On Thu, 3 Nov 2022, Jason A. Donenfeld wrote:
> On Wed, Nov 02, 2022 at 06:17:04PM +0100, Julia Lawall wrote:
> >
> >
> > On Wed, 26 Oct 2022, Jason A. Donenfeld wrote:
> >
> > > On Wed, Oct 26, 2022 at 03:50:25AM +0200, Jason A. Donenfeld wrote:
> > > > The traditional objdump comparison does work, though. It produces a good
> > >
> > > Another thing that appears to work well is just using Coccinelle
> > > scripts. I've had some success just scrolling through the results of:
> > >
> > > @@
> > > char c;
> > > expression E;
> > > @@
> > > (
> > > * E > c
> > > |
> > > * E >= c
> > > |
> > > * E < c
> > > |
> > > * E <= c
> > > )
> > >
> > > That also triggers on explicitly signed chars, and examining those
> > > reveals that quite a bit of code in the tree already does do the right
> > > thing, which is good.
> > >
> > > From looking at this and objdump output, it looks like most naked-char
> > > usage that isn't for strings is actually already assuming it's unsigned,
> > > using it as a byte. I'll continue to churn, and I'm sure I'll miss a few
> > > things here and there, but all and all, I don't think this is looking as
> > > terrible as I initially feared.
> > >
> > > I'm CC'ing the Coccinelle people to see if they have any nice ideas on
> > > improvements. Specifically, the thing we're trying to identify is:
> > >
> > > - Usage of vanilla `char`, without a `signed` or `unsigned` qualifier,
> > > where:
> >
> > Try putting
> >
> > disable optional_qualifier
> >
> > between the initial @@, to avoid the implicit matching of signed and
> > unsigned.
>
> Hmm, this doesn't quite work. Here are my rules:
>
> @disable optional_qualifier@
> char c;
> expression E;
> @@
> (
> * E > c
> |
> * E >= c
> |
> * E < c
> |
> * E <= c
> )
>
> @disable optional_qualifier@
> char c;
> @@
> * c == -1
>
> @disable optional_qualifier@
> char c;
> @@
> * c = -1
>
> This produces, for example:
>
> diff -u -p ./sound/firewire/bebob/bebob_focusrite.c /tmp/nothing/sound/firewire/bebob/bebob_focusrite.c
> --- ./sound/firewire/bebob/bebob_focusrite.c
> +++ /tmp/nothing/sound/firewire/bebob/bebob_focusrite.c
> @@ -192,7 +192,6 @@ saffirepro_both_clk_src_get(struct snd_b
>
> /* In a case that this driver cannot handle the value of register. */
> value &= SAFFIREPRO_CLOCK_SOURCE_SELECT_MASK;
> - if (value >= SAFFIREPRO_CLOCK_SOURCE_COUNT || map[value] < 0) {
> err = -EIO;
> goto end;
> }
>
> Except map is defined as:
>
> const signed char *map;
>
> So this would be one of those cases that I had hoped `disable
> optional_qualifier` would exclude. (I think internally coccinelle might
> be assuming `char` is signed, by the way.)
OK, I see the problem. Coccinelle isn't taking the "disable
optional_qualifier" into account when it checks types on expressions. It
would work if you put, eg:
char x;
... when any
* x < 0
But that would be much slower and less general. I will fix it.
>
> > > - It's not being used for characters; and
> > > - It's doing something that assumes it is signed, such as various
> > > types of comparisons or decrements.
> >
> > I took a quick look at the article, but I'm not completely sure what you
> > are getting at here. Could you give some examples of what you do and
> > don't want to find?
> >
> > You don't want the case where c is 'x', for some x?
>
> Something I would want to find is `if (c < 0)`. Something I wouldn't
> want to find is `if (c < '9')`. IOW, I'm looking for code that assumes
> `c` is signed, and would become incorrect if `c` suddenly became
> unsigned. Most things involving actual characters are fine. But most
> things involving signed arithmetic or comparisons with numbers isn't
> find.
This seems to do what you want:
@disable optional_qualifier@
constant char cc;
expression e;
char c;
@@
(
c < cc
|
* c < e
)
It highlights only the two return lines in:
int main () {
char x;
if (x < 'd')
return x < 0;
else return x < y;
}
julia
>
> Jason
>
