Re: [PATCH] lib: vsprintf: Fix handling of number field widths in vsscanf

[Petr Mladek]

On Fri 2020-11-20 10:07:05, 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.

Fair enough.

I have suggested to remove it because it was somehow hard to parse for
me. It confused me more than helped. It is funny because I often write
these long and complicated sentences as well. And I really confuse
people from time to time.

I wonder if the following is more strightforward:

	* Convert non-negative integer string representation in
	* explicitly given radix to an integer.
	*
	* @max_chars limits the number of converted characters.
	* Use INT_MAX when unlimited. It is an arbitrary, big enough,
	* number that is used also by vsnprintf().



> > >   * 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)
> > 
> > 
> > >  {


> > > 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).

Yup, it looks like LONG_LONG_MAX.

> But yeah, we could very much have cp - startp > max_chars.

Thanks a lot for testing it.

Best Regards,
Petr