Re: [PATCH 05/17] arm64: entry: add a call_on_stack helper

From: Anshuman Khandual <anshuman.khandual@arm.com>
To: Mark Rutland <mark.rutland@arm.com>,
	linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com,
	will@kernel.org, james.morse@arm.com
Cc: keescook@chromium.org, maz@kernel.org, broonie@kernel.org,
	labbott@redhat.com, robin.murphy@arm.com,
	julien.thierry.kdev@gmail.com, alex.popov@linux.com
Subject: Re: [PATCH 05/17] arm64: entry: add a call_on_stack helper
Date: Thu, 9 Jan 2020 13:30:02 +0530	[thread overview]
Message-ID: <98534fbd-5600-5ae2-a117-3bad76ddbe04@arm.com> (raw)
In-Reply-To: <20200108185634.1163-6-mark.rutland@arm.com>

On 01/09/2020 12:26 AM, Mark Rutland wrote:
> In some cases, we want to call a function from C code, using an
> alternative stack. Add a helper that we can use in such cases.
> 
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: James Morse <james.morse@arm.com>
> Cc: Will Deacon <will@kernel.org>
> ---
>  arch/arm64/include/asm/exception.h |  2 ++
>  arch/arm64/kernel/entry.S          | 21 +++++++++++++++++++++
>  2 files changed, 23 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/exception.h b/arch/arm64/include/asm/exception.h
> index b87c6e276ab1..a49038fa4faf 100644
> --- a/arch/arm64/include/asm/exception.h
> +++ b/arch/arm64/include/asm/exception.h
> @@ -31,6 +31,8 @@ static inline u32 disr_to_esr(u64 disr)
>  	return esr;
>  }
>  
> +asmlinkage void call_on_stack(struct pt_regs *, void (*)(struct pt_regs *),
> +			      unsigned long);
>  asmlinkage void enter_from_user_mode(void);
>  void do_mem_abort(unsigned long addr, unsigned int esr, struct pt_regs *regs);
>  void do_sp_pc_abort(unsigned long addr, unsigned int esr, struct pt_regs *regs);
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 53ce1877a4aa..184313c773ea 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -901,6 +901,27 @@ ENTRY(ret_from_fork)
>  ENDPROC(ret_from_fork)
>  NOKPROBE(ret_from_fork)
>  
> +/*
> + * x0 = argument to function

A small nit. Though the definition here itself does not limit the
argument type, it might worth to mention that to be struct pt_regs
per the previous declaration.

> + * x1 = function to call
> + * x2 = new stack pointer
> + */
> +ENTRY(call_on_stack)
> +	/* Create a frame record to save our LR and SP (implicit in FP) */
> +	stp	x29, x30, [sp, #-16]!
> +	mov	x29, sp
> +
> +	/* Move to the new stack and call the function there */
> +	mov	sp, x2
> +	blr	x1
> +
> +	/* Restore SP from the FP, FP and LR from the record, and return */
> +	mov	sp, x29
> +	ldp	x29, x30, [sp], #16
> +	ret
> +ENDPROC(call_on_stack)
> +NOKPROBE(call_on_stack)
> +
>  #ifdef CONFIG_ARM_SDE_INTERFACE
>  
>  #include <asm/sdei.h>
> 

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>

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