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Re: [PATCH 1/3] utils: Improve qemu_strtosz() to have 64 bits of precisi

From: Eric Blake
Subject: Re: [PATCH 1/3] utils: Improve qemu_strtosz() to have 64 bits of precision
Date: Fri, 5 Feb 2021 08:15:23 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0

On 2/5/21 4:28 AM, Richard W.M. Jones wrote:
> On Thu, Feb 04, 2021 at 01:07:06PM -0600, Eric Blake wrote:
>> We have multiple clients of qemu_strtosz (qemu-io, the opts visitor,
>> the keyval visitor), and it gets annoying that edge-case testing is
>> impacted by implicit rounding to 53 bits of precision due to parsing
>> with strtod().  As an example posted by Rich Jones:
>>  $ nbdkit memory $(( 2**63 - 2**30 )) --run \
>>    'build/qemu-io -f raw "$uri" -c "w -P 3 $(( 2**63 - 2**30 - 512 )) 512" '
>>  write failed: Input/output error
>> because 9223372035781033472 got rounded to 0x7fffffffc0000000 which is
>> out of bounds.
>> It is also worth noting that our existing parser, by virtue of using
>> strtod(), accepts decimal AND hex numbers, even though test-cutils
>> previously lacked any coverage of the latter.  We do have existing
>> clients that expect a hex parse to work (for example, iotest 33 using
>> qemu-io -c "write -P 0xa 0x200 0x400"), but strtod() parses "08" as 8
>> rather than as an invalid octal number, so we know there are no
>> clients that depend on octal.  Our use of strtod() also means that
>> "0x1.8k" would actually parse as 1536 (the fraction is 8/16), rather
>> than 1843 (if the fraction were 8/10); but as this was not covered in
>> the testsuite, I have no qualms forbidding hex fractions as invalid,
>> so this patch declares that the use of fractions is only supported
>> with decimal input, and enhances the testsuite to document that.
>> Our previous use of strtod() meant that -1 parsed as a negative; now
>> that we parse with strtoull(), negative values can wrap around module
> ^^ modulo
> The patch looked fine to me although Vladimir found some problems
> which I didn't spot.  I have a question: What happens with leading or
> trailing whitespace?  Is that ignored, rejected or impossible?

leading whitespace: ignored (because both strtod() pre-patch, and now
strtoull() post-patch, do so for free).  And that is why we have to
memchr() (and not strchr(), as pointed out by Vladimir) for a '-' sign,
because merely checking *nptr=='-' would be wrong in the presence of
leading space.

trailing whitespace: treated the same as any other trailing garbage
(again, what strtod() and strtoull() give you for free).  If endptr was
non-NULL, then *endptr now points to that trailing space; if it was
NULL, the parse is rejected because of the trailing garbage.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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