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

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Date:   Wed, 26 Oct 2022 14:58:34 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Kees Cook <keescook@...omium.org>, cocci@...ia.fr
Cc:     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, andriy.shevchenko@...ux.intel.com,
        Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH -mm] -funsigned-char, x86: make struct
 p4_event_bind::cntr signed array

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?

Jason