Re: [Qemu-block] [Qemu-devel] [PATCH] xen_disk: convert discard input to byte ranges

[Olaf Hering]

On Fri, Nov 18, Eric Blake wrote:

> On 11/18/2016 04:24 AM, Olaf Hering wrote:
> > +    /* Overflowing byte limit? */
> > +    if ((sec_start + sec_count) > ((INT64_MAX + INT_MAX) >> 
> > BDRV_SECTOR_BITS)) {
> This is undefined.  INT64_MAX + anything non-negative overflows int64,

The expanded value used to be stored into a uint64_t before it was used
here. A "cleanup" introduced this error. Thanks for spotting.

> If you are trying to detect guests that make a request that would cover
> more than INT64_MAX bytes, you can simplify.  Besides, for as much
> storage as there is out there, I seriously doubt ANYONE will ever have
> 2^63 bytes addressable through a single device.  Why not just write it as:
> 
> if ((INT64_MAX >> BDRV_SECTOR_BITS) - sec_count < sec_start) {

That would always be false I think. I will resubmit with this:
if ((sec_start + sec_count) > (INT64_MAX >> BDRV_SECTOR_BITS)) {

Regarding the cast for ->req, it has type blkif_request_t, but the
pointer needs to be assigned to type blkif_request_discard_t.


Olaf

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