[sheepdog] [PATCH v2 1/2] sheep: handle VID overflow correctly

[FUKUMOTO Yoshifumi]

On 2015/02/26 15:47, Hitoshi Mitake wrote:
> Current sheep cannot handle a case like this:
> 1. iterate snapshot creation and let latest working VDI have VID 0xffffff
> 2. create one more snapshot
>
> This problem comes from invalid usage of find_next_zero_bit() of sheep.
> Current sheep checks its VID bitmap with find_next_zero_bit(). But the
> function has a subtle point we must care about. If we check a bitmap
> and whose right most bit is 1, it returns a number of the bit though
> the bit is not 0. It means
>      /* the right most bit of sys->vdi_inuse is 1*/
>      find_next_zero_bit(sys->vdi_inuse, SD_NR_VDIS, 0);
> will return SD_NR_VDIS - 1 (not SD_NR_VDIS, of course). So it is not
> possible to check the right most bit is used or not by simply calling
> find_next_zero_bit(). So the existing code cannot handle the case of
> overflow.
>
> This patch solves the problem by let 0xffffff be an invalid VID (and
> as I describe later, 0x000000 will also be invalid). With this
> modification, we can simply ignore the return value 0xffffff of
> find_next_zero_bit() and the right most bit vdi_inuse is already used.
>
> And this patch also let 0x000000 be an invalid VID. It is for VID
> recycling. In some places of sheepdog, parent_vid is used as a
> value which indicates that the VDI has a parent (clone or snapshot) or
> not (working VDI). So this patch lets the first VID 0x000001 and
> prevent this sort of confusion.

I tried some border value tests.
The invalid VDIs (0x000000 and 0xffffff) could be created and creating snapshots 
of them failed, so creating invalid VDIs should be avoided.

example 1:
  $ dog vdi create 01909709 1G  (create 0x000000)
  $ dog vdi snapshot 01909709
  Failed to create snapshot for 01909709

example 2:
  $ dog vdi create 05421217 1G  (create 0xffffff)
  $ dog vdi snapshot 05421217
  $ dog vdi list -r
  s 05421217 1 1073741824 0 0 DATE ffffff 3 22  (only this line)

Thanks,
Yoshifumi

> Signed-off-by: Hitoshi Mitake <mitake.hitoshi at lab.ntt.co.jp>
> ---
>   sheep/vdi.c | 22 ++++++++++++++++------
>   1 file changed, 16 insertions(+), 6 deletions(-)
>
> diff --git a/sheep/vdi.c b/sheep/vdi.c
> index 8114fb5..7a9baa6 100644
> --- a/sheep/vdi.c
> +++ b/sheep/vdi.c
> @@ -1345,7 +1345,7 @@ out:
>   /*
>    * Return SUCCESS (range of bits set):
>    * Iff we get a bitmap range [left, right) that VDI might be set between. if
> - * right < start, this means a wrap around case where we should examine the
> + * right < left, this means a wrap around case where we should examine the
>    * two split ranges, [left, SD_NR_VDIS - 1] and [0, right). 'Right' is the free
>    * bit that might be used by newly created VDI.
>    *
> @@ -1360,10 +1360,10 @@ static int get_vdi_bitmap_range(const char *name, unsigned long *left,
>   	if (*left == *right)
>   		return SD_RES_NO_VDI;
>
> -	if (*right == SD_NR_VDIS) {
> +	if (*right == SD_NR_VDIS - 1) {
>   		/* Wrap around */
> -		*right = find_next_zero_bit(sys->vdi_inuse, SD_NR_VDIS, 0);
> -		if (*right == SD_NR_VDIS)
> +		*right = find_next_zero_bit(sys->vdi_inuse, SD_NR_VDIS, 1);
> +		if (*right == SD_NR_VDIS - 1)
>   			return SD_RES_FULL_VDI;
>   	}
>   	return SD_RES_SUCCESS;
> @@ -1404,7 +1404,7 @@ static int fill_vdi_info_range(uint32_t left, uint32_t right,
>   		ret = SD_RES_NO_MEM;
>   		goto out;
>   	}
> -	for (i = right - 1; i >= left; i--) {
> +	for (i = right - 1; i >= left && i; i--) {
>   		ret = sd_read_object(vid_to_vdi_oid(i), (char *)inode,
>   				     SD_INODE_HEADER_SIZE, 0);
>   		if (ret != SD_RES_SUCCESS)
> @@ -1448,10 +1448,20 @@ static int fill_vdi_info(unsigned long left, unsigned long right,
>   {
>   	int ret;
>
> +	assert(left != right);
> +	/*
> +	 * If left == right, fill_vdi_info() shouldn't called by vdi_lookup().
> +	 * vdi_lookup() must return SD_RES_NO_VDI to its caller.
> +	 */
> +
>   	if (left < right)
>   		return fill_vdi_info_range(left, right, iocb, info);
>
> -	ret = fill_vdi_info_range(0, right, iocb, info);
> +	if (likely(1 < right))
> +		ret = fill_vdi_info_range(1, right, iocb, info);
> +	else
> +		ret = SD_RES_NO_VDI;
> +
>   	switch (ret) {
>   	case SD_RES_NO_VDI:
>   	case SD_RES_NO_TAG:
>