Re: [PATCH] block: fix get_max_segment_size() overflow on 32bit arch

[Ming Lei <ming.lei@redhat.com>]

On Wed, Jan 08, 2020 at 02:38:02AM +0000, Damien Le Moal wrote:
> On 2020/01/08 11:34, Jens Axboe wrote:
> > On 1/7/20 7:06 PM, Damien Le Moal wrote:
> >> On 2020/01/08 10:25, Ming Lei wrote:
> >>> Commit 429120f3df2d starts to take account of segment's start dma address
> >>> when computing max segment size, and data type of 'unsigned long'
> >>> is used to do that. However, the segment mask may be 0xffffffff, so
> >>> the figured out segment size may be overflowed because DMA address can
> >>> be 64bit on 32bit arch.
> >>>
> >>> Fixes the issue by using 'unsigned long long' to compute max segment
> >>> size.
> >>>
> >>> Fixes: 429120f3df2d ("block: fix splitting segments on boundary masks")
> >>> Reported-by: Guenter Roeck <linux@roeck-us.net>
> >>> Tested-by: Guenter Roeck <linux@roeck-us.net>
> >>> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> >>> ---
> >>>  block/blk-merge.c | 4 ++--
> >>>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/block/blk-merge.c b/block/blk-merge.c
> >>> index 347782a24a35..b0fcc72594cb 100644
> >>> --- a/block/blk-merge.c
> >>> +++ b/block/blk-merge.c
> >>> @@ -159,12 +159,12 @@ static inline unsigned get_max_io_size(struct request_queue *q,
> >>>  
> >>>  static inline unsigned get_max_segment_size(const struct request_queue *q,
> >>>  					    struct page *start_page,
> >>> -					    unsigned long offset)
> >>> +					    unsigned long long offset)
> >>>  {
> >>>  	unsigned long mask = queue_segment_boundary(q);
> >>>  
> >>>  	offset = mask & (page_to_phys(start_page) + offset);
> >>
> >> Shouldn't mask be an unsigned long long too for this to give the
> >> expected correct result ?
> > 
> > Don't think so, and the seg boundary is a ulong to begin with as well.
> > 
> 
> I was referring to 32bits arch were ulong is 32bits. So we would have
> 
> offset = 32bits & 64bits;
> 
> with the patch applied. But I am not sure how gcc handles that and if
> this can be a problem.

Both are 'unsigned', so C compiler will do zero extension for 32bits.


Thanks,
Ming