Oops, forgot to CC:
----- Forwarded message from Dave Martin dave.martin@linaro.org -----
Date: Mon, 28 Feb 2011 17:38:12 +0000 From: Dave Martin dave.martin@linaro.org To: linux-arm-kernel@lists.infradead.org Cc: Dave Martin dave.martin@linaro.org Subject: [PATCH] ARM: 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: Dave Martin dave.martin@linaro.org --- arch/arm/Kconfig | 31 +++++++++++++++++++++++++++++++ arch/arm/Makefile | 4 ++++ 2 files changed, 35 insertions(+), 0 deletions(-)
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig index 5cff165..196f6d2 100644 --- a/arch/arm/Kconfig +++ b/arch/arm/Kconfig @@ -1371,6 +1371,37 @@ 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
diff --git a/arch/arm/Makefile b/arch/arm/Makefile index c22c1ad..ef5105a 100644 --- a/arch/arm/Makefile +++ b/arch/arm/Makefile @@ -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