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