[PATCH v2] x86/purgatory: Change compiler flags to avoid relocation errors.

Steve Wahl
2019-09-05T20:24:06Z

From the error message, relocation type 11 is R_X86_64_32S. The
x86_64 abi says: "The R_X86_64_32 and R_X86_64_32S relocations
truncate the computed value to 32-bits. The linker must verify that
the generated value for the R_X86_64_32 (R_X86_64_32S) relocation
zero-extends (sign-extends) to the original 64-bit value."

This type of relocation doesn't work when kexec chooses to place the
purgatory binary in memory that is not reachable with 32 bit
addresses.

The compiler flag -mcmodel=kernel allows those type of relocations to
be emitted, so revert to using -mcmodel=large as was done before.

Also restore the -ffreestanding and -fno-zero-initialized-bss flags
because they are appropriate for a stand alone piece of object code
which doesn't explicitly zero the bss, and one other report has said
undefined symbols are encountered without -ffreestanding.

These identical compiler flag changes need to happen for every object
that becomes part of the purgatory.ro object, so gather them together
first into PURGATORY_CFLAGS_REMOVE and PURGATORY_CFLAGS, and then
apply them to each of the objects that have C source. Do not apply
any of these flags to kexec-purgatory.o, which is not part of the
standalone object but part of the kernel proper.

Fixes: b059f801a937 ("x86/purgatory: Use CFLAGS_REMOVE rather than reset KBUILD_CFLAGS")
Signed-off-by: Steve Wahl <steve.wahl@hpe.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Vaibhav Rustagi <vaibhavrustagi@google.com>
Tested-by: Andreas Smas <andreas@lonelycoder.com>
---
PATCH v2: Incorporated suggested changes to the commit message.

arch/x86/purgatory/Makefile | 35 +++++++++++++++++++----------------
1 file changed, 19 insertions(+), 16 deletions(-)

diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
index 8901a1f89cf5..10fb42da0007 100644
--- a/arch/x86/purgatory/Makefile
+++ b/arch/x86/purgatory/Makefile
@@ -18,37 +18,40 @@ targets += purgatory.ro
KASAN_SANITIZE := n
KCOV_INSTRUMENT := n

+# These are adjustments to the compiler flags used for objects that
+# make up the standalone purgatory.ro
+
+PURGATORY_CFLAGS_REMOVE := -mcmodel=kernel
+PURGATORY_CFLAGS := -mcmodel=large -ffreestanding -fno-zero-initialized-in-bss
+
# Default KBUILD_CFLAGS can have -pg option set when FTRACE is enabled. That
# in turn leaves some undefined symbols like __fentry__ in purgatory and not
# sure how to relocate those.
ifdef CONFIG_FUNCTION_TRACER
-CFLAGS_REMOVE_sha256.o += $(CC_FLAGS_FTRACE)
-CFLAGS_REMOVE_purgatory.o += $(CC_FLAGS_FTRACE)
-CFLAGS_REMOVE_string.o += $(CC_FLAGS_FTRACE)
-CFLAGS_REMOVE_kexec-purgatory.o += $(CC_FLAGS_FTRACE)
+PURGATORY_CFLAGS_REMOVE += $(CC_FLAGS_FTRACE)
endif

ifdef CONFIG_STACKPROTECTOR
-CFLAGS_REMOVE_sha256.o += -fstack-protector
-CFLAGS_REMOVE_purgatory.o += -fstack-protector
-CFLAGS_REMOVE_string.o += -fstack-protector
-CFLAGS_REMOVE_kexec-purgatory.o += -fstack-protector
+PURGATORY_CFLAGS_REMOVE += -fstack-protector
endif

ifdef CONFIG_STACKPROTECTOR_STRONG
-CFLAGS_REMOVE_sha256.o += -fstack-protector-strong
-CFLAGS_REMOVE_purgatory.o += -fstack-protector-strong
-CFLAGS_REMOVE_string.o += -fstack-protector-strong
-CFLAGS_REMOVE_kexec-purgatory.o += -fstack-protector-strong
+PURGATORY_CFLAGS_REMOVE += -fstack-protector-strong
endif

ifdef CONFIG_RETPOLINE
-CFLAGS_REMOVE_sha256.o += $(RETPOLINE_CFLAGS)
-CFLAGS_REMOVE_purgatory.o += $(RETPOLINE_CFLAGS)
-CFLAGS_REMOVE_string.o += $(RETPOLINE_CFLAGS)
-CFLAGS_REMOVE_kexec-purgatory.o += $(RETPOLINE_CFLAGS)
+PURGATORY_CFLAGS_REMOVE += $(RETPOLINE_CFLAGS)
endif

+CFLAGS_REMOVE_purgatory.o += $(PURGATORY_CFLAGS_REMOVE)
+CFLAGS_purgatory.o += $(PURGATORY_CFLAGS)
+
+CFLAGS_REMOVE_sha256.o += $(PURGATORY_CFLAGS_REMOVE)
+CFLAGS_sha256.o += $(PURGATORY_CFLAGS)
+
+CFLAGS_REMOVE_string.o += $(PURGATORY_CFLAGS_REMOVE)
+CFLAGS_string.o += $(PURGATORY_CFLAGS)
+
$(obj)/purgatory.ro: $(PURGATORY_OBJS) FORCE
$(call if_changed,ld)

--
2.21.0

--
Steve Wahl, Hewlett Packard Enterprise