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

[Nick Desaulniers]

On Tue, Sep 28, 2021 at 8:42 AM 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

does `mov pc, lr` work here, with a preprocessor guard on CPU_32v4? Or
better yet...

If `ret` is just an assembler macro
(arch/arm/include/asm/assembler.h#L449), then perhaps always just
using `ret` would be preferable here? In that case, it looks like
`usr_ret` could be outright replaced with just expansions of `ret`?

Just spitballing ideas; like you said, maybe not worth fixing.

> that a kernel supporting both ARMv4 and a later CPU architecture cannot
> run THUMB binaries.
>
> This turned up while build-testing with clang; for some reason,
> gcc never triggered the problem.

I suspect this is a Clang's integrated assembler vs GAS difference
(compiler irrelevant); clang's assembler is more strict that `bx lr`
requires armvt (CPU_32v4T).  Though I can reproduce that exact message
in local testing with GAS:

$ cat foo.s
bx lr
$ arm-linux-gnueabi-as -march=armv4 -c foo.s
foo.s: Assembler messages:
foo.s:1: Error: selected processor does not support `bx lr' in ARM mode
$ arm-linux-gnueabi-as -march=armv4t -c foo.s
$ arm-linux-gnueabi-objdump -dr a.out
...
00000000 <.text>:
   0:   e12fff1e        bx      lr
                        0: R_ARM_V4BX   *ABS*

The `<instantiation>:2:2` makes me think of inline asm.

>
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>

Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

> ---
>  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
>


-- 
Thanks,
~Nick Desaulniers