linux-kernel - Re: [PATCH v2] kexec: copy only happens before uchunk goes to zero

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Message-ID: <Zdb7ogmu/7IOBN3U@MiWiFi-R3L-srv>
Date: Thu, 22 Feb 2024 15:45:38 +0800
From: Baoquan He <bhe@...hat.com>
To: "yang.zhang" <gaoshanliukou@....com>
Cc: ebiederm@...ssion.com, kexec@...ts.infradead.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	"yang.zhang" <yang.zhang@...intek.com>
Subject: Re: [PATCH v2] kexec: copy only happens before uchunk goes to zero

On 02/20/24 at 10:11am, yang.zhang wrote:
> From: "yang.zhang" <yang.zhang@...intek.com>
> 
> When load segments, all of the copying and the rest
> only happens before uchunk goes to zero.

The code change looks good, while the log can be improved with more
details so that people can more easily get what's wrong and fixed.

===
When loading segments, ubytes is <= mbytes. When ubytes is exhausted,
there could be remaining mbytes. Then in the while loop, the buf pointer
advancing with mchunk will causing meaningless reading even though it
doesn't harm. 

So let's change to make sure that all of the copying and the rest only
happens before uchunk goes to zero.
===

Please take above words as reference to udpate patch log and post v3,
and you can add:

Acked-by: Baoquan He <bhe@...hat.com>

> 
> Signed-off-by: yang.zhang <yang.zhang@...intek.com>
> 
> ---
> v1 -> v2:
> - Only copy before uchunk goes to zero
> 
> V1: https://lore.kernel.org/lkml/20240130101802.23850-1-gaoshanliukou@163.com/
> ---
>  kernel/kexec_core.c | 44 ++++++++++++++++++++++++--------------------
>  1 file changed, 24 insertions(+), 20 deletions(-)
> 
> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> index d08fc7b5db97..2fc3d0e3715a 100644
> --- a/kernel/kexec_core.c
> +++ b/kernel/kexec_core.c
> @@ -800,22 +800,24 @@ static int kimage_load_normal_segment(struct kimage *image,
>  				PAGE_SIZE - (maddr & ~PAGE_MASK));
>  		uchunk = min(ubytes, mchunk);
>  
> -		/* For file based kexec, source pages are in kernel memory */
> -		if (image->file_mode)
> -			memcpy(ptr, kbuf, uchunk);
> -		else
> -			result = copy_from_user(ptr, buf, uchunk);
> +		if (uchunk) {
> +			/* For file based kexec, source pages are in kernel memory */
> +			if (image->file_mode)
> +				memcpy(ptr, kbuf, uchunk);
> +			else
> +				result = copy_from_user(ptr, buf, uchunk);
> +			ubytes -= uchunk;
> +			if (image->file_mode)
> +				kbuf += uchunk;
> +			else
> +				buf += uchunk;
> +		}
>  		kunmap_local(ptr);
>  		if (result) {
>  			result = -EFAULT;
>  			goto out;
>  		}
> -		ubytes -= uchunk;
>  		maddr  += mchunk;
> -		if (image->file_mode)
> -			kbuf += mchunk;
> -		else
> -			buf += mchunk;
>  		mbytes -= mchunk;
>  
>  		cond_resched();
> @@ -866,11 +868,18 @@ static int kimage_load_crash_segment(struct kimage *image,
>  			memset(ptr + uchunk, 0, mchunk - uchunk);
>  		}
>  
> -		/* For file based kexec, source pages are in kernel memory */
> -		if (image->file_mode)
> -			memcpy(ptr, kbuf, uchunk);
> -		else
> -			result = copy_from_user(ptr, buf, uchunk);
> +		if (uchunk) {
> +			/* For file based kexec, source pages are in kernel memory */
> +			if (image->file_mode)
> +				memcpy(ptr, kbuf, uchunk);
> +			else
> +				result = copy_from_user(ptr, buf, uchunk);
> +			ubytes -= uchunk;
> +			if (image->file_mode)
> +				kbuf += uchunk;
> +			else
> +				buf += uchunk;
> +		}
>  		kexec_flush_icache_page(page);
>  		kunmap_local(ptr);
>  		arch_kexec_pre_free_pages(page_address(page), 1);
> @@ -878,12 +887,7 @@ static int kimage_load_crash_segment(struct kimage *image,
>  			result = -EFAULT;
>  			goto out;
>  		}
> -		ubytes -= uchunk;
>  		maddr  += mchunk;
> -		if (image->file_mode)
> -			kbuf += mchunk;
> -		else
> -			buf += mchunk;
>  		mbytes -= mchunk;
>  
>  		cond_resched();
> -- 
> 2.25.1
>