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.)
> > - 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.
Jason
