From: Leonard Michlmayr <leonard.michlmayr@gmail.com>
Subject: Re: [PATCH 11/28] ext4: correctly calculate number of blocks for
 fiemap
Date: Thu, 04 Mar 2010 22:44:58 +0100
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To: Akira Fujita <a-fujita@rs.jp.nec.com>, tytso@mit.edu
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Akira Fujita:
> <kernel-space>
> fs/ioctl.c ioctl_fimap():
> 
> filemap_check_ranges():
> 	len(~0ULL)
> 	new_len(4398046511103 = s_maxbytes)  <--- Because 'len > s_maxbytes'
> 
> fs/ext4/extents.c ext4_fiemap():
> 	last_blk = start(0) + len(4398046511103) - 1 >> s_blocksize_bits(11)
> 	         = 4294967295 (0xFFFFFFFF)
> 	len_blks = 4294967295 + 1 (0xFFFFFFFF + 0x00000001)
> 		 = 4294967296 (0x100000000)  <--- _OVERFLOW!!_
> 	
> ext4_ext_walk_space():
> 	num = 0
> 
> This overflow leads to incorrect output like the below,
> even though 2 extents exist.
> 

Thank you for pointing this out. I had not checked s_maxbytes.
Appearently s_maxbytes can be 1<<(32 + s_blocksize_bits) - 1. Therefore
the number of blocks in a file cannot be stored in a 32bit integer.

I have a patch that should fix it for fiemap. I have just compiled it
and I will do some testing and double checking tomorrow. I will send a
separate email with the patch.

regards
Leonard