linux-kernel - Re: __getblk infinite loop

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Date:	Fri, 5 Sep 2008 11:20:17 +0200
From:	Eric Sesterhenn <snakebyte@....de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Bob Copeland <me@...copeland.com>, linux-kernel@...r.kernel.org
Subject: Re: __getblk infinite loop

hi,

* Andrew Morton (akpm@...ux-foundation.org) wrote:
> On Thu, 4 Sep 2008 23:24:11 -0400 Bob Copeland <me@...copeland.com> wrote:
> 
> > Hi all,
> > 
> > Eric Sesterhenn and I were puzzling over a lockup found by his fsfuzzer.
> > 
> > sb_bread() calls __getblk, which says:
> > 
> > /*
> >  * __getblk will locate (and, if necessary, create) the buffer_head
> >  * which corresponds to the passed block_device, block and size. The
> >  * returned buffer has its reference count incremented.
> >  *
> >  * __getblk() cannot fail - it just keeps trying.  If you pass it an
> >  * illegal block number, __getblk() will happily return a buffer_head
> >  * which represents the non-existent block.  Very weird.
> >  *
> >  * __getblk() will lock up the machine if grow_dev_page's try_to_free_buffers()
> >  * attempt is failing.  FIXME, perhaps?
> >  */
> > 
> > In fact the following will cause an infinite loop when mounting omfs 
> > loopback (on 32 bit x86 at least):
> > 
> > diff --git a/fs/omfs/inode.c b/fs/omfs/inode.c
> > index a95fe59..80eacc8 100644
> > --- a/fs/omfs/inode.c
> > +++ b/fs/omfs/inode.c
> > @@ -413,6 +413,15 @@ static int omfs_fill_super(struct super_block *sb, void *data, int silent)
> >  	sector_t start;
> >  	int ret = -EINVAL;
> >  
> > +	if (1) {
> > +		sector_t foo = 0x1d4000004ULL;
> > +
> > +		sb_set_blocksize(sb, 2048);
> > +		bh = sb_bread(sb, foo);
> > +		brelse(bh);
> > +		goto end;
> > +	}
> > +
> >  	save_mount_options(sb, (char *) data);
> >  
> >  	sbi = kzalloc(sizeof(struct omfs_sb_info), GFP_KERNEL);
> > 
> > What's supposed to happen here?  I would have thought that sb_bread
> > would realize foo was outside the block dev and bail out, but instead
> > it just gets stuck.  Do I need to bounds-check anything passed to
> > sb_bread?
> 
> That loop does lock up on people occasionally - last time was in isofs,
> because it had done an insane set_blocksize() earlier on.
> 
> Yes, it's always a case of garbage in, garbage out (or nothing out, as
> the case may be).
> 
> No, it's not particularly programmer-friendly behaviour.

Wouldnt it make sense to limit the loop in __getblk_slow()?

Like only try five times and drop a warning?
Yesterday I changed free_more_memory() to
return the number of pages freed by try_to_free_pages() and abort
if no pages where freed. But it seems it always frees exactly one
page...

Greetings, Eric
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