On Mon, 2015-09-14 at 14:57 +0200, Vitaly Kuznetsov wrote:
> Some string_get_size() calls (e.g.:
> string_get_size(1, 512, STRING_UNITS_10, ..., ...)
> string_get_size(15, 64, STRING_UNITS_10, ..., ...)
> ) result in an infinite loop. The problem is that if size is equal to
> divisor[units]/blk_size and is smaller than divisor[units] we'll end
> up with size == 0 when we start doing sf_cap calculations:
>
> For string_get_size(1, 512, STRING_UNITS_10, ..., ...) case:
> ...
> remainder = do_div(size, divisor[units]); -> size is 0, remainder is 1
> remainder *= blk_size; -> remainder is 512
> ...
> size *= blk_size; -> size is still 0
> size += remainder / divisor[units]; -> size is still 0
>
> The caller causing the issue is sd_read_capacity(), the problem was noticed
> on Hyper-V, such weird size was reported by host when scanning collides
> with device removal. This is probably a separate issue worth fixing, this
> patch is intended to prevent the library routing from infinite looping.
>
> Signed-off-by: Vitaly Kuznetsov <vkuzn...@redhat.com>
> ---
> lib/string_helpers.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/lib/string_helpers.c b/lib/string_helpers.c
> index c98ae81..d0e607c 100644
> --- a/lib/string_helpers.c
> +++ b/lib/string_helpers.c
> @@ -59,7 +59,7 @@ void string_get_size(u64 size, u64 blk_size, const enum
> string_size_units units,
> }
>
> exp = divisor[units] / (u32)blk_size;
Add a comment here explaining to those who come after why strict greater
than is necessary. Something like:
/* size must be strictly greater than exp here to ensure
* that remainder is greater than divisor[units] coming out of the if */
> - if (size >= exp) {
> + if (size > exp) {
> remainder = do_div(size, divisor[units]);
> remainder *= blk_size;
> i++;
So when pushed, you finally found the algorithmic problem. Well done.
Acked-by: James Bottomley <jbottom...@odin.com>
James
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