Re: [PATCH 06/14] ARM: disallow CONFIG_THUMB with ARMv4
From: Ard Biesheuvel <ardb@kernel.org>
Date: 2021-10-06 16:01:37
Also in:
linux-arm-kernel, lkml
On Tue, 28 Sept 2021 at 17:42, Arnd Bergmann [off-list ref] 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>
quoted hunk ↗ jump to hunk
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