lib: vsprintf: Fix handling of number field widths in vsscanf

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().

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210514161206.30821-2-rf@opensource.cirrus.com

1 files changed, 10 insertions, 3 deletions

diff --git a/lib/kstrtox.c b/lib/kstrtox.c
index a118b0b1e9b2..0b5fe8b41173 100644
--- a/lib/kstrtox.c
+++ b/lib/kstrtox.c
@@ -39,20 +39,22 @@ const char *_parse_integer_fixup_radix(const char *s, unsigned int *base)
 
 /*
  * Convert non-negative integer string representation in explicitly given radix
- * to an integer.
+ * to an integer. A maximum of max_chars characters will be converted.
+ *
  * Return number of characters consumed maybe or-ed with overflow bit.
  * If overflow occurs, result integer (incorrect) is still returned.
  *
  * Don't you dare use this function.
  */
-unsigned int _parse_integer(const char *s, unsigned int base, unsigned long long *p)
+unsigned int _parse_integer_limit(const char *s, unsigned int base, unsigned long long *p,
+				  size_t max_chars)
 {
 	unsigned long long res;
 	unsigned int rv;
 
 	res = 0;
 	rv = 0;
-	while (1) {
+	while (max_chars--) {
 		unsigned int c = *s;
 		unsigned int lc = c | 0x20; /* don't tolower() this line */
 		unsigned int val;
@@ -82,6 +84,11 @@ unsigned int _parse_integer(const char *s, unsigned int base, unsigned long long
 	return rv;
 }
 
+unsigned int _parse_integer(const char *s, unsigned int base, unsigned long long *p)
+{
+	return _parse_integer_limit(s, base, p, INT_MAX);
+}
+
 static int _kstrtoull(const char *s, unsigned int base, unsigned long long *res)
 {
 	unsigned long long _res;