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

["Daniel P. Berrangé" <berrange@redhat.com>]

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
> 2^64, so we have to explicitly check whether the user passed in a '-'.
> 
> We also had no testsuite coverage of "1.1e0k", which happened to parse
> under strtod() but is unlikely to occur in practice; as long as we are
> making things more robust, it is easy enough to reject the use of
> exponents in a strtod parse.
> 
> The fix is done by breaking the parse into an integer prefix (no loss
> in precision), rejecting negative values (since we can no longer rely
> on strtod() to do that), determining if a decimal or hexadecimal parse
> was intended (with the new restriction that a fractional hex parse is
> not allowed), and where appropriate, using a floating point fractional
> parse (where we also scan to reject use of exponents in the fraction).
> The bulk of the patch is then updates to the testsuite to match our
> new precision, as well as adding new cases we reject (whether they
> were rejected or inadvertenly accepted before).
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> 


> diff --git a/util/cutils.c b/util/cutils.c
> index 0b5073b33012..0234763bd70b 100644
> --- a/util/cutils.c
> +++ b/util/cutils.c
> @@ -241,10 +241,21 @@ static int64_t suffix_mul(char suffix, int64_t unit)
>  }
> 
>  /*
> - * Convert string to bytes, allowing either B/b for bytes, K/k for KB,
> - * M/m for MB, G/g for GB or T/t for TB. End pointer will be returned
> - * in *end, if not NULL. Return -ERANGE on overflow, and -EINVAL on
> - * other error.
> + * Convert size string to bytes.
> + *
> + * Allow either B/b for bytes, K/k for KB, M/m for MB, G/g for GB or
> + * T/t for TB, with scaling based on @unit, and with @default_suffix
> + * implied if no explicit suffix was given.
> + *
> + * The end pointer will be returned in *end, if not NULL.  If there is
> + * no fraction, the input can be decimal or hexadecimal; if there is a
> + * fraction, then the input must be decimal and there must be a suffix
> + * (possibly by @default_suffix) larger than Byte, and the fractional
> + * portion may suffer from precision loss or rounding.  The input must
> + * be positive.

Even though the test suite gives some illustrations, I think we should
document here the patterns we're intending to support. IIUC, we aim for

[quote]
The size parsing supports the following syntaxes

 - 12345   - decimal, bytes
 - 12345{bBkKmMgGtT} - decimal, scaled bytes
 - 12345.678 - fractional decimal, bytes
 - 12345.678{bBkKmMgGtT} - fractional decimal, scaled bytes
 - 0x7FEE  - hex, bytes

The following are intentionally not supported

 - octal
 - fractional hex
 - floating point exponents
[/quote]

> + *
> + * Return -ERANGE on overflow (with *@end advanced), and -EINVAL on
> + * other error (with *@end left unchanged).
>   */

Regards,
Daniel
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