by Ard Biesheuvel

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

On Tue, 28 Sept 2021 at 17:42, Arnd Bergmann <[email protected]> wrote:
>
> From: Arnd Bergmann <[email protected]>
>
> 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 <[email protected]>

> This turned up while build-testing with clang; for some reason,
> gcc never triggered the problem.
>
> Signed-off-by: Arnd Bergmann <[email protected]>
> ---
> 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
>

2022-01-22 09:45:04

by Nathan Chancellor

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Subject: Re: [PATCH 13/14] ARM: use .arch directives instead of assembler command line flags

Hi Arnd,

On Tue, Sep 28, 2021 at 05:41:42PM +0200, Arnd Bergmann wrote:
> From: Nick Desaulniers <[email protected]>
>
> Similar to commit a6c30873ee4a ("ARM: 8989/1: use .fpu assembler
> directives instead of assembler arguments").
>
> GCC and GNU binutils support setting the "sub arch" via -march=,
> -Wa,-march, target function attribute, and .arch assembler directive.
>
> Clang's integrated assembler does not support -Wa,-march (and the logic
> to overrule one when multiple of the above are used), and this can
> cause annoying warnings such as:
>
> clang: warning: argument unused during compilation: '-march=armv6k' [-Wunused-command-line-argument]
> clang: warning: argument unused during compilation: '-march=armv6k' [-Wunused-command-line-argument]
> clang: warning: argument unused during compilation: '-march=armv6k' [-Wunused-command-line-argument]
>
> Since most assembler is non-conditionally assembled with one sub arch
> (modulo arch/arm/delay-loop.S which conditionally is assembled as armv4
> based on CONFIG_ARCH_RPC, and arch/arm/mach-at91/pm-suspend.S which is
> conditionally assembled as armv7-a based on CONFIG_CPU_V7), prefer the
> .arch assembler directive.
>
> Link: https://bugs.llvm.org/show_bug.cgi?id=48894
> Link: https://github.com/ClangBuiltLinux/linux/issues/1195
> Suggested-by: Arnd Bergmann <[email protected]>
> Signed-off-by: Nick Desaulniers <[email protected]>
> [arnd] add a few more instances found in compile testing
> Signed-off-by: Arnd Bergmann <[email protected]>

Did anything ever come of this patch (or series)? I would like to add
-Werror=unused-command-line-argument to CLANG_FLAGS so that invalid
flags will not cause cascading cc-option failures but we cannot not do
that until this is resolved.

Cheers,
Nathan