Re: [PATCH] ARM: fix gcc-10 thumb2-kernel regression

[Ard Biesheuvel]

On Wed, 12 May 2021 at 10:13, Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
>
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> When building the kernel wtih gcc-10 or higher using the
> CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE=y flag, the compiler picks a slightly
> different set of registers for the inline assembly in cpu_init() that
> subsequently results in a corrupt kernel stack as well as remaining in
> FIQ mode. If a banked register is used for the last argument, the wrong
> version of that register gets loaded into CPSR_c.  When building in Arm
> mode, the arguments are passed as immediate values and the bug cannot
> happen.
>
> This got introduced when Daniel reworked the FIQ handling and was
> technically always broken, but happened to work with both clang and gcc
> before gcc-10 as long as they picked one of the lower registers.
> This is probably an indication that still very few people build the
> kernel in Thumb2 mode.
>
> Marek pointed out the problem on IRC, Arnd narrowed it down to this
> inline assembly and Russell pinpointed the exact bug.
>
> Change the constraints to force the final mode switch to use a non-banked
> register for the argument to ensure that the correct constant gets loaded.
> Another alternative would be to always use registers for the constant
> arguments to avoid the #ifdef that has now become more complex.
>
> Cc: <stable@xxxxxxxxxxxxxxx> # v3.18+
> Cc: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
> Reported-by: Marek Vasut <marek.vasut@xxxxxxxxx>
> Fixes: c0e7f7ee717e ("ARM: 8150/3: fiq: Replace default FIQ handler")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>

Nice bug!

Acked-by: Ard Biesheuvel <ardb@xxxxxxxxxx>

> ---
>  arch/arm/kernel/setup.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
>
> diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
> index 1a5edf562e85..73ca7797b92f 100644
> --- a/arch/arm/kernel/setup.c
> +++ b/arch/arm/kernel/setup.c
> @@ -545,9 +545,11 @@ void notrace cpu_init(void)
>          * In Thumb-2, msr with an immediate value is not allowed.
>          */
>  #ifdef CONFIG_THUMB2_KERNEL
> -#define PLC    "r"
> +#define PLC_l  "l"
> +#define PLC_r  "r"
>  #else
> -#define PLC    "I"
> +#define PLC_l  "I"
> +#define PLC_r  "I"
>  #endif
>
>         /*
> @@ -569,15 +571,15 @@ void notrace cpu_init(void)
>         "msr    cpsr_c, %9"
>             :
>             : "r" (stk),
> -             PLC (PSR_F_BIT | PSR_I_BIT | IRQ_MODE),
> +             PLC_r (PSR_F_BIT | PSR_I_BIT | IRQ_MODE),
>               "I" (offsetof(struct stack, irq[0])),
> -             PLC (PSR_F_BIT | PSR_I_BIT | ABT_MODE),
> +             PLC_r (PSR_F_BIT | PSR_I_BIT | ABT_MODE),
>               "I" (offsetof(struct stack, abt[0])),
> -             PLC (PSR_F_BIT | PSR_I_BIT | UND_MODE),
> +             PLC_r (PSR_F_BIT | PSR_I_BIT | UND_MODE),
>               "I" (offsetof(struct stack, und[0])),
> -             PLC (PSR_F_BIT | PSR_I_BIT | FIQ_MODE),
> +             PLC_r (PSR_F_BIT | PSR_I_BIT | FIQ_MODE),
>               "I" (offsetof(struct stack, fiq[0])),
> -             PLC (PSR_F_BIT | PSR_I_BIT | SVC_MODE)
> +             PLC_l (PSR_F_BIT | PSR_I_BIT | SVC_MODE)
>             : "r14");
>  #endif
>  }
> --
> 2.29.2
>