Re: [PATCH v7 2/4] lib: vsprintf: Fix handling of number field widths in vsscanf

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On 29/03/2021 14:36, Andy Shevchenko wrote:
On Mon, Mar 29, 2021 at 01:08:22PM +0100, 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.

Note that where a very large number is used to mean "unlimited", the value
INT_MAX is used for consistency with the behaviour of vsnprintf().

...

  unsigned long simple_strtoul(const char *cp, char **endp, unsigned int base)
  {
-	return simple_strtoull(cp, endp, base);
+	return simple_strntoull(cp, INT_MAX, endp, base);

Why do you need this change?


I agree it's not necessary. I changed it between V1 and V2 but I can't
remember what the reason was.




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