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

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Date:	Mon, 2 Jun 2008 19:47:19 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Theodore Tso <tytso@....edu>, cmm@...ibm.com,
	linux-ext4@...r.kernel.org, alex@...sterfs.com, adilger@....com
Subject: Re: [PATCH] ext4: Fix use of uninitialized data

On Mon, Jun 02, 2008 at 08:42:24AM -0500, Eric Sandeen 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;
> >  };
> 
> Hm, ok, so what's going on here:
> 
> ext4_mb_normalize_group_request()
> {
> ...
>         if (EXT4_SB(sb)->s_stripe)
>                 ac->ac_g_ex.fe_len = EXT4_SB(sb)->s_stripe;
>         else
>                 ac->ac_g_ex.fe_len = EXT4_SB(sb)->s_mb_group_prealloc;
> ...
> }
> 
> and that's a long:
> 
>         unsigned long s_mb_group_prealloc;
> 
> Oh, but that's only ever assigned as
> 
>         sbi->s_mb_group_prealloc = MB_DEFAULT_GROUP_PREALLOC;
> 
> which is
> 
> /*
>  * default group prealloc size 512 blocks
>  */
> #define MB_DEFAULT_GROUP_PREALLOC       512
> 
> 
> so it's fine... but why are we carrying around a field in the sbi to
> hold a constant that cannot be changed runtime?

We can tune that via MB_PROC_FOPS(group_prealloc);


-aneesh

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