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Subject[PATCH v6 2/4] lib: vsprintf: Fix handling of number field widths in vsscanf
Date
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>
---
Changed since v5:
- Use INT_MAX to mean "unlimited length".
- Rework simple_strntoull() to avoid the goto
- Rewrite the comment in simple_strntoll()
- In vsscanf() change the calls to simple_strnto[u]ll() so that a field_width
of 0 is not treated as unlimited.

Changed since v3:
- Use INT_MAX to mean "unlimited length".
- Use while-loop instead of for-loop in _parse_integer_limit().
- Keep the existing arguments for _parse_integer() on their original line.
And the corresponding arguments to _parse_integer_limit() formatted/wrapped
the same way as _parse_integer().
- Remove redundant check for (max_chars == 0) in simple_strntoull().
- Fixed "vsscanf" -> "vsscanf()" in commit message.
---
lib/kstrtox.c | 13 ++++++--
lib/kstrtox.h | 2 ++
lib/vsprintf.c | 88 +++++++++++++++++++++++++++++---------------------
3 files changed, 63 insertions(+), 40 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;
diff --git a/lib/kstrtox.h b/lib/kstrtox.h
index 3b4637bcd254..158c400ca865 100644
--- a/lib/kstrtox.h
+++ b/lib/kstrtox.h
@@ -4,6 +4,8 @@

#define KSTRTOX_OVERFLOW (1U << 31)
const char *_parse_integer_fixup_radix(const char *s, unsigned int *base);
+unsigned int _parse_integer_limit(const char *s, unsigned int base, unsigned long long *res,
+ size_t max_chars);
unsigned int _parse_integer(const char *s, unsigned int base, unsigned long long *res);

#endif
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 28bb26cd1f67..794ecc373bf9 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -53,29 +53,43 @@
#include <linux/string_helpers.h>
#include "kstrtox.h"

-/**
- * simple_strtoull - convert a string to an unsigned long long
- * @cp: The start of the string
- * @endp: A pointer to the end of the parsed string will be placed here
- * @base: The number base to use
- *
- * This function has caveats. Please use kstrtoull instead.
- */
-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;
+ size_t prefix_chars;
unsigned int rv;

- cp = _parse_integer_fixup_radix(cp, &base);
- rv = _parse_integer(cp, base, &result);
- /* FIXME */
- cp += (rv & ~KSTRTOX_OVERFLOW);
+ cp = _parse_integer_fixup_radix(startp, &base);
+ prefix_chars = cp - startp;
+ if (prefix_chars < max_chars) {
+ rv = _parse_integer_limit(cp, base, &result, max_chars - prefix_chars);
+ /* FIXME */
+ cp += (rv & ~KSTRTOX_OVERFLOW);
+ } else {
+ /* Field too short for prefix + digit, skip over without converting */
+ cp = startp + max_chars;
+ }

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

return result;
}
+
+/**
+ * simple_strtoull - convert a string to an unsigned long long
+ * @cp: The start of the string
+ * @endp: A pointer to the end of the parsed string will be placed here
+ * @base: The number base to use
+ *
+ * This function has caveats. Please use kstrtoull instead.
+ */
+unsigned long long simple_strtoull(const char *cp, char **endp, unsigned int base)
+{
+ return simple_strntoull(cp, INT_MAX, endp, base);
+}
EXPORT_SYMBOL(simple_strtoull);

/**
@@ -88,7 +102,7 @@ EXPORT_SYMBOL(simple_strtoull);
*/
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);
}
EXPORT_SYMBOL(simple_strtoul);

@@ -109,6 +123,21 @@ long simple_strtol(const char *cp, char **endp, unsigned int base)
}
EXPORT_SYMBOL(simple_strtol);

+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 cp[0] == '-' && max_chars == 1.
+ * If max_chars == 0 we can drop through and pass it to simple_strntoull()
+ * and the content of *cp is irrelevant.
+ */
+ if (*cp == '-' && max_chars > 0)
+ return -simple_strntoull(cp + 1, max_chars - 1, endp, base);
+
+ return simple_strntoull(cp, max_chars, endp, base);
+}
+
/**
* simple_strtoll - convert a string to a signed long long
* @cp: The start of the string
@@ -119,10 +148,7 @@ EXPORT_SYMBOL(simple_strtol);
*/
long long simple_strtoll(const char *cp, char **endp, unsigned int base)
{
- if (*cp == '-')
- return -simple_strtoull(cp + 1, endp, base);
-
- return simple_strtoull(cp, endp, base);
+ return simple_strntoll(cp, INT_MAX, endp, base);
}
EXPORT_SYMBOL(simple_strtoll);

@@ -3449,25 +3475,13 @@ int vsscanf(const char *buf, const char *fmt, va_list args)
break;

if (is_sign)
- val.s = qualifier != 'L' ?
- simple_strtol(str, &next, base) :
- simple_strtoll(str, &next, base);
+ val.s = simple_strntoll(str,
+ field_width >= 0 ? field_width : INT_MAX,
+ &next, base);
else
- val.u = qualifier != 'L' ?
- simple_strtoul(str, &next, base) :
- simple_strtoull(str, &next, base);
-
- if (field_width > 0 && next - str > field_width) {
- if (base == 0)
- _parse_integer_fixup_radix(str, &base);
- while (next - str > field_width) {
- if (is_sign)
- val.s = div_s64(val.s, base);
- else
- val.u = div_u64(val.u, base);
- --next;
- }
- }
+ val.u = simple_strntoull(str,
+ field_width >= 0 ? field_width : INT_MAX,
+ &next, base);

switch (qualifier) {
case 'H': /* that's 'hh' in format */
--
2.20.1
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