[OpenWrt-Devel] [PATCH v2] Allow kernel modules to set build ID debug symbol

[Toke Høiland-Jørgensen]

This change adds support for specifying a build ID for kernel modules.
This is done by setting PKG_BUILD_ID to a hexadecimal string, which will
then be passed to the kernel linker. In addition, when this flag is set,
the build ID debug symbol (.note.gnu.build-id) will not be stripped from
the kernel module. This symbol is exported in sysfs by the kernel (if
the kernel is compiled with CONFIG_KALLSYMS) and so can be used to
uniquely identify a version of a kernel module in a running kernel. This
is useful for keeping track of different versions of a module when doing
experiments and development.

Modules that specify the build ID will be ~100 bytes larger (depending
on the length of the build ID specified). There is no size difference
for kernel modules that do not set this variable.

Signed-off-by: Toke Høiland-Jørgensen <toke at toke.dk>
---
 include/kernel-defaults.mk | 3 ++-
 rules.mk                   | 3 ++-
 scripts/strip-kmod.sh      | 5 ++++-
 3 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/include/kernel-defaults.mk b/include/kernel-defaults.mk
index 24d2630..d37787a 100644
--- a/include/kernel-defaults.mk
+++ b/include/kernel-defaults.mk
@@ -11,7 +11,8 @@ KERNEL_MAKEOPTS := -C $(LINUX_DIR) \
 	ARCH="$(LINUX_KARCH)" \
 	KBUILD_HAVE_NLS=no \
 	CONFIG_SHELL="$(BASH)" \
-	$(if $(findstring c,$(OPENWRT_VERBOSE)),V=1,V='')
+	$(if $(findstring c,$(OPENWRT_VERBOSE)),V=1,V='') \
+	$(if $(PKG_BUILD_ID),LDFLAGS_MODULE=--build-id=0x$(PKG_BUILD_ID))
 
 ifdef CONFIG_STRIP_KERNEL_EXPORTS
   KERNEL_MAKEOPTS += \
diff --git a/rules.mk b/rules.mk
index 819bea5..7710fb4 100644
--- a/rules.mk
+++ b/rules.mk
@@ -273,8 +273,9 @@ else
       STRIP:=$(STAGING_DIR_HOST)/bin/sstrip
     endif
   endif
-  RSTRIP:= \
+  RSTRIP= \
     export CROSS="$(TARGET_CROSS)" \
+		$(if $(PKG_BUILD_ID),KEEP_BUILD_ID=1) \
 		$(if $(CONFIG_KERNEL_KALLSYMS),NO_RENAME=1) \
 		$(if $(CONFIG_KERNEL_PROFILING),KEEP_SYMBOLS=1); \
     NM="$(TARGET_CROSS)nm" \
diff --git a/scripts/strip-kmod.sh b/scripts/strip-kmod.sh
index 13e6b58..e3f13be 100755
--- a/scripts/strip-kmod.sh
+++ b/scripts/strip-kmod.sh
@@ -18,11 +18,14 @@ else
 	ARGS="-x -G __this_module --strip-unneeded"
 fi
 
+if [ -z "$KEEP_BUILD_ID" ]; then
+    ARGS="$ARGS -R .note.gnu.build-id"
+fi
+
 ${CROSS}objcopy \
 	-R .comment \
 	-R .pdr \
 	-R .mdebug.abi32 \
-	-R .note.gnu.build-id \
 	-R .gnu.attributes \
 	-R .reginfo \
 	$ARGS \
-- 
2.6.2
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