linux-kernel - Re: [PATCH v2] add -Wpointer-arith sparse flag to toggle sizeof(void) warnings

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Message-ID: <20180409225101.uzztpdv6bbjmlwwr@gmail.com>
Date:   Mon, 9 Apr 2018 12:51:01 -1000
From:   Joey Pabalinas <joeypabalinas@...il.com>
To:     Luc Van Oostenryck <luc.vanoostenryck@...il.com>
Cc:     Joey Pabalinas <joeypabalinas@...il.com>,
        linux-sparse@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Martin Uecker <Martin.Uecker@....uni-goettingen.de>,
        Al Viro <viro@...IV.linux.org.uk>,
        Christopher Li <sparse@...isli.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] add -Wpointer-arith sparse flag to toggle
 sizeof(void) warnings

On Sun, Apr 08, 2018 at 09:48:24AM +0200, Luc Van Oostenryck wrote:
> With the warning disabled by default (for the moment), I had to adapt
> the testsuite with:

Ah, so should I include that change in the patch itself when I make a V3?

> > +Warn about anything that depends on the \fBsizeof\fR a function type or of void.
> 
> Maybe it would be useful to add something along the line of "like directly using
> the sizeof operator on void or doing pointer arithmetic on a void pointer" ?

I actually just took the explanation straight from the GCC man page
since I figured the explanation should match (as the flag itself is
basicallt copied).

But I do sort of like your wording of it more, so if no one else sees
any reasons to _not_ to diverge from GCC's wording here I have no problem
changing that.

> > +Although non-standard (and somewhat illogical), constructs such as \fBsizeof(void)\fR
> > +are often useful when the intent is to operate on an expression without evaluating
> > +it, e.g. in the following integer constant expression predicate:
> > +.nf
> > +#define __is_constexpr(x) \\
> > +	(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
> > +.fi
> 
> I think that pointer arithmetic is much more useful than taking the size of void
> (being able to take the size of *any* thing is somewhere in the middle, IMO).
> But in all case, I don't think this part should belong to the man page.

Also have no problem eliding this section if no one else has any
good arguments for keeping it.

-- 
Cheers,
Joey Pabalinas

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