Re: BTRFS_SEARCH_ARGS_BUFSIZE too small

[David Sterba <dsterba@suse.cz>]

On Fri, Jan 10, 2014 at 12:44:17AM +0100, Gerhard Heift wrote:
> I'm playing around with the BTRFS_IOC_SEARCH_TREE to extract the csums
> of the physical blocks. During the tests some item_header had len = 0,
> which indicates the buffer was to small to hold the item. I added a
> printk into the kernel to get the original size of the item and it was
> around 6600 bytes.

This can happen with the metadata blocks bigger than 4k, the search
ioctl does not have buffer large enough as you've found. The EXTENT_CSUM
items seem to cap at 16k, no matter what's the node size (tried with
64k).

> Is there another way to get the item? Otherwise I would suggest to
> create an ioctl, which is a little bit more flexible, something like
> 
> struct btrfs_ioctl_search_args2 {
>         struct btrfs_ioctl_search_key key;
>         __u64 buf_len
>         char buf[0];
> };

Yeah, a V2 ioctl has to be introduced, the structure could look like
this. Kernel would have uo directly copy_to_user to the buffer without
the intermediate memcpy to the in-kernel copy of the ioctl structure.


david