linux-kernel - Re: mount -o loop with truncated dosdisk.img uses 100% cpu and can't be killed

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Date:	Fri, 13 Jul 2012 17:43:54 +0200
From:	Jan Kara <jack@...e.cz>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:	"Steven J. Magnani" <steve@...idescorp.com>,
	Stevie Trujillo <stevie.trujillo@...il.com>,
	linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
	Jeff Moyer <jmoyer@...hat.com>
Subject: Re: mount -o loop with truncated dosdisk.img uses 100% cpu and
 can't be killed

On Fri 13-07-12 04:21:44, OGAWA Hirofumi wrote:
> "Steven J. Magnani" <steve@...idescorp.com> writes:
> 
> > On Thu, 2012-07-12 at 16:28 +0200, Stevie Trujillo wrote: 
> >> Hello,
> >> 
> >> I was trying to create a bootdisk to update my BIOS, and accidentially
> >> made a 512byte image with only the FreeDOS header in it.
> >> 
> >> ( Linux 3.4.4 )
> >> # mount -o loop dosdisk.img /tmp
> >> ^C^C^C
> >> It uses 100% CPU and doesn't listen to me when I do ^C, kill -9 etc. I
> >> think this means it's stuck in the kernel?
> >
> > Here is the stack trace I get for the mount process:
> >
> > [root@...ezart smagnani]# cat /proc/1334/stack
> > [<ffffffff81077a5a>] __cond_resched+0x2a/0x40
> > [<ffffffff8110c4bb>] find_lock_page+0x3b/0x80
> > [<ffffffff8110cbaf>] find_or_create_page+0x3f/0xb0
> > [<ffffffff8119a692>] __getblk+0xf2/0x2a0
> > [<ffffffff8119a893>] __bread+0x13/0xb0
> > [<ffffffffa00fd1fe>] fat__get_entry+0x14e/0x220 [fat]
> > [<ffffffffa00fd596>] fat_get_short_entry+0x66/0xc0 [fat]
> > [<ffffffffa00ff765>] fat_subdirs+0x55/0x80 [fat]
> > [<ffffffffa0103d30>] fat_fill_super+0x810/0xa80 [fat]
> > [<ffffffffa00e821a>] vfat_fill_super+0x1a/0x20 [vfat]
> > [<ffffffff8116d61b>] mount_bdev+0x1cb/0x210
> > [<ffffffffa00e81f5>] vfat_mount+0x15/0x20 [vfat]
> > [<ffffffff8116e410>] mount_fs+0x20/0xd0
> > [<ffffffff81186e4f>] vfs_kern_mount+0x6f/0x100
> > [<ffffffff81187804>] do_kern_mount+0x54/0x110
> > [<ffffffff811891d6>] do_mount+0x306/0x8b0
> > [<ffffffff811898cd>] sys_mount+0x8d/0xe0
> > [<ffffffff81583629>] system_call_fastpath+0x16/0x1b
> > [<ffffffffffffffff>] 0xffffffffffffffff
> >
> > I would think this is more likely a bug in the loopback driver than the
> > FAT filesystem...
> 
> It looks like the bug of __getblk_slow(). If requested block was beyond
> end of device, __find_get_block() will find buffer_mapped()'s buffer,
> but block >= end_block is unmapped. So, it can be loop.
> 
> The following patch fixes it? If it fix, there are some options to check
> it.
> 
> a) Check it like this patch and warn.
> b) (a), but without warn.
> c) Check it in init_page_buffers() and return -EIO or such
> 
> Well, anyway, Cc to Jens.
> 
> Signed-off-by: OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
  I think Jeff Moyer has sent a similar fix recently. It may even be
already queued in Jens' tree. Jeff?

								Honza
> ---
> 
>  fs/buffer.c |    7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff -puN fs/buffer.c~debug fs/buffer.c
> --- tux3fs/fs/buffer.c~debug	2012-07-13 04:10:40.000000000 +0900
> +++ tux3fs-hirofumi/fs/buffer.c	2012-07-13 04:11:50.000000000 +0900
> @@ -1055,6 +1055,13 @@ __getblk_slow(struct block_device *bdev,
>  		dump_stack();
>  		return NULL;
>  	}
> +	if (block >= blkdev_max_block(I_BDEV(bdev->bd_inode))) {
> +		printk(KERN_ERR "getblk(): block %llu, end_block %llu\n",
> +		       (unsigned long long)block,
> +		       (unsigned long long)blkdev_max_block(I_BDEV(bdev->bd_inode)));
> +		dump_stack();
> +		return NULL;
> +	}
>  
>  	for (;;) {
>  		struct buffer_head * bh;
> _
> 
> -- 
> OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
> --
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-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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