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

[Eric Blake]

On 2/4/21 1:07 PM, 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

> 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).

inadvertently

> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> 
> ---
> 

Is it a bad sign when I review my own code?

> 
>  /*
> - * 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.

Reformatted existing text, but incomplete; we also support 'P' and 'E'.

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

If we take patch 2 and 3, this contract should also be updated to
mention what we consider to be deprecated.

>   */
>  static int do_strtosz(const char *nptr, const char **end,
>                        const char default_suffix, int64_t unit,
> @@ -253,40 +264,66 @@ static int do_strtosz(const char *nptr, const char 
> **end,
>      int retval;
>      const char *endptr;
>      unsigned char c;
> -    int mul_required = 0;
> -    double val, mul, integral, fraction;
> +    bool mul_required = false;
> +    uint64_t val;
> +    int64_t mul;
> +    double fraction = 0.0;
> 
> -    retval = qemu_strtod_finite(nptr, &endptr, &val);
> +    /* Parse integral portion as decimal. */
> +    retval = qemu_strtou64(nptr, &endptr, 10, &val);
>      if (retval) {
>          goto out;
>      }
> -    fraction = modf(val, &integral);
> -    if (fraction != 0) {
> -        mul_required = 1;
> +    if (strchr(nptr, '-') != NULL) {
> +        retval = -ERANGE;
> +        goto out;
> +    }
> +    if (val == 0 && (*endptr == 'x' || *endptr == 'X')) {
> +        /* Input looks like hex, reparse, and insist on no fraction. */
> +        retval = qemu_strtou64(nptr, &endptr, 16, &val);
> +        if (retval) {
> +            goto out;
> +        }
> +        if (*endptr == '.') {
> +            endptr = nptr;
> +            retval = -EINVAL;
> +            goto out;
> +        }
> +    } else if (*endptr == '.') {
> +        /* Input is fractional, insist on 0 <= fraction < 1, with no 
> exponent */
> +        retval = qemu_strtod_finite(endptr, &endptr, &fraction);
> +        if (retval) {
> +            endptr = nptr;
> +            goto out;
> +        }
> +        if (fraction >= 1.0 || memchr(nptr, 'e', endptr - nptr)
> +            || memchr(nptr, 'E', endptr - nptr)) {
> +            endptr = nptr;
> +            retval = -EINVAL;
> +            goto out;
> +        }
> +        if (fraction != 0) {
> +            mul_required = true;
> +        }
>      }
>      c = *endptr;
>      mul = suffix_mul(c, unit);
> -    if (mul >= 0) {
> +    if (mul > 0) {
>          endptr++;
>      } else {
>          mul = suffix_mul(default_suffix, unit);
> -        assert(mul >= 0);
> +        assert(mul > 0);
>      }
>      if (mul == 1 && mul_required) {
> +        endptr = nptr;
>          retval = -EINVAL;
>          goto out;
>      }
> -    /*
> -     * Values near UINT64_MAX overflow to 2**64 when converting to double
> -     * precision.  Compare against the maximum representable double precision
> -     * value below 2**64, computed as "the next value after 2**64 (0x1p64) in
> -     * the direction of 0".
> -     */
> -    if ((val * mul > nextafter(0x1p64, 0)) || val < 0) {
> +    if (val > UINT64_MAX / mul) {

Hmm, do we care about:
15.9999999999999999999999999999E
where the fractional portion becomes large enough to actually bump our
sum below to 16E which indeed overflows?  Then again, we rejected a
fraction of 1.0 above, and 0.9999999999999999999999999999 parses to 1.0
due to rounding.
Maybe it's just worth a good comment why the overflow check here works
without consulting fraction.

>          retval = -ERANGE;
>          goto out;
>      }
> -    *result = val * mul;
> +    *result = val * mul + (uint64_t) (fraction * mul);
>      retval = 0;
> 
>  out:
-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org