linux-kernel - Re: [PATCH 2/2] nds32: use get_kernel_nofault in dump

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Date:   Tue, 21 Jul 2020 19:07:17 +0800
From:   Nick Hu <nickhu@...estech.com>
To:     Christoph Hellwig <hch@....de>
CC:     <green.hu@...il.com>, <deanbo422@...il.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] nds32: use get_kernel_nofault in dump_mem

On Mon, Jul 20, 2020 at 01:44:48PM +0200, Christoph Hellwig wrote:
> Use the proper get_kernel_nofault helper to access an unsafe kernel
> pointer without faulting instead of playing with set_fs and get_user.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
>  arch/nds32/kernel/traps.c | 15 +++------------
>  1 file changed, 3 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/nds32/kernel/traps.c b/arch/nds32/kernel/traps.c
> index b66f889bc6df9b..ee0d9ae192a504 100644
> --- a/arch/nds32/kernel/traps.c
> +++ b/arch/nds32/kernel/traps.c
> @@ -25,17 +25,8 @@ extern void show_pte(struct mm_struct *mm, unsigned long addr);
>  void dump_mem(const char *lvl, unsigned long bottom, unsigned long top)
>  {
>  	unsigned long first;
> -	mm_segment_t fs;
>  	int i;
>  
> -	/*
> -	 * We need to switch to kernel mode so that we can use __get_user
> -	 * to safely read from kernel space.  Note that we now dump the
> -	 * code first, just in case the backtrace kills us.
> -	 */
> -	fs = get_fs();
> -	set_fs(KERNEL_DS);
> -
>  	pr_emerg("%s(0x%08lx to 0x%08lx)\n", lvl, bottom, top);
>  
>  	for (first = bottom & ~31; first < top; first += 32) {
> @@ -48,7 +39,9 @@ void dump_mem(const char *lvl, unsigned long bottom, unsigned long top)
>  		for (p = first, i = 0; i < 8 && p < top; i++, p += 4) {
>  			if (p >= bottom && p < top) {
>  				unsigned long val;
> -				if (__get_user(val, (unsigned long *)p) == 0)
> +
> +				if (get_kernel_nofault(val,
> +						(unsigned long *)p) == 0)
>  					sprintf(str + i * 9, " %08lx", val);
>  				else
>  					sprintf(str + i * 9, " ????????");
> @@ -56,8 +49,6 @@ void dump_mem(const char *lvl, unsigned long bottom, unsigned long top)
>  		}
>  		pr_emerg("%s%04lx:%s\n", lvl, first & 0xffff, str);
>  	}
> -
> -	set_fs(fs);
>  }
>  
>  EXPORT_SYMBOL(dump_mem);
> -- 
> 2.27.0
>
Hi Christoph,

Thank you!
Acked-by: Nick Hu <nickhu@...estech.com>