linux-ext4 - Re: [PATCH 2/2 v2] debugfs: dump a sparse file as a new sparse file

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Date:	Tue, 1 Jan 2013 15:38:58 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Zheng Liu <gnehzuil.liu@...il.com>
Cc:	linux-ext4@...r.kernel.org, George Spelvin <linux@...izon.com>,
	Zheng Liu <wenqing.lz@...bao.com>
Subject: Re: [PATCH 2/2 v2] debugfs: dump a sparse file as a new sparse file

On Tue, Jan 01, 2013 at 08:30:15PM +0800, Zheng Liu wrote:
> +errcode_t ext2fs_file_read2(ext2_file_t file, void *buf,
> +			    unsigned int wanted, unsigned int *got,
> +			    ext2_off64_t *seek)

I'm a bit concenred about this abstraction.  Consider what happens if
wanted is greater than a block size --- for example, consider if
wanted is 16k, and every other 1k block is uninitialized.

Then ext2fs_file_read2() will return *got set to 8k, and *seek set to
8k, and the buffer will contain the blocks that are initialized packed
up right against each other.

Worse, ext2fs_file_read() will do the same thing, so this commit
changes how ext2fs_file_read() functions, and a program which expects
to get the correct contents from the file will malfunction.

       	   	   	    	     	       - Ted
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