Re: [PATCH 2/2] hw/block/nvme: fix resource leak in nvme_format_ns

[Max Reitz <mreitz@redhat.com>]

On 22.03.21 11:48, Klaus Jensen wrote:
> On Mar 22 11:02, Max Reitz wrote:
>> On 22.03.21 07:19, Klaus Jensen wrote:
>>> From: Klaus Jensen <k.jensen@samsung.com>
>>>
>>> In nvme_format_ns(), if the namespace is of zero size (which might be
>>> useless, but not invalid), the `count` variable will leak. Fix this by
>>> returning early in that case.
>>
>> When looking at the Coverity report, something else caught my eye: As far as
>> I’m aware, blk_aio_pwrite_zeroes() may invoke the CB before returning (if
>> blk_do_pwritev_part() returns without yielding).  I don’t think that will
>> happen with real hardware (who knows, though), but it should be possible to
>> see with the null-co block driver.
>>
>> nvme_format_ns() doesn’t quite look like it takes that into account. For
>> example, because *count starts at 1 and is decremented after the while (len)
>> loop, all nvme_aio_format_cb() invocations (if they are invoked before their
>> blk_aio_pwrite_zeroes() returns) will see
>> *count == 2, and thus not free it, or call nvme_enqueue_req_completion().
>>
>> I don’t know whether the latter is problematic, but not freeing `count`
>> doesn’t seem right.  Perhaps this could be addressed by adding a condition
>> to the `(*count)--` to see whether `(*count)-- == 1` (or rather `--(*count)
>> == 0`), which would indicate that there are no AIO functions still in
>> flight?
> 
> Hi Max,
> 
> Thanks for glossing over this.
> 
> And, yeah, you are right, nice catch. The reference counting is exactly
> to guard against pwrite_zeroes invoking the CB before returning, but if
> it happens it is not correctly handling it anyway :(
> 
> This stuff is exactly why I proposed converting all this into the
> "proper" BlockAIOCB approach (see [1]), but it need a little more work.
> 
> I'll v2 this with a fix for this! Thanks!
> 
> 
>    [1]: https://lore.kernel.org/qemu-devel/20210302111040.289244-1-its@irrelevant.dk/

OK, thanks! :)

That rewrite does look more in-line with how AIO is handled elsewhere, yes.

Max