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

* [PATCH v2] x86/purgatory: Change compiler flags to avoid relocation errors.
@ 2019-09-05 20:23 Steve Wahl
  2019-09-06 18:55 ` [tip: x86/urgent] x86/purgatory: Change compiler flags from -mcmodel=kernel to -mcmodel=large to fix kexec " tip-bot2 for Steve Wahl
  2019-09-06 18:55 ` tip-bot2 for Steve Wahl
  0 siblings, 2 replies; 3+ messages in thread
From: Steve Wahl @ 2019-09-05 20:23 UTC (permalink / raw)
  To: linux-kernel, Thomas Gleixner, Ingo Molnar, Borislav Petkov,
	H. Peter Anvin, x86, Nick Desaulniers, Steve Wahl,
	clang-built-linux, vaibhavrustagi, russ.anderson,
	dimitri.sivanich, mike.travis

The last change to this Makefile caused relocation errors when loading
a kdump kernel.  Restore -mcmodel=large (not -mcmodel=kernel),
-ffreestanding, and -fno-zero-initialized-bsss, without reverting to
the former practice of resetting KBUILD_CFLAGS.

Purgatory.ro is a standalone binary that is not linked against the
rest of the kernel.  Its image is copied into an array that is linked
to the kernel, and from there kexec relocates it wherever it desires.

With the previous change to compiler flags, the error "kexec: Overflow
in relocation type 11 value 0x11fffd000" was encountered when trying
to load the crash kernel.  This is from kexec code trying to relocate
the purgatory.ro object.

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

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