2 files changed, 35 insertions, 0 deletions

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 2fec7073f687..f871f2e1dd7c 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -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
 
diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index da525bc96145..6d611f407ce6 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