Re: [PATCH] ARM: entry: add .ltorg directive to keep literals in range

[Nathan Chancellor <nathan@kernel.org>]

On Mon, May 30, 2022 at 01:24:17PM +0200, Ard Biesheuvel wrote:
> LKP reports a build issue on Clang, related to a literal load of
> __current issued through the ldr_va macro. This turns out to be due to
> the fact that group relocations are disabled when CONFIG_COMPILE_TEST=y,
> which means that the ldr_va macro resolves to a pair of LDR
> instructions, the first one being a literal load issued too far from its
> literal pool.

For the record, I do see a similar failure with GNU as, not sure if it
is worth calling that out in the commit message or not:

arch/arm/kernel/entry-common.S: Assembler messages:
arch/arm/kernel/entry-common.S:149: Error: invalid literal constant: pool needs to be closer

> Due to the introduction of a couple of new uses of this macro in commit
> 508074607c7b95b2 ("ARM: 9195/1: entry: avoid explicit literal loads"),
> the literal pools end up getting rearranged in a way that causes the
> literal for __current to go out of range. Let's fix this up by putting a
> .ltorg directive in a suitable place in the code.
> 
> Cc: Nathan Chancellor <nathan@kernel.org>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Fixes: 508074607c7b95b2 ("ARM: 9195/1: entry: avoid explicit literal loads")
> Reported-by: kernel test robot <lkp@intel.com>
> Link: https://lore.kernel.org/all/202205290805.1vZLAr36-lkp@intel.com/
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

With the problematic configuration, I no longer see issues with either
clang or GCC:

Tested-by: Nathan Chancellor <nathan@kernel.org>

> ---
>  arch/arm/kernel/entry-common.S | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
> index 7aa3ded4af92..6a447ac67d80 100644
> --- a/arch/arm/kernel/entry-common.S
> +++ b/arch/arm/kernel/entry-common.S
> @@ -298,14 +298,15 @@ local_restart:
>  9001:
>  	sub	lr, saved_pc, #4
>  	str	lr, [sp, #S_PC]
>  	get_thread_info tsk
>  	b	ret_fast_syscall
>  #endif
>  ENDPROC(vector_swi)
> +	.ltorg
>  
>  	/*
>  	 * This is the really slow path.  We're going to be doing
>  	 * context switches, and waiting for our parent to respond.
>  	 */
>  __sys_trace:
>  	add	r0, sp, #S_OFF
> -- 
> 2.30.2
> 

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