Re: [PATCH] lib: vsprintf: Fix handling of number field widths in vsscanf
From: Richard Fitzgerald
Date: Fri Nov 20 2020 - 11:04:44 EST
On 20/11/2020 15:07, Steven Rostedt wrote:
On Fri, 20 Nov 2020 12:05:25 +0100
Petr Mladek <pmladek@xxxxxxxx> wrote:
On Mon 2020-11-16 14:32:52, Richard Fitzgerald wrote:
The existing code attempted to handle numbers by doing a strto[u]l(),
ignoring the field width, and then bitshifting the field out of the
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 bitshifting can recover the correct value.
This patch fixes vsscanf to obey number field widths.
A new _parse_integer_limit() is added that takes a limit for the number
of characters to parse. A length of INT_MAX is effectively unlimited, as
we are not likely to need parsing of digit strings >INT_MAX length.
The number field conversion in vsscanf is changed to use this new
_parse_integer_limit() function so that field widths are obeyed when
parsing the number. Note also that the conversion is always done as a
long long - as there's currently no overflow checking there is no point
implementing separate long and long long conversions.
diff --git a/lib/kstrtox.c b/lib/kstrtox.c
index a14ccf905055..9867501a4ab0 100644
--- a/lib/kstrtox.c
+++ b/lib/kstrtox.c
@@ -39,20 +39,23 @@ 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. The maximum number of characters to convert can be given.
+ * A character limit of INT_MAX is effectively unlimited since a string that
+ * long is unreasonable.
The INT_MAX value meaning is obvious. It does not need to be
mentioned. It is the same as with vsnprintf().
Yeah, but I never think that restating the obvious is a bad idea.
Especially when something that is obvious to us, is not obvious to a new
comer. There's been lots of times I wish someone mentioned the obvious in a
comment somewhere, because it wasn't obvious to me ;-)
I vote to keep it in.
* 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,
+ int max_chars)
Please, use size_t. Passing negative value usually means
that the caller did not handle some situation correctly.
And it actually happened in this patch, see below.
nit: better ballance the length of the lines above. I mean to move
*p to the next line:
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) {
+ for (; max_chars > 0; max_chars--) {
unsigned int c = *s;
unsigned int lc = c | 0x20; /* don't tolower() this line */
unsigned int val;
@@ -82,6 +85,11 @@ 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/vsprintf.c b/lib/vsprintf.c
index 14c9a6af1b23..8ec47b5da2cb 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -53,6 +53,25 @@
#include <linux/string_helpers.h>
#include "kstrtox.h"
+static unsigned long long simple_strntoull(const char *startp, int max_chars,
+ char **endp, unsigned int base)
+{
+ const char *cp;
+ unsigned long long result;
+ unsigned int rv;
+
+ cp = _parse_integer_fixup_radix(startp, &base);
+ max_chars -= (cp - startp);
Negative value means that _parse_integer_fixup_radix() already
proceed more characters than allowed. I would handle this
the following way:
if (cp - startp > max_chars) {
cp = startp + max_chars;
result = 0LL;
goto out;
Agreed. I was looking at what sscanf() in user space does.
And testing it with the following:
char *line = "0x123456789abcdef0123456789\n";
int i;
for (i = 0; i < 10; i++) {
char str[32];
long a, b;
if (i)
sprintf(str, "%%%dli%%9lx", i);
else
strcpy(str, "%li%6lx");
ret = sscanf(line, str, &a, &b);
switch (ret) {
case 1:
printf("read 1 '%s': %lx\n", str, a);
break;
case 2:
printf("read 2 '%s': %lx %lx\n", str, a, b);
break;
default:
printf("Failed to read: '%s' ret = %d\n", str, ret);
}
}
And the above produced:
read 1 '%li%6lx': 7fffffffffffffff
read 1 '%1li%9lx': 0
read 2 '%2li%9lx': 0 123456789
read 2 '%3li%9lx': 1 23456789a
read 2 '%4li%9lx': 12 3456789ab
read 2 '%5li%9lx': 123 456789abc
read 2 '%6li%9lx': 1234 56789abcd
read 2 '%7li%9lx': 12345 6789abcde
read 2 '%8li%9lx': 123456 789abcdef
read 2 '%9li%9lx': 1234567 89abcdef0
The first line I'm assuming is because %li overflowed (more digits than a
64 bit could hold).
But yeah, we could very much have cp - startp > max_chars.
My code handles the prefix overflow, but I did it by having
__parse_integer_limit() simply give 0 if max_chars <= 0.
So if the field width isn't enough for the prefix/leading '-' and at
least one digit, subtracting the prefix length from the field length
will give a max_chars <= 0. And you'll get a result of 0 as in your
'%2li%9lx' test case.
I thought this was nice because it didn't need to add code to check
for the prefix overflow - it comes inherently from the loop in
__parse_integer_limit(). But I'm willing to change
__parse_integer_limit() to take an unsigned and add explicit checks for
the prefix/'-' overflow cases.
+ rv = _parse_integer_limit(cp, base, &result, max_chars);
+ /* FIXME */
+ cp += (rv & ~KSTRTOX_OVERFLOW);
out:
+ if (endp)
+ *endp = (char *)cp;
+
+ return result;
+}
+
/**
* simple_strtoull - convert a string to an unsigned long long
* @cp: The start of the string
@@ -126,6 +134,15 @@ long long simple_strtoll(const char *cp, char **endp, unsigned int base)
}
EXPORT_SYMBOL(simple_strtoll);
+static long long simple_strntoll(const char *cp, int max_chars, char **endp,
+ unsigned int base)
+{
+ if (*cp == '-')
+ return -simple_strntoull(cp + 1, max_chars - 1, endp, base);
+
+ return simple_strntoull(cp, max_chars, endp, base);
+}
Please, use this in simple_strtoll() like it is already done in
simple_strtoull(). I mean:
long long simple_strtoll(const char *cp, char **endp, unsigned int base)
{
return simple_strntoll(cp, INT_MAX, endp, base);
}
Agreed.
+
static noinline_for_stack
int skip_atoi(const char **s)
{
Finally, it would be great to add some selftests for this into
lib/test_printf.c.
Thanks a lot for working on this. I like this approach.
+1
-- Steve