[tip:efi/urgent] efi/arm: Fix absolute relocation detection for older toolchains

From: tip-bot for Ard Biesheuvel
Date: Wed Oct 19 2016 - 10:43:33 EST

Commit-ID: b0dddf6c147e6fe61374d625c4bb2b7c52018639
Gitweb: http://git.kernel.org/tip/b0dddf6c147e6fe61374d625c4bb2b7c52018639
Author: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
AuthorDate: Tue, 18 Oct 2016 16:53:11 +0100
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 19 Oct 2016 14:49:44 +0200

efi/arm: Fix absolute relocation detection for older toolchains

When building the ARM kernel with CONFIG_EFI=y, the following build
error may occur when using a less recent version of binutils (2.23 or

STUBCPY drivers/firmware/efi/libstub/lib-sort.stub.o
00000000 R_ARM_ABS32 sort
00000004 R_ARM_ABS32 __ksymtab_strings
drivers/firmware/efi/libstub/lib-sort.stub.o: absolute symbol references not allowed in the EFI stub

(and when building with debug symbols, the list above is much longer, and
contains all the internal references between the .debug sections and the
actual code)

This issue is caused by the fact that objcopy v2.23 or earlier does not
support wildcards in its -R and -j options, which means the following
line from the Makefile:

STUBCOPY_FLAGS-y := -R .debug* -R *ksymtab* -R *kcrctab*

fails to take effect, leaving harmless absolute relocations in the binary
that are indistinguishable from relocations that may cause crashes at
runtime due to the fact that these relocations are resolved at link time
using the virtual address of the kernel, which is always different from
the address at which the EFI firmware loads and invokes the stub.

So, as a workaround, disable debug symbols explicitly when building the
stub for ARM, and strip the ksymtab and kcrctab symbols for the only
exported symbol we currently reuse in the stub, which is 'sort'.

Tested-by: Jon Hunter <jonathanh@xxxxxxxxxx>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
Reviewed-by: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: linux-efi@xxxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/1476805991-7160-2-git-send-email-ard.biesheuvel@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
drivers/firmware/efi/libstub/Makefile | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index c069451..5e23e2d 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile
@@ -11,7 +11,7 @@ cflags-$(CONFIG_X86) += -m$(BITS) -D__KERNEL__ $(LINUX_INCLUDE) -O2 \
-mno-mmx -mno-sse

cflags-$(CONFIG_ARM64) := $(subst -pg,,$(KBUILD_CFLAGS))
-cflags-$(CONFIG_ARM) := $(subst -pg,,$(KBUILD_CFLAGS)) \
+cflags-$(CONFIG_ARM) := $(subst -pg,,$(KBUILD_CFLAGS)) -g0 \
-fno-builtin -fpic -mno-single-pic-base

cflags-$(CONFIG_EFI_ARMSTUB) += -I$(srctree)/scripts/dtc/libfdt
@@ -79,5 +79,6 @@ quiet_cmd_stubcopy = STUBCPY $@
# decompressor. So move our .data to .data.efistub, which is preserved
# explicitly by the decompressor linker script.
-STUBCOPY_FLAGS-$(CONFIG_ARM) += --rename-section .data=.data.efistub
+STUBCOPY_FLAGS-$(CONFIG_ARM) += --rename-section .data=.data.efistub \
+ -R ___ksymtab+sort -R ___kcrctab+sort