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

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Message-ID: <20130104040505.GA27833@gmail.com>
Date:	Fri, 4 Jan 2013 12:05:05 +0800
From:	Zheng Liu <gnehzuil.liu@...il.com>
To:	Theodore Ts'o <tytso@....edu>
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 03:38:58PM -0500, Theodore Ts'o wrote:
> 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.

Hi Ted, 

I wonder why wanted is 16k.  If a program calls ext2fs_file_read()
function, seek will be 0 and SEEK flag won't be marked.  The behavior of
ext2fs_file_read() is the same as before.  If ext2fs_file_read2() is
called by dump_file(), seek won't be 0 and wanted is always equal to
block size.  That is why I fix the hard-coded buffer length in dump_file().
If I miss something, please let me know.

Thanks,
                                                - Zheng

> 
> 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.
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