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Date:   Thu, 3 Nov 2022 01:08:53 +0100
From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     Julia Lawall <julia.lawall@inria.fr>
Cc:     Kees Cook <keescook@chromium.org>, cocci@inria.fr,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        mm-commits@vger.kernel.org, masahiroy@kernel.org,
        gregkh@linuxfoundation.org, andriy.shevchenko@linux.intel.com,
        Stephen Rothwell <sfr@canb.auug.org.au>
Subject: Re: [cocci] [PATCH -mm] -funsigned-char, x86: make struct
 p4_event_bind::cntr signed array
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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