From 8b5bd5adf9e6d073e538e2e9de810f2f25379c3b Mon Sep 17 00:00:00 2001 From: Arnd Bergmann Date: Mon, 18 Oct 2021 15:30:07 +0100 Subject: ARM: 9137/1: disallow CONFIG_THUMB with ARMv4 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 :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. This turned up while build-testing with clang; for some reason, gcc never triggered the problem. Reviewed-by: Nick Desaulniers Reviewed-by: Ard Biesheuvel Signed-off-by: Arnd Bergmann Signed-off-by: Russell King (Oracle) --- arch/arm/mm/Kconfig | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'arch/arm/mm/Kconfig') 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 -- cgit v1.2.3