Re: [PATCH v3 04/12] livepatch/klp-build: build LoongArch with -fPIC to keep GOT-indirect symbol references

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

On Tue, Jul 07, 2026 at 03:20:23PM +0800, George Guo wrote:
> From: George Guo <guodongtai@kylinos.cn>
> 
> On LoongArch, klp-build livepatch modules panic when a patched function
> references a global defined in the same compilation unit (e.g.
> SYSCALL_DEFINE1(newuname) -> 'uts_sem' in kernel/sys.c).
> 
> With CONFIG_RELOCATABLE=y the kernel is already -fPIE, so this is not
> absolute addressing; the problem is GOT indirection. For a same-unit
> global, -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_*).
> klp-build extracts the patched function into a separate module while
> 'uts_sem' stays in the core kernel, and the klp relocation machinery can
> only redirect such a cross-object reference through a GOT entry. The
> direct -fPIE reference has no GOT slot to fix up, so once the function is
> relocated its target is wrong and it faults.
> 
> Force -fPIC for LoongArch KLP builds; -fPIE is not enough, as it
> optimizes away the very GOT indirection KLP relies on.
> 
> This depends on the preceding patch: -fPIC is passed via KCFLAGS, but the
> arch adds -fPIE via KBUILD_CFLAGS_KERNEL, which kbuild applies after
> KCFLAGS (so -fPIE would win). That patch's command-line
> KBUILD_CFLAGS_KERNEL= assignment replaces the arch value and drops -fPIE,
> letting -fPIC take effect. The two patches must stay together.
> 

This might be my personal preference, but I think it would be quicker to
review patches: ("livepatch/klp-build: disable direct-extern-access for
LoongArch to fix kernel panic") ("livepatch/klp-build: build LoongArch
with -fPIC to keep GOT-indirect symbol references") if they were
squashed into one patch.

With that, the commit messages can drop the forward/backward references,
while the combined commit remains complete thought, i.e. for LoongArch
KLP builds, replace the arch's KBUILD_CFLAGS_KERNEL (dropping -fPIE and
-mdirect-extern-access) and add -fPIC via KCFLAGS instead.

> Co-developed-by: Kexin Liu <liukexin@kylinos.cn>
> Signed-off-by: Kexin Liu <liukexin@kylinos.cn>
> Signed-off-by: George Guo <guodongtai@kylinos.cn>
> ---
>  scripts/livepatch/klp-build | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/scripts/livepatch/klp-build b/scripts/livepatch/klp-build
> index 27fe8824ef12..42cd58aff3d8 100755
> --- a/scripts/livepatch/klp-build
> +++ b/scripts/livepatch/klp-build
> @@ -557,8 +557,15 @@ build_kernel() {
>  	local cmd=()
>  
>  	local ARCH_KBUILD_CFLAGS_KERNEL=""
> +	local ARCH_KCFLAGS=""
>  
>  	if [[ -v CONFIG_LOONGARCH && "$CONFIG_LOONGARCH" == "y" ]]; then
> +		# -fPIC replaces the kernel's -fPIE (added under CONFIG_RELOCATABLE);
> +		# without that config there is no -fPIE to replace.
> +		[[ "${CONFIG_RELOCATABLE:-}" == "y" ]] || \
> +			die "LoongArch klp-build requires CONFIG_RELOCATABLE=y"
> +		ARCH_KCFLAGS="-fPIC"
> +
>  		# -mdirect-extern-access only exists under explicit relocs, and this
>  		# function replaces KBUILD_CFLAGS_KERNEL wholesale (safe only then;
>  		# the non-explicit build puts -Wa,-mla-global-with-pcrel there).
> @@ -599,8 +606,16 @@ build_kernel() {
>  		cmd+=("-s")
>  	fi
>  	cmd+=("-j$JOBS")
> -	cmd+=("KCFLAGS=-ffunction-sections -fdata-sections")
> -	cmd+=("KBUILD_CFLAGS_KERNEL=$ARCH_KBUILD_CFLAGS_KERNEL")
> +	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")
>  

I'd also pull out the configuration checks into validate_config() to be
consistent:

  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

and then build_kernel() only needs to adjust the cmd / flags accordingly.

--
Joe