[PATCH v4 03/14] livepatch/klp-build: use -fPIC and drop direct-extern-access on LoongArch
From: George Guo <hidden>
Date: 2026-07-24 11:42:45
Also in:
lkml, llvm, loongarch
Subsystem:
live patching, the rest · Maintainers:
Josh Poimboeuf, Jiri Kosina, Miroslav Benes, Petr Mladek, Linus Torvalds
From: George Guo <redacted>
On LoongArch, the klp relocation machinery redirects a livepatch's
reference to a core-kernel symbol through a GOT entry. klp-build extracts
the patched function into a separate module while the referenced symbol
stays in the core kernel, so the reference must be GOT-indirect for that
redirect to work. Two default compiler behaviours defeat this and make a
loaded livepatch module fault.
The first is a same-unit global (the syscall test patches sys_newuname()
in kernel/sys.c, which reads 'uts_sem', a rw_semaphore defined in the
same file). With CONFIG_RELOCATABLE=y the kernel is built -fPIE. For
a global defined in the same unit, -fPIE emits a direct PC-relative
reference (R_LARCH_PCALA_*) and skips the GOT, while -fPIC routes it
through the GOT (R_LARCH_GOT_PC_*). Once klp-build moves the patched
function into the livepatch module, the direct reference has no GOT
slot to redirect and faults.
The second is an extern global. -mdirect-extern-access replaces GOT-based
external symbol access with direct addressing. A livepatch reference to
such a symbol then has no GOT slot to fix up, and the wrong address faults
the kernel. This optimization was added by commit 38b10b269d04
("LoongArch: Tweak CFLAGS for Clang compatibility") as a nice-to-have that
reduces GOT accesses.
For LoongArch KLP builds, keep both forms GOT-indirect:
- Add -fPIC via KCFLAGS. -fPIE is not enough; it optimizes away the very
GOT indirection KLP relies on.
- Disable direct-extern-access: -mno-direct-extern-access for GCC,
-fno-direct-access-external-data for Clang.
Co-developed-by: Kexin Liu <redacted>
Signed-off-by: Kexin Liu <redacted>
Signed-off-by: George Guo <redacted>
---
scripts/livepatch/klp-build | 43 ++++++++++++++++++++++++++++++++++++-
1 file changed, 42 insertions(+), 1 deletion(-)
diff --git a/scripts/livepatch/klp-build b/scripts/livepatch/klp-build
index e83973567c87..27463def08f8 100755
--- a/scripts/livepatch/klp-build
+++ b/scripts/livepatch/klp-build@@ -278,6 +278,13 @@ validate_config() { [[ -x "$OBJTOOL" ]] && "$OBJTOOL" klp 2>&1 | command grep -q "not implemented" && \ die "objtool not built with KLP support; install xxhash-devel/libxxhash-dev (version >= 0.8) and recompile" + if [[ -v CONFIG_LOONGARCH ]]; then + [[ -v CONFIG_AS_HAS_EXPLICIT_RELOCS ]] || \ + die "LoongArch klp-build requires CONFIG_AS_HAS_EXPLICIT_RELOCS=y" + [[ -v CONFIG_RELOCATABLE ]] || \ + die "LoongArch klp-build requires CONFIG_RELOCATABLE=y" + fi + return 0 }
@@ -556,6 +563,31 @@ build_kernel() { local log="$TMP_DIR/build.log" local cmd=() + local ARCH_KBUILD_CFLAGS_KERNEL="" + local ARCH_KCFLAGS="" + + # For KLP, LoongArch symbol references must stay GOT-indirect so the klp + # relocation machinery can redirect a cross-object reference through a + # GOT entry. Two default behaviours defeat that (CONFIG_RELOCATABLE and + # CONFIG_AS_HAS_EXPLICIT_RELOCS are checked in validate_config()): + # + # - -fPIE (added under CONFIG_RELOCATABLE) emits a direct PC-relative + # reference for a same-unit global and skips the GOT; -fPIC routes it + # through the GOT. Add -fPIC via KCFLAGS. + # - -mdirect-extern-access replaces GOT access for an extern global with + # direct addressing. Disable it. + if [[ -v CONFIG_LOONGARCH ]]; then + ARCH_KCFLAGS="-fPIC" + + if [[ "${CONFIG_CC_IS_CLANG:-}" == "y" ]]; then + ARCH_KBUILD_CFLAGS_KERNEL="-fno-direct-access-external-data" + else + ARCH_KBUILD_CFLAGS_KERNEL="-mno-direct-extern-access" + fi + + status "LoongArch detected: adding $ARCH_KBUILD_CFLAGS_KERNEL to KBUILD_CFLAGS_KERNEL" + fi + cmd=("make") # When a patch to a kernel module references a newly created unexported
@@ -581,7 +613,16 @@ build_kernel() { cmd+=("-s") fi cmd+=("-j$JOBS") - cmd+=("KCFLAGS=-ffunction-sections -fdata-sections") + cmd+=("KCFLAGS=-ffunction-sections -fdata-sections${ARCH_KCFLAGS:+ $ARCH_KCFLAGS}") + # -fPIC is added for KLP via KCFLAGS above; the arch adds -fPIE via + # KBUILD_CFLAGS_KERNEL, which kbuild places after KCFLAGS on the + # built-in compile line. -fPIC/-fPIE is last-one-wins, so -fPIE would + # win. Setting KBUILD_CFLAGS_KERNEL on the command line replaces the + # arch value (not append), which drops -fPIE and lets -fPIC win. Only + # do this when an arch needs it (LoongArch). + if [[ -n "$ARCH_KBUILD_CFLAGS_KERNEL" ]]; then + cmd+=("KBUILD_CFLAGS_KERNEL=$ARCH_KBUILD_CFLAGS_KERNEL") + fi cmd+=("vmlinux") cmd+=("modules")
--
2.53.0