linux-kernel - Re: [PATCH v2] arm64/traps: Avoid unnecessary kernel/user pointer conversion

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Date:   Fri, 17 Sep 2021 10:36:28 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Amit Daniel Kachhap <amit.kachhap@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Vincenzo Frascino <Vincenzo.Frascino@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH v2] arm64/traps: Avoid unnecessary kernel/user pointer
 conversion

On Fri, Sep 17, 2021 at 11:28:11AM +0530, Amit Daniel Kachhap wrote:
> Annotating a pointer from kernel to __user and then back again requires
> an extra __force annotation to silent sparse warning. In call_undef_hook()
> this unnecessary complexity can be avoided by modifying the intermediate
> user pointer to unsigned long.
> 
> This way there is no inter-changeable use of user and kernel pointers
> and the code is consistent.
> 
> Note: This patch adds no functional changes to code.
> 
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Signed-off-by: Amit Daniel Kachhap <amit.kachhap@....com>

I like that this clarifies that `pc` is not necessarily a user pointer,
and this makes the relevant lines shorter and eaier to read, so FWIW:

Acked-by: Mark Rutland <mark.rutland@....com>

Thanks,
Mark.

> ---
> Changes from v1:
> * Instead of directly using instruction_pointer() for kernel pointers,
>   modified the pc type from void __user * to unsigned long as suggested
>   by Mark Rutland.
> 
>  arch/arm64/kernel/traps.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> index b03e383d944a..09236751283e 100644
> --- a/arch/arm64/kernel/traps.c
> +++ b/arch/arm64/kernel/traps.c
> @@ -400,11 +400,11 @@ static int call_undef_hook(struct pt_regs *regs)
>  	unsigned long flags;
>  	u32 instr;
>  	int (*fn)(struct pt_regs *regs, u32 instr) = NULL;
> -	void __user *pc = (void __user *)instruction_pointer(regs);
> +	unsigned long pc = instruction_pointer(regs);
>  
>  	if (!user_mode(regs)) {
>  		__le32 instr_le;
> -		if (get_kernel_nofault(instr_le, (__force __le32 *)pc))
> +		if (get_kernel_nofault(instr_le, (__le32 *)pc))
>  			goto exit;
>  		instr = le32_to_cpu(instr_le);
>  	} else if (compat_thumb_mode(regs)) {
> -- 
> 2.17.1
>