[PATCH] ARM: Thumb-2: Add local symbols to work around gas behaviour

[Nicolas Pitre]

On Thu, 3 Mar 2011, Dave Martin wrote:

> 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>

I'd suggest making the shadow local symbol truly local by prefixing it 
with .L so gas won't even store it in the symbol table which would 
otherwise "pollute" backtrace annotations and the like.

> ---
>  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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