linux-ext4 - Re: [PATCH] ext4: Fix use of uninitialized data

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Message-ID: <4843C52D.20400@cn.fujitsu.com>
Date:	Mon, 02 Jun 2008 18:02:21 +0800
From:	Shen Feng <shen@...fujitsu.com>
To:	Theodore Tso <tytso@....edu>
CC:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	cmm@...ibm.com, sandeen@...hat.com, linux-ext4@...r.kernel.org,
	alex@...sterfs.com, adilger@....com
Subject: Re: [PATCH] ext4: Fix use of uninitialized data



Theodore Tso Wrote:
> On Thu, May 15, 2008 at 12:17:11AM +0530, Aneesh Kumar K.V wrote:
>> @@ -3134,8 +3135,7 @@ static void ext4_mb_use_inode_pa(struct ext4_allocation_context *ac,
>>  static void ext4_mb_use_group_pa(struct ext4_allocation_context *ac,
>>  				struct ext4_prealloc_space *pa)
>>  {
>> -	unsigned len = ac->ac_o_ex.fe_len;
>> -
>> +	unsigned int len = ac->ac_o_ex.fe_len;
>>  	ext4_get_group_no_and_offset(ac->ac_sb, pa->pa_pstart,
>>  					&ac->ac_b_ex.fe_group,
>>  					&ac->ac_b_ex.fe_start);
>> -- 
> 
> This change had nothing to do with fixing the use of unitialized data,
> but when I started looking more closely, it raised a potential signed
> vs. unsigned issue: ac_o_ex is a struct ext4_free_extent, and fe_len
> is an int.
> 
> So here we are assigning an int to an unsigned int.  Later, len is
> assigned to ac_b_ex.len, which means assigning an unsigned int to an
> int.  In other places, fe_len (an int) is compared against pa_free
> (which is an unsigned short), and fe_len gets assined to pa_free, once
> again mixing signed and unsigned.
> 
> Can someone who is really familiar with this code check this out?  I
> think the following pseudo-patch to mballoc.h might be in order:
> 
>  struct ext4_free_extent {
>  	ext4_lblk_t fe_logical;
>  	ext4_grpblk_t fe_start;
>  	ext4_group_t fe_group;
> -	int fe_len;
> +	unsigned int fe_len;
>  };
> 

I'm studying the ext4 code these days.
The data types always confuse me.

The length of a ext4_extent ee_len is define as unsigned short.

struct ext4_extent {
	__le32	ee_block;	/* first logical block extent covers */
	__le16	ee_len;		/* number of blocks covered by extent */
	__le16	ee_start_hi;	/* high 16 bits of physical block */
	__le32	ee_start_lo;	/* low 32 bits of physical block */
};

So I think fe_len should also be defined as unsigned short.
Is that right?

-Shen Feng
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