linux-kernel - Re: [PATCH v2 03/12] ARM: Clean up definition of ARM_HAS_GROUP

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Message-Id: <35178205-7cff-4b4b-abdd-b4cfb9e69dc2@app.fastmail.com>
Date: Fri, 22 Aug 2025 09:05:38 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Nathan Chancellor" <nathan@...nel.org>, linux-kernel@...r.kernel.org
Cc: "Kees Cook" <kees@...nel.org>,
 "Nick Desaulniers" <nick.desaulniers+lkml@...il.com>,
 "Bill Wendling" <morbo@...gle.com>, "Justin Stitt" <justinstitt@...gle.com>,
 llvm@...ts.linux.dev, patches@...ts.linux.dev,
 "Russell King" <linux@...linux.org.uk>, "Ard Biesheuvel" <ardb@...nel.org>,
 linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 03/12] ARM: Clean up definition of ARM_HAS_GROUP_RELOCS

On Thu, Aug 21, 2025, at 23:15, Nathan Chancellor wrote:
> Now that the minimum supported version of LLVM for building the kernel
> has been bumped to 15.0.0, the first depends line of
> ARM_HAS_GROUP_RELOCS is always true, so it can be safely removed.
> Combine the !COMPILE_TEST dependency into the 'def_bool' line and update
> the comment as well.
>
> Reviewed-by: Kees Cook <kees@...nel.org>
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>

Acked-by: Arnd Bergmann <arnd@...db.de>

>.
> 
>  config ARM_HAS_GROUP_RELOCS
> -	def_bool y
> -	depends on !LD_IS_LLD || LLD_VERSION >= 140000
> -	depends on !COMPILE_TEST
> +	def_bool !COMPILE_TEST
>  	help
>  	  Whether or not to use R_ARM_ALU_PC_Gn or R_ARM_LDR_PC_Gn group
> -	  relocations, which have been around for a long time, but were not
> -	  supported in LLD until version 14. The combined range is -/+ 256 MiB,
> -	  which is usually sufficient, but not for allyesconfig, so we disable
> -	  this feature when doing compile testing.

The change is obviously correct by itself, but can we revisit the
question of whether the COMPILE_TEST check is still needed?

Trying it out, I single link issue using llvm-21:

ld.lld-21: error: vmlinux.a(arch/arm/kernel/entry-armv.o):(function __bad_stack: .text+0x110): relocation R_ARM_LDR_PC_G2 out of range: 10168 is not in [0, 4095]; references 'overflow_stack_ptr'
>>> defined in vmlinux.a(arch/arm/kernel/traps.o)

which is from this line

arch/arm/kernel/entry-armv.S:   ldr_this_cpu_armv6 ip, overflow_stack_ptr

with the macro expanding to

        .macro          ldr_this_cpu_armv6, rd:req, sym:req
        this_cpu_offset \rd
        .globl          \sym 
        .reloc          .L0_\@, R_ARM_ALU_PC_G0_NC, \sym
        .reloc          .L1_\@, R_ARM_ALU_PC_G1_NC, \sym
        .reloc          .L2_\@, R_ARM_LDR_PC_G2, \sym
        add             \rd, \rd, pc
.L0_\@: sub             \rd, \rd, #4
.L1_\@: sub             \rd, \rd, #0
.L2_\@: ldr             \rd, [\rd, #4]
        .endm

Would it be possible to either change the macro or to move
the overflow_stack_ptr closer in order to completely eliminate
the CONFIG_ARM_HAS_GROUP_RELOCS symbol and have VMAP_STACK
enabled for all CONFIG_MMU builds?

Are there any other build testing issues with ARM_HAS_GROUP_RELOCS
besides the one I saw here?

      Arnd