Re: [PATCH v7 32/32] iotests: Add tests for qcow2 images with extended L2 entries

[Eric Blake <eblake@redhat.com>]

On 5/29/20 10:07 AM, Alberto Garcia wrote:
> On Wed 27 May 2020 08:30:06 PM CEST, Eric Blake wrote:
>>> +    offset=$(($offset + 8))
>>> +    bitmap=`peek_file_be "$TEST_IMG" $offset 8`
>>> +
>>> +    expected_bitmap=0
>>> +    for bit in $expected_alloc; do
>>> +        expected_bitmap=$(($expected_bitmap | (1 << $bit)))
>>> +    done
>>> +    for bit in $expected_zero; do
>>> +        expected_bitmap=$(($expected_bitmap | (1 << (32 + $bit))))
>>> +    done
>>> +    expected_bitmap=`printf "%llu" $expected_bitmap`
>>
>> Dead statement - expected_bitmap is already a 64-bit decimal number
>> without reprinting it to itself.
> 
> Not quite... it seems that simply expanding the variable treats the
> value as signed so echo $((1 << 63)) returns INT64_MIN. The printf call
> makes it unsigned,

Ah, yes, then that makes sense.  Still, you could shave a fork or 
comment the action by doing:

printf -v expected_bitmap %llu $expected_bitmap # convert to unsigned

> but even though I tried that in a 32-bit system and
> it works now I'm actually wondering about the portability of the whole
> thing.

Bash supports 64-bit numbers even on 32-bit platforms, and has done for 
years.  Since we are running the test only under bash, that's all the 
more we have to worry about.

> 
> Looking at the source it seems that bash uses intmax_t:
> 
>     https://git.savannah.gnu.org/cgit/bash.git/tree/variables.h?h=bash-5.0#n68
> 
> But if this is a problem then peek_file_* would also be affected, it
> also uses printf %llu and a few iotests are already reading 64bit values
> (grep 'peek_file_.* 8').

In cases where a negative number is read but the 64-bit pattern is the 
same, it doesn't matter; but here, you are using it for output, and so 
you do want the unsigned representation instead of bash's default signed 
representation.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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