objtool/klp: Add test for vmlinux relocs in a patched module

The kernel refuses a module-targeted klp relocation which names a vmlinux
symbol.  An EXPORT_SYMBOL_FOR_MODULES() symbol needs a klp relocation, so
patching a module function that references one only loads if klp diff files
that relocation under vmlinux rather than under the patched module.

This fails at load, not at build: klp-build produces a module and static
checks of it find nothing wrong.  Hence the assertion on which object the
relocation is filed against.

Reported by Dylan Hatch, whose mod-ns-lp branch carries the kernel-side
half of this case.

Co-developed-by: Dylan Hatch <dylanbhatch@google.com>
Signed-off-by: Dylan Hatch <dylanbhatch@google.com>
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Assisted-by: Claude:claude-opus-5
Link: https://patch.msgid.link/20260916184351.2720310-30-song@kernel.org

1 files changed, 40 insertions, 0 deletions

diff --git a/tools/objtool/tests/generic/test-module-vmlinux-reloc.sh b/tools/objtool/tests/generic/test-module-vmlinux-reloc.sh
new file mode 100755
index 000000000000..635a75f6b8ba
--- /dev/null
+++ b/tools/objtool/tests/generic/test-module-vmlinux-reloc.sh
@@ -0,0 +1,40 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Patching a module, where the patched code references a vmlinux symbol which
+# needs a klp relocation.
+#
+# The kernel does not allow a module-targeted klp relocation to reference a
+# vmlinux symbol, and a symbol exported with EXPORT_SYMBOL_FOR_MODULES gets a
+# klp relocation.  Put together, filing that relocation under the patched
+# module produces a patch the kernel refuses to apply to its target.
+#
+# So it goes under vmlinux instead, and is applied when the patch module loads
+# rather than when the patched module does.  That is the opposite of the rule
+# for a reference to a module's symbol, which test-module-object covers; this
+# is the other branch of the same decision.
+
+. "$(dirname "$0")/../lib.sh"
+
+setup
+# The object being patched is a module ...
+build_module_pair cross_module.c klp_testmod
+
+# ... and the symbol it references belongs to vmlinux, exported in a way that
+# still requires a klp relocation.
+export_syms
+add_exports_ns vmlinux module:kvm other_mod_func
+run_diff
+
+# Filed against vmlinux, applied when the patch loads.
+assert_section __klp_relocs.vmlinux
+assert_klp_sym other_mod_func vmlinux
+
+# Not against the patched module: that is the relocation the kernel rejects.
+assert_no_section __klp_relocs.klp_testmod
+
+run_post_link
+assert_klp_rela vmlinux .text.target
+assert_no_section ".klp.rela.klp_testmod..text.target"
+
+pass "klp relocation to a vmlinux symbol filed under vmlinux, not the patched module"