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Commit 6f685c5c authored by Dave Martin's avatar Dave Martin Committed by Russell King
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ARM: 6781/1: Thumb-2: Work around buggy Thumb-2 short branch relocations in gas 

Various binutils versions can resolve Thumb-2 branches to
locally-defined, preemptible global symbols as short-range "b.n"
branch instructions.

This is a problem, because there's no guarantee the final
destination of the symbol, or any candidate locations for a
trampoline, are within range of the branch.  For this reason, the
kernel does not support fixing up the R_ARM_THM_JUMP11 (102)
relocation in modules at all, and it makes little sense to add
support.

The symptom is that the kernel fails with an "unsupported
relocation" error when loading some modules.

Until fixed tools are available, passing
-fno-optimize-sibling-calls to gcc should prevent gcc generating
code which hits this problem, at the cost of a bit of extra runtime
stack usage in some cases.

The problem is described in more detail at:
    https://bugs.launchpad.net/binutils-linaro/+bug/725126



Only Thumb-2 kernels are affected.

This patch adds a new CONFIG_THUMB2_AVOID_R_ARM_THM_JUMP11 config
option which adds -fno-optimize-sibling-calls to CFLAGS_MODULE
when building a Thumb-2 kernel.

Signed-off-by: default avatarDave Martin <dave.martin@linaro.org>
Acked-by: default avatarNicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
parent 23bfdacf

arch/arm/Kconfig

+31 −0
Original line number Diff line number Diff line
@@ -1379,6 +1379,37 @@ config THUMB2_KERNEL 


	  If unsure, say N.



config THUMB2_AVOID_R_ARM_THM_JUMP11

	bool "Work around buggy Thumb-2 short branch relocations in gas"

	depends on THUMB2_KERNEL && MODULES

	default y

	help

	  Various binutils versions can resolve Thumb-2 branches to

	  locally-defined, preemptible global symbols as short-range "b.n"

	  branch instructions.



	  This is a problem, because there's no guarantee the final

	  destination of the symbol, or any candidate locations for a

	  trampoline, are within range of the branch.  For this reason, the

	  kernel does not support fixing up the R_ARM_THM_JUMP11 (102)

	  relocation in modules at all, and it makes little sense to add

	  support.



	  The symptom is that the kernel fails with an "unsupported

	  relocation" error when loading some modules.



	  Until fixed tools are available, passing

	  -fno-optimize-sibling-calls to gcc should prevent gcc generating

	  code which hits this problem, at the cost of a bit of extra runtime

	  stack usage in some cases.



	  The problem is described in more detail at:

	      https://bugs.launchpad.net/binutils-linaro/+bug/725126



	  Only Thumb-2 kernels are affected.



	  Unless you are sure your tools don't have this problem, say Y.



config ARM_ASM_UNIFIED

	bool

arch/arm/Makefile

+4 −0
Original line number Diff line number Diff line
@@ -105,6 +105,10 @@ AFLAGS_AUTOIT :=$(call as-option,-Wa$(comma)-mimplicit-it=always,-Wa$(comma)-mau 
AFLAGS_NOWARN	:=$(call as-option,-Wa$(comma)-mno-warn-deprecated,-Wa$(comma)-W)

CFLAGS_THUMB2	:=-mthumb $(AFLAGS_AUTOIT) $(AFLAGS_NOWARN)

AFLAGS_THUMB2	:=$(CFLAGS_THUMB2) -Wa$(comma)-mthumb

# Work around buggy relocation from gas if requested:

ifeq ($(CONFIG_THUMB2_AVOID_R_ARM_THM_JUMP11),y)

CFLAGS_MODULE	+=-fno-optimize-sibling-calls

endif

endif



# Need -Uarm for gcc < 3.x