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

From: Richard W.M. Jones
Subject: Re: [PATCH 1/3] utils: Improve qemu_strtosz() to have 64 bits of precision
Date: Fri, 5 Feb 2021 10:28:05 +0000
User-agent: Mutt/1.5.21 (2010-09-15)

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?


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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