linux-kernel - Re: [PATCH -next 1/1] iomap: Fix a false positive of UBSAN in iomap_seek

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Date:   Fri, 2 Jul 2021 19:50:25 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     "Darrick J . Wong" <djwong@...nel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next 1/1] iomap: Fix a false positive of UBSAN in
 iomap_seek_data()



On 2021/7/2 17:34, Christoph Hellwig wrote:
> We might as well just kill off the length variable while we're at it:

Hi, Christoph:
  Maybe you need to write a separate patch. Because the patch I sent is
to modify function iomap_seek_data(). I didn't look at the other functions.
In fact, both iomap_seek_data() and iomap_seek_hole() need to be modified.
The iomap_seek_data() may not be intuitive to delete the variable 'length'.

I'm now analyzing if the "if (length <= 0)" statement in iomap_seek_data()
is redundant (the condition is never true).

> 
> 
> diff --git a/fs/iomap/seek.c b/fs/iomap/seek.c
> index dab1b02eba5b7f..942e354e9e13e6 100644
> --- a/fs/iomap/seek.c
> +++ b/fs/iomap/seek.c
> @@ -35,23 +35,21 @@ loff_t
>  iomap_seek_hole(struct inode *inode, loff_t offset, const struct iomap_ops *ops)
>  {
>  	loff_t size = i_size_read(inode);
> -	loff_t length = size - offset;
>  	loff_t ret;
>  
>  	/* Nothing to be found before or beyond the end of the file. */
>  	if (offset < 0 || offset >= size)
>  		return -ENXIO;
>  
> -	while (length > 0) {
> -		ret = iomap_apply(inode, offset, length, IOMAP_REPORT, ops,
> -				  &offset, iomap_seek_hole_actor);
> +	while (offset < size) {
> +		ret = iomap_apply(inode, offset, size - offset, IOMAP_REPORT,
> +				  ops, &offset, iomap_seek_hole_actor);
>  		if (ret < 0)
>  			return ret;
>  		if (ret == 0)
>  			break;
>  
>  		offset += ret;
> -		length -= ret;
>  	}
>  
>  	return offset;
> 
> .
>