Re: [PATCH 06/14] ARM: disallow CONFIG_THUMB with ARMv4

[Ard Biesheuvel]

On Tue, 28 Sept 2021 at 17:42, Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
>
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> We can currently build a multi-cpu enabled kernel that allows both ARMv4
> and ARMv5 CPUs, and also supports THUMB mode in user space.
>
> However, returning to user space in this configuration with the usr_ret
> macro requires the use of the 'bx' instruction, which is refused by
> the assembler:
>
> arch/arm/kernel/entry-armv.S: Assembler messages:
> arch/arm/kernel/entry-armv.S:937: Error: selected processor does not support `bx lr' in ARM mode
> arch/arm/kernel/entry-armv.S:960: Error: selected processor does not support `bx lr' in ARM mode
> arch/arm/kernel/entry-armv.S:1003: Error: selected processor does not support `bx lr' in ARM mode
> <instantiation>:2:2: note: instruction requires: armv4t
>  bx lr
>
> While it would be possible to handle this correctly in principle, doing so
> seems to not be worth it, if we can simply avoid the problem by enforcing
> that a kernel supporting both ARMv4 and a later CPU architecture cannot
> run THUMB binaries.
>

I had a quick look, and it seems that the only way to handle this
correctly is to emit the BX instructions, and use code patching to
change them into 'mov pc, <reg>' if the runtime detected CPU is not
Thumb capable. The usual approach (emitting 'tst <reg>, #1; moveq pc,
<reg>; bx lr') is not feasible here, since we run out of space in the
kuser helper slots.

Reviewed-by: Ard Biesheuvel <ardb@xxxxxxxxxx>

> This turned up while build-testing with clang; for some reason,
> gcc never triggered the problem.
>
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  arch/arm/mm/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig
> index 82aa990c4180..58afba346729 100644
> --- a/arch/arm/mm/Kconfig
> +++ b/arch/arm/mm/Kconfig
> @@ -675,7 +675,7 @@ config ARM_PV_FIXUP
>
>  config ARM_THUMB
>         bool "Support Thumb user binaries" if !CPU_THUMBONLY && EXPERT
> -       depends on CPU_THUMB_CAPABLE
> +       depends on CPU_THUMB_CAPABLE && !CPU_32v4
>         default y
>         help
>           Say Y if you want to include kernel support for running user space
> --
> 2.29.2
>