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SubjectRe: [RFC PATCH V3 2/2] fs : Add sanity checks for block size > PAGE_SIZE
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().



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