[PATCH v2 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 emits
PC-relative relocations. However there are cases where the compiler may
still emit absolute relocations, as they are valid in regular PIC DSO code.
These would be resolved by the linker and will 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.

Debug information can contain legal absolute relocations and readelf can
not print relocations from the .text section only. Make use of the fact
that all global vDSO symbols follow the naming pattern "vdso_u_".

Link: https://lore.kernel.org/lkml/aApGPAoctq_eoE2g@t14ultra/
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120002
Signed-off-by: Thomas Weißschuh <thomas.weissschuh at linutronix.de>
---
 lib/vdso/Makefile.include | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/lib/vdso/Makefile.include b/lib/vdso/Makefile.include
index cedbf15f80874d4bb27c097244bc5b11272f261c..04257d0f28c0ed324e31adbb68497181085752f8 100644
--- a/lib/vdso/Makefile.include
+++ b/lib/vdso/Makefile.include
@@ -12,7 +12,13 @@ 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 against global symbols are present
+# in the object files.
 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, $(real-prereqs)) | grep -q " R_\w*_ABS.*vdso_u_"; \
+		       then (echo >&2 "$@: absolute relocations are not supported"; \
 			     rm -f $@; /bin/false); fi

-- 
2.49.0