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Date:   Wed, 9 Mar 2022 13:51:13 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Marc Zyngier <maz@...nel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Yanteng Si <siyanteng01@...il.com>, linux-mips@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] MIPS: Only use current_stack_pointer on GCC

Nit: I think the subject needs to be updated (I assume this was written
before Nick's fix?).

On Wed, Mar 09, 2022 at 12:45:37PM -0800, Kees Cook wrote:
> Unfortunately, Clang did not have support for "sp" as a global register
> definition, and was crashing after the addition of current_stack_pointer.
> This has been fixed in Clang 15, but earlier Clang versions need to
> avoid this code, so add a versioned test and revert back to the
> open-coded asm instances. Fixes Clang build error:
> 
> fatal error: error in backend: Invalid register name global variable
> 
> Fixes: 200ed341b864 ("mips: Implement "current_stack_pointer"")
> Reported-by: Nathan Chancellor <nathan@...nel.org>
> Cc: Thomas Bogendoerfer <tsbogend@...ha.franken.de>
> Cc: Marc Zyngier <maz@...nel.org>
> Cc: Guenter Roeck <linux@...ck-us.net>
> Cc: Yanteng Si <siyanteng01@...il.com>
> Cc: linux-mips@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>

Thanks for sending this!

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

> ---
>  arch/mips/Kconfig                   | 2 +-
>  arch/mips/include/asm/thread_info.h | 2 ++
>  arch/mips/kernel/irq.c              | 3 ++-
>  arch/mips/lib/uncached.c            | 4 +++-
>  4 files changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
> index 3f58b45fc953..15769013f46e 100644
> --- a/arch/mips/Kconfig
> +++ b/arch/mips/Kconfig
> @@ -4,7 +4,7 @@ config MIPS
>  	default y
>  	select ARCH_32BIT_OFF_T if !64BIT
>  	select ARCH_BINFMT_ELF_STATE if MIPS_FP_SUPPORT
> -	select ARCH_HAS_CURRENT_STACK_POINTER
> +	select ARCH_HAS_CURRENT_STACK_POINTER if !CC_IS_CLANG || CLANG_VERSION >= 150000

Nit: This can be 140000, as release/14.x has received the fix:

https://github.com/llvm/llvm-project/commit/0826716786cd4a8c7cbcb8c01e4d9fac46b7a17a

>  	select ARCH_HAS_DEBUG_VIRTUAL if !64BIT
>  	select ARCH_HAS_FORTIFY_SOURCE
>  	select ARCH_HAS_KCOV
> diff --git a/arch/mips/include/asm/thread_info.h b/arch/mips/include/asm/thread_info.h
> index 4463348d2372..ecae7470faa4 100644
> --- a/arch/mips/include/asm/thread_info.h
> +++ b/arch/mips/include/asm/thread_info.h
> @@ -69,7 +69,9 @@ static inline struct thread_info *current_thread_info(void)
>  	return __current_thread_info;
>  }
>  
> +#ifdef CONFIG_ARCH_HAS_CURRENT_STACK_POINTER
>  register unsigned long current_stack_pointer __asm__("sp");
> +#endif
>  
>  #endif /* !__ASSEMBLY__ */
>  
> diff --git a/arch/mips/kernel/irq.c b/arch/mips/kernel/irq.c
> index fc313c49a417..5e11582fe308 100644
> --- a/arch/mips/kernel/irq.c
> +++ b/arch/mips/kernel/irq.c
> @@ -75,8 +75,9 @@ void __init init_IRQ(void)
>  #ifdef CONFIG_DEBUG_STACKOVERFLOW
>  static inline void check_stack_overflow(void)
>  {
> -	unsigned long sp = current_stack_pointer;
> +	unsigned long sp;
>  
> +	__asm__ __volatile__("move %0, $sp" : "=r" (sp));
>  	sp &= THREAD_MASK;
>  
>  	/*
> diff --git a/arch/mips/lib/uncached.c b/arch/mips/lib/uncached.c
> index f8d4ca046c3e..f80a67c092b6 100644
> --- a/arch/mips/lib/uncached.c
> +++ b/arch/mips/lib/uncached.c
> @@ -40,7 +40,9 @@ unsigned long run_uncached(void *func)
>  	register long ret __asm__("$2");
>  	long lfunc = (long)func, ufunc;
>  	long usp;
> -	long sp = current_stack_pointer;
> +	long sp;
> +
> +	__asm__("move %0, $sp" : "=r" (sp));
>  
>  	if (sp >= (long)CKSEG0 && sp < (long)CKSEG2)
>  		usp = CKSEG1ADDR(sp);
> -- 
> 2.32.0
> 

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