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SubjectRe: [PATCH v5 2/4] lib: vsprintf: Fix handling of number field widths in vsscanf
On Mon, Feb 08, 2021 at 02:01:52PM +0000, Richard Fitzgerald wrote:
> The existing code attempted to handle numbers by doing a strto[u]l(),
> ignoring the field width, and then repeatedly dividing to extract the
> field out of the full converted value. If the string contains a run of
> valid digits longer than will fit in a long or long long, this would
> overflow and no amount of dividing can recover the correct value.
>
> This patch fixes vsscanf() to obey number field widths when parsing
> the number.
>
> A new _parse_integer_limit() is added that takes a limit for the number
> of characters to parse. The number field conversion in vsscanf is changed
> to use this new function.
>
> If a number starts with a radix prefix, the field width must be long
> enough for at last one digit after the prefix. If not, it will be handled
> like this:
>
> sscanf("0x4", "%1i", &i): i=0, scanning continues with the 'x'
> sscanf("0x4", "%2i", &i): i=0, scanning continues with the '4'
>
> This is consistent with the observed behaviour of userland sscanf.
>
> Note that this patch does NOT fix the problem of a single field value
> overflowing the target type. So for example:
>
> sscanf("123456789abcdef", "%x", &i);
>
> Will not produce the correct result because the value obviously overflows
> INT_MAX. But sscanf will report a successful conversion.


I have a few nit-picks, but it's up to you and maintainers how to proceed.

...

> -unsigned long long simple_strtoull(const char *cp, char **endp, unsigned int base)
> +static unsigned long long simple_strntoull(const char *startp, size_t max_chars,
> + char **endp, unsigned int base)
> {
> - unsigned long long result;
> + const char *cp;
> + unsigned long long result = 0ULL;
> unsigned int rv;
>
> - cp = _parse_integer_fixup_radix(cp, &base);
> - rv = _parse_integer(cp, base, &result);
> + cp = _parse_integer_fixup_radix(startp, &base);
> + if ((cp - startp) >= max_chars) {
> + cp = startp + max_chars;
> + goto out;
> + }
> +
> + max_chars -= (cp - startp);
> + rv = _parse_integer_limit(cp, base, &result, max_chars);
> /* FIXME */
> cp += (rv & ~KSTRTOX_OVERFLOW);
>
> +out:
> if (endp)
> *endp = (char *)cp;
>
> return result;
> }

A nit-pick: What if we rewrite above as

static unsigned long long simple_strntoull(const char *cp, size_t max_chars,
char **endp, unsigned int base)
{
unsigned long long result = 0ULL;
const char *startp = cp;
unsigned int rv;
size_t chars;

cp = _parse_integer_fixup_radix(cp, &base);
chars = cp - startp;
if (chars >= max_chars) {
/* We hit the limit */
cp = startp + max_chars;
} else {
rv = _parse_integer_limit(cp, base, &result, max_chars - chars);
/* FIXME */
cp += (rv & ~KSTRTOX_OVERFLOW);
}

if (endp)
*endp = (char *)cp;

return result;
}

...

> +static long long simple_strntoll(const char *cp, size_t max_chars, char **endp,
> + unsigned int base)
> +{
> + /*
> + * simple_strntoull safely handles receiving max_chars==0 in the
> + * case we start with max_chars==1 and find a '-' prefix.

A nit-pick: Spaces surrounding '=='? simple_strntoull -> simple_strntoull()?

> + */

Above misses to add something like:

"Otherwise we hit the '-' as an illegal number in the following
simple_strntoull() call."

> + if (*cp == '-' && max_chars > 0)
> + return -simple_strntoull(cp + 1, max_chars - 1, endp, base);
> +
> + return simple_strntoull(cp, max_chars, endp, base);


> +}

...

> + val.s = simple_strntoll(str,
> + field_width > 0 ? field_width : SIZE_MAX,
> + &next, base);

A nit-pick: Wouldn't be negative field_width "big enough" to just being used as
is? Also, is field_width == 0 should be treated as "parse to the MAX"?

...

> + val.u = simple_strntoull(str,
> + field_width > 0 ? field_width : SIZE_MAX,
> + &next, base);

Ditto.

--
With Best Regards,
Andy Shevchenko


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