[PATCH 2/2] vdso: Reject absolute relocations during build

[Thomas Weißschuh]

All vDSO code needs to be completely position independent.
Symbol references are marked as hidden so the compiler is forced to emit
PC-relative relocations.
If the compiler does however emit an absolute relocation this will be
resolved by the linker and break at runtime.

Introduce a build-time check for absolute relocations.
The check is done on the object files as the relocations will not exist
anymore in the final DSO. As there is no extension point for the
compilation of each object file perform the validation in vdso_check.

Link: https://lore.kernel.org/lkml/aApGPAoctq_eoE2g@t14ultra/
Signed-off-by: Thomas Weißschuh <thomas.weissschuh at linutronix.de>
---
 lib/vdso/Makefile.include | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/lib/vdso/Makefile.include b/lib/vdso/Makefile.include
index cedbf15f80874d4bb27c097244bc5b11272f261c..18fcc94e4d6bf7abf374c7a953349e7ad75f8a18 100644
--- a/lib/vdso/Makefile.include
+++ b/lib/vdso/Makefile.include
@@ -12,7 +12,12 @@ c-getrandom-$(CONFIG_VDSO_GETRANDOM) := $(addprefix $(GENERIC_VDSO_DIR), getrand
 #
 # As a workaround for some GNU ld ports which produce unneeded R_*_NONE
 # dynamic relocations, ignore R_*_NONE.
+#
+# Also validate that no absolute relocations are present in the object files themselves.
 quiet_cmd_vdso_check = VDSOCHK $@
       cmd_vdso_check = if $(READELF) -rW $@ | grep -v _NONE | grep -q " R_\w*_"; \
 		       then (echo >&2 "$@: dynamic relocations are not supported"; \
+			     rm -f $@; /bin/false); fi && \
+		       if $(READELF) -rW $(filter %.o, $(real-prereqs)) | grep -q " R_\w*_ABS"; \
+		       then (echo >&2 "$@: absolute relocations are not supported"; \
 			     rm -f $@; /bin/false); fi

-- 
2.49.0