(int x, int len)
{
	int i, sum = x;

	for (i = 0; i < len; i++)
		sum += i + 5;
	if (sum > 1000)
		sum = 0;
	return sum;
}

static int value_a;

int klp_test_ambig_func_a(int x)
{
	value_a = __helper(value_a, x);
	return value_a;
}
EXPORT_SYMBOL_GPL(klp_test_ambig_func_a);

MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("ThinLTO demangled ambiguity test module");

==> klp_test_ambig_b.c <==
#include <linux/module.h>
#include <linux/kernel.h>
#include "klp_test_ambig.h"

static noinline int __helper(int x, int len)
{
	int i, sum = x;

	for (i = 0; i < len; i++)
		sum += i + 10;
	if (sum > 1000)
		sum = 0;
	return sum;
}

static int value_b;

int klp_test_ambig_func_b(int x)
{
	value_b = __helper(value_b, x);
	return value_b;
}
EXPORT_SYMBOL_GPL(klp_test_ambig_func_b);

==> klp_test_ambig_c.c <==
#include <linux/module.h>
#include <linux/kernel.h>
#include "klp_test_ambig.h"

static int value_c;

int klp_test_ambig_func_c(int x)
{
	value_c = klp_test_ambig_func_a(x) + klp_test_ambig_func_b(x);
	return value_c;
}
EXPORT_SYMBOL_GPL(klp_test_ambig_func_c);

==> klp_test_ambig.h <==
#ifndef _KLP_TEST_AMBIG_H
#define _KLP_TEST_AMBIG_H

int klp_test_ambig_func_a(int x);
int klp_test_ambig_func_b(int x);
int klp_test_ambig_func_c(int x);

#endif


==> 0001-thin-lto-demangled-ambiguity.patch <==
From 2d208e686739b1ccccfe385e837b5a4a04a9526f Mon Sep 17 00:00:00 2001
From: klp-build-test <test@example.com>
Date: Sat, 28 Mar 2026 13:55:56 +0000
Subject: [PATCH] thin-lto-demangled-ambiguity

ThinLTO two __helper.llvm.* symbols with disjoint hashes; objtool correlates

---
 klp_test_ambig_a.c | 2 +-
 klp_test_ambig_b.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/klp_test_ambig_a.c b/klp_test_ambig_a.c
index cc4db70..4ee26a5 100644
--- a/klp_test_ambig_a.c
+++ b/klp_test_ambig_a.c
@@ -7,7 +7,7 @@ static noinline int __helper(int x, int len)
 	int i, sum = x;
 
 	for (i = 0; i < len; i++)
-		sum += i + 5;
+		sum += i * 2 + 5;
 	if (sum > 1000)
 		sum = 0;
 	return sum;
diff --git a/klp_test_ambig_b.c b/klp_test_ambig_b.c
index ca114b0..1b7ce1a 100644
--- a/klp_test_ambig_b.c
+++ b/klp_test_ambig_b.c
@@ -7,7 +7,7 @@ static noinline int __helper(int x, int len)
 	int i, sum = x;
 
 	for (i = 0; i < len; i++)
-		sum += i + 10;
+		sum += i * 2 + 10;
 	if (sum > 1000)
 		sum = 0;
 	return sum;
-- 
2.53.0

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Applies on top of:
tree:   git://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git
branch: klp-build-arm64
commit: fffce0ac08e0 ("klp-build: Add arm64 syscall patching macro")

Joe Lawrence (4):
  objtool/klp: add --symvers option to klp diff
  objtool/klp: allow special section entry size overrides
  objtool/klp: add --arch option to display target architecture
  livepatch/klp-build: add basic out-of-tree module patching support

 scripts/livepatch/klp-build | 90 ++++++++++++++++++++++++++++---------
 tools/objtool/Makefile      |  3 +-
 tools/objtool/klp-diff.c    | 49 +++++++++++++++++---
 3 files changed, 115 insertions(+), 27 deletions(-)

-- 
2.53.0