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

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

On Mon, Jun 02, 2008 at 06:02:21PM +0800, Shen Feng wrote:
> 
> 
> 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?

Extents and each prealloc space have at max 2**16 blocks. So the length
of both should be unsigned short. With respect to ext4_free_extent we
use fe_len to store the number of blocks requested for allocation.
( ext4_mb_initialize_context )

The allocated extent will definitely have <= 2**16. But the requested
number of blocks may not.

-aneesh