From: Ard Biesheuvel <ardb@kernel.org>
To: Arnd Bergmann <arnd@kernel.org>
Cc: Russell King <linux@armlinux.org.uk>,
Linux ARM <linux-arm-kernel@lists.infradead.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Arnd Bergmann <arnd@arndb.de>,
Linus Walleij <linus.walleij@linaro.org>,
Nathan Chancellor <nathan@kernel.org>,
Nick Desaulniers <ndesaulniers@google.com>,
llvm@lists.linux.dev
Subject: Re: [PATCH 06/14] ARM: disallow CONFIG_THUMB with ARMv4
Date: Wed, 6 Oct 2021 18:01:22 +0200 [thread overview]
Message-ID: <CAMj1kXF-iJj2cg=M8BD96OE8ozA_vB8yEN+f8HvRHY=VYb0pwg@mail.gmail.com> (raw)
In-Reply-To: <20210928154143.2106903-7-arnd@kernel.org>
On Tue, 28 Sept 2021 at 17:42, Arnd Bergmann <arnd@kernel.org> wrote:
>
> From: Arnd Bergmann <arnd@arndb.de>
>
> 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@kernel.org>
> This turned up while build-testing with clang; for some reason,
> gcc never triggered the problem.
>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> 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
>
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next prev parent reply other threads:[~2021-10-06 16:03 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-28 15:41 [PATCH 00/14] ARM: randconfig build fixes Arnd Bergmann
2021-09-28 15:41 ` [PATCH 01/14] ARM: RiscPC needs older gcc version Arnd Bergmann
2021-09-28 16:03 ` Arnd Bergmann
2021-09-29 8:45 ` Arnd Bergmann
2021-09-28 15:41 ` [PATCH 02/14] ARM: patch: fix BE32 compilation Arnd Bergmann
2021-09-28 21:33 ` Linus Walleij
2021-09-28 15:41 ` [PATCH 03/14] ARM: remove duplicate memcpy() definition Arnd Bergmann
2021-09-28 21:39 ` Linus Walleij
2021-09-28 15:41 ` [PATCH 04/14] ARM: kprobes: address gcc -Wempty-body warning Arnd Bergmann
2021-09-28 15:41 ` [PATCH 05/14] ARM: ARMv7-M uses BE-8, not BE-32 Arnd Bergmann
2021-09-28 15:41 ` [PATCH 06/14] ARM: disallow CONFIG_THUMB with ARMv4 Arnd Bergmann
2021-09-29 18:52 ` Nick Desaulniers
2021-10-06 16:01 ` Ard Biesheuvel [this message]
2021-09-28 15:41 ` [PATCH 07/14] ARM: fix link warning with XIP + frame-pointer Arnd Bergmann
2021-09-28 15:41 ` [PATCH 08/14] ARM: kprobes: fix arch_init_kprobes() prototype Arnd Bergmann
2021-09-28 15:41 ` [PATCH 09/14] ARM: allow compile-testing without machine record Arnd Bergmann
2021-09-28 15:41 ` [PATCH 10/14] ARM: only warn about XIP address when not compile testing Arnd Bergmann
2021-09-28 15:41 ` [PATCH 11/14] ARM: kasan: work around LPAE build warning Arnd Bergmann
2021-09-28 15:41 ` [PATCH 12/14] ARM: add CONFIG_PHYS_OFFSET default values Arnd Bergmann
2021-09-28 16:02 ` Nicolas Pitre
2021-09-28 21:44 ` Linus Walleij
2021-09-28 15:41 ` [PATCH 13/14] ARM: use .arch directives instead of assembler command line flags Arnd Bergmann
2021-09-28 17:10 ` Nick Desaulniers
2021-09-28 18:32 ` Arnd Bergmann
2021-09-29 18:11 ` Nick Desaulniers
2022-01-21 22:17 ` Nathan Chancellor
2021-09-28 15:41 ` [PATCH 14/14] [RFC] ARM: forbid ftrace with clang and thumb2_kernel Arnd Bergmann
2021-09-28 17:20 ` Nick Desaulniers
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