linux-kernel - Re: [PATCH -mm] -funsigned-char, x86: make struct p4_event

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Message-ID: <Y1kzUMzFdMiHpEnF@smile.fi.intel.com>
Date:   Wed, 26 Oct 2022 16:17:04 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Kees Cook <keescook@...omium.org>, cocci@...ia.fr,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        mm-commits@...r.kernel.org, masahiroy@...nel.org,
        gregkh@...uxfoundation.org, Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH -mm] -funsigned-char, x86: make struct
 p4_event_bind::cntr signed array

+Cc: Rasmus as he has done a lot regarding library stuff and optimizations and
     he knows Coccinelle (to some extent as far as I can tell).

On Wed, Oct 26, 2022 at 02:58:34PM +0200, 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:
>   - 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.
> 
> LWN wrote a summary of the general problem, in case that helps describe
> what would be useful: https://lwn.net/SubscriberLink/911914/f90c2ed1af23cbc4/
> 
> Any nice Cocci tricks for this?

-- 
With Best Regards,
Andy Shevchenko