[PATCH] ARM: Thumb-2: Add local symbols to work around gas behaviour
Dave Martin
dave.martin at linaro.org
Thu Mar 3 07:39:06 EST 2011
All current versions of gas at the time of writing have issues
fixing up pc-relative instructions which reference global symbols,
due to the potential need to support symbol preemption.
Even though symbol preemption is not relevant to the Linux kernel,
there is no way to inform the tools of this, so we get the problem.
Most pc-relative forms in ARM, and all pc-relative forms in
Thumb, will cause the assembler to fail with various fixup error
messages when used to reference global symbols.
The legacy behaviour is for ADR and plain LDR instructions in ARM
which reference global symbols to be fixed up silently with no
relocation emitted. This means that building the kernel in ARM
currently works without problems, but it may be a bug, and the
behaviour could change in the future.
After discussion with Richard Earnshaw, it seems that there is
no single obvious remedy for this inconsistent behaviour,
so there is not likely to be a comprehensive upstream fix for
a while.
A workaround which should be valid for all past and all
foreseeable future versions of gas is to express the need for
a local fixup explicitly, by declaring a shadow local symbol
for any global symbol which needs to be addressed using ADR
or any pc-relative LDR variant.
This patch implements this workaround for the one part of the
main kernel currently known to be affected. The resulting code
builds and works correctly in ARM and Thumb.
Similar fixes may be needed in mach-specific assembler.
Signed-off-by: Dave Martin <dave.martin at linaro.org>
---
arch/arm/kernel/relocate_kernel.S | 12 ++++++++----
1 files changed, 8 insertions(+), 4 deletions(-)
diff --git a/arch/arm/kernel/relocate_kernel.S b/arch/arm/kernel/relocate_kernel.S
index 9cf4cbf..a2599e7 100644
--- a/arch/arm/kernel/relocate_kernel.S
+++ b/arch/arm/kernel/relocate_kernel.S
@@ -7,8 +7,8 @@
.globl relocate_new_kernel
relocate_new_kernel:
- ldr r0,kexec_indirection_page
- ldr r1,kexec_start_address
+ ldr r0,__kexec_indirection_page
+ ldr r1,__kexec_start_address
/*
* If there is no indirection page (we are doing crashdumps)
@@ -55,27 +55,31 @@ relocate_new_kernel:
/* Jump to relocated kernel */
mov lr,r1
mov r0,#0
- ldr r1,kexec_mach_type
- ldr r2,kexec_boot_atags
+ ldr r1,__kexec_mach_type
+ ldr r2,__kexec_boot_atags
mov pc,lr
.align
.globl kexec_start_address
kexec_start_address:
+__kexec_start_address:
.long 0x0
.globl kexec_indirection_page
kexec_indirection_page:
+__kexec_indirection_page:
.long 0x0
.globl kexec_mach_type
kexec_mach_type:
+__kexec_mach_type:
.long 0x0
/* phy addr of the atags for the new kernel */
.globl kexec_boot_atags
kexec_boot_atags:
+__kexec_boot_atags:
.long 0x0
relocate_new_kernel_end:
--
1.7.1
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