kernel: build the powerpc vDSO after the stack protector flags

The powerpc kernel of the WD My Book Live differs between builds. The
difference sits in the vDSO, which the kernel embeds with its debug
information as vdso32.so.dbg:

  -GNU C11 14.4.0 ... -mstack-protector-guard=tls -mstack-protector-guard-reg=r2 -mstack-protector-guard-offset=872 -m32 -g -O2 ...
  +GNU C11 14.4.0 ... -m32 -g -O2 ...

stack_protector_prepare appends these flags to KBUILD_CFLAGS with $(eval)
from its recipe, while vdso_prepare builds the vDSO from a sibling
prerequisite of prepare. Nothing orders the two, so whether the flags
end up in DW_AT_producer depends on the order in which make starts them.

Add a pending patch that makes vdso_prepare depend on
stack_protector_prepare.

Link: https://github.com/openwrt/openwrt/pull/25429
Signed-off-by: Paul Spooren <mail@aparcar.org>

2 files changed, 78 insertions, 0 deletions

diff --git a/target/linux/generic/pending-6.12/305-powerpc-build-the-vDSO-after-the-stack-protector-flags.patch b/target/linux/generic/pending-6.12/305-powerpc-build-the-vDSO-after-the-stack-protector-flags.patch
new file mode 100644
index 0000000000..0c83e9884a
--- /dev/null
+++ b/target/linux/generic/pending-6.12/305-powerpc-build-the-vDSO-after-the-stack-protector-flags.patch
@@ -0,0 +1,39 @@
+From ed7fc59a8dde6059bdbb55cf836a32beb4dd26de Mon Sep 17 00:00:00 2001
+From: Paul Spooren <mail@aparcar.org>
+Date: Fri, 25 Sep 2026 09:54:16 +0200
+Subject: [PATCH] powerpc: build the vDSO after the stack protector flags are
+ set
+
+stack_protector_prepare appends the -mstack-protector-guard flags to
+KBUILD_CFLAGS with $(eval) from within its recipe, so the flags only
+reach commands that make starts after that recipe. vdso_prepare builds
+the vDSO from a sibling prerequisite of prepare. Nothing orders the two,
+so the vDSO is built with or without the flags depending on the order in
+which make starts the recipes.
+
+The vDSO is built with -fno-stack-protector, so its code is the same
+either way. GCC records the command line in DW_AT_producer, though, and
+the kernel embeds vdso32.so.dbg with its debug information. Two builds
+of the same kernel therefore differ:
+
+  -GNU C11 14.4.0 ... -mstack-protector-guard=tls -mstack-protector-guard-reg=r2 -mstack-protector-guard-offset=872 -m32 -g -O2 ...
+  +GNU C11 14.4.0 ... -m32 -g -O2 ...
+
+Make vdso_prepare depend on stack_protector_prepare, so the vDSO is
+always built with the final KBUILD_CFLAGS.
+
+Signed-off-by: Paul Spooren <mail@aparcar.org>
+---
+ arch/powerpc/Makefile | 1 +
+ 1 file changed, 1 insertion(+)
+
+--- a/arch/powerpc/Makefile
++++ b/arch/powerpc/Makefile
+@@ -391,6 +391,7 @@ archheaders:
+ 
+ ifdef CONFIG_STACKPROTECTOR
+ prepare: stack_protector_prepare
++vdso_prepare: stack_protector_prepare
+ 
+ PHONY += stack_protector_prepare
+ stack_protector_prepare: prepare0
diff --git a/target/linux/generic/pending-6.18/305-powerpc-build-the-vDSO-after-the-stack-protector-flags.patch b/target/linux/generic/pending-6.18/305-powerpc-build-the-vDSO-after-the-stack-protector-flags.patch
new file mode 100644
index 0000000000..e30f917ea3
--- /dev/null
+++ b/target/linux/generic/pending-6.18/305-powerpc-build-the-vDSO-after-the-stack-protector-flags.patch
@@ -0,0 +1,39 @@
+From a4b68ad654bedc6b401e648fd359701e62321f79 Mon Sep 17 00:00:00 2001
+From: Paul Spooren <mail@aparcar.org>
+Date: Fri, 25 Sep 2026 09:54:16 +0200
+Subject: [PATCH] powerpc: build the vDSO after the stack protector flags are
+ set
+
+stack_protector_prepare appends the -mstack-protector-guard flags to
+KBUILD_CFLAGS with $(eval) from within its recipe, so the flags only
+reach commands that make starts after that recipe. vdso_prepare builds
+the vDSO from a sibling prerequisite of prepare. Nothing orders the two,
+so the vDSO is built with or without the flags depending on the order in
+which make starts the recipes.
+
+The vDSO is built with -fno-stack-protector, so its code is the same
+either way. GCC records the command line in DW_AT_producer, though, and
+the kernel embeds vdso32.so.dbg with its debug information. Two builds
+of the same kernel therefore differ:
+
+  -GNU C11 14.4.0 ... -mstack-protector-guard=tls -mstack-protector-guard-reg=r2 -mstack-protector-guard-offset=872 -m32 -g -O2 ...
+  +GNU C11 14.4.0 ... -m32 -g -O2 ...
+
+Make vdso_prepare depend on stack_protector_prepare, so the vDSO is
+always built with the final KBUILD_CFLAGS.
+
+Signed-off-by: Paul Spooren <mail@aparcar.org>
+---
+ arch/powerpc/Makefile | 1 +
+ 1 file changed, 1 insertion(+)
+
+--- a/arch/powerpc/Makefile
++++ b/arch/powerpc/Makefile
+@@ -398,6 +398,7 @@ archheaders:
+ 
+ ifdef CONFIG_STACKPROTECTOR
+ prepare: stack_protector_prepare
++vdso_prepare: stack_protector_prepare
+ 
+ PHONY += stack_protector_prepare
+ stack_protector_prepare: prepare0