On 10/20/2017 10:03 AM, Kevin Wolf wrote: > Am 12.10.2017 um 05:47 hat Eric Blake geschrieben: >> We are gradually moving away from sector-based interfaces, towards >> byte-based. In the common case, allocation is unlikely to ever use >> values that are not naturally sector-aligned, but it is possible >> that byte-based values will let us be more precise about allocation >> at the end of an unaligned file that can do byte-based access. >>
>> int bdrv_block_status(BlockDriverState *bs, int64_t offset, int64_t bytes,
>> int64_t *pnum, int64_t *map, BlockDriverState **file)
>> {
>> - int64_t ret;
>> - int n;
>> -
>> - assert(QEMU_IS_ALIGNED(offset | bytes, BDRV_SECTOR_SIZE));
>> - assert(pnum);
>> - /*
>> - * The contract allows us to return pnum smaller than bytes, even
>> - * if the next query would see the same status; we truncate the
>> - * request to avoid overflowing the driver's 32-bit interface.
>> - */
>> - bytes = MIN(bytes, BDRV_REQUEST_MAX_BYTES);
>
> Is the limitation to BDRV_REQUEST_MAX_BYTES going away without being
> replaced by a new one in bdrv_co_block_status()? What protects us now
> from 32-bit truncation?
As of this patch, we have this 64-bit-clean call chain:
bdrv_block_status()
bdrv_block_status_above()
bdrv_common_block_status_above()
bdrv_block_status_above_co_entry()
bdrv_co_block_status_above()
bdrv_co_block_status()
which in turn has:
/*
* The contract allows us to return pnum smaller than bytes, even
* if the next query would see the same status; we truncate the
* request to avoid overflowing the driver's 32-bit interface.
*/
bytes = MIN(bytes, BDRV_REQUEST_MAX_BYTES);
added earlier in 8/24.
At the end of series 3, the truncation prevention is still in place when
calling into the drivers, per the changes to bdrv_co_block_status() in
21/24.
Later, in series 4, all drivers are converted to 64-bit interfaces,
where the drivers are either made 64-bit clean, or do their own
truncation to 32 bits.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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