ARM: 6781/1: Thumb-2: Work around buggy Thumb-2 short branch relocations in gas (6f685c5c) · Commits · 戴 / test

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:

Dave Martin <dave.martin@linaro.org> Acked-by:

Nicolas Pitre <nicolas.pitre@linaro.org> Signed-off-by:

Russell King <rmk+kernel@arm.linux.org.uk>

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

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