Re: [RFC PATCH V3 2/2] fs : Add sanity checks for block size > PAGE_SIZE

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 21 Jan 2014 17:00:00 +0530 Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com> wrote:

> We could hit null pointer dereference error during alloc_page_buffers
> in : (1) block size > PAGE_SIZE (2) low memory.
> Add sanity check for that.
> 
> Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
> ---
>  fs/block_dev.c | 1 +
>  fs/buffer.c    | 6 ++++++
>  2 files changed, 7 insertions(+)
> 
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 1e86823..2481d42 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -1027,6 +1027,7 @@ void bd_set_size(struct block_device *bdev, loff_t size)
>  			break;
>  		bsize <<= 1;
>  	}
> +	BUG_ON(bsize > PAGE_SIZE);
>  	bdev->bd_block_size = bsize;
>  	bdev->bd_inode->i_blkbits = blksize_bits(bsize);
>  }

alloc_page_buffers() will always return NULL if passed size >=
PAGE_SIZE.  So if we're going to add a check, it would be better to add
it to alloc_page_buffers() because that will catch errors from the
widest range of callsites.

But alloc_page_buffers() is pretty frequently called and I'd be
inclined to not add any check - most callers will just go oops and that
will provide basically the same information.

> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -1571,6 +1571,12 @@ void create_empty_buffers(struct page *page,
>  	struct buffer_head *bh, *head, *tail;
>  
>  	head = alloc_page_buffers(page, blocksize, 1);
> +
> +	/*
> + 	 * alloc_page_buffers() could return NULL on (1) bs > PAGE_SIZE
> + 	 * (2) low memory case. Ensure that we don't dereference null ptr
> + 	 */  
> +	BUG_ON(!head);

This is unneeded.

- bs > PAGE_SIZE can be checked elsewhere in a direct fashion

- low memory case can't happen - we passed retry=1

- create_empty_buffers() will immediately go oops if head==NULL. 
  That oops contains the same info as is presented by a BUG().