From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <SRS0=dk12=4L=gmail.com=jwakely.gcc@sourceware.org>
Received: from mail-ej1-x629.google.com (mail-ej1-x629.google.com [IPv6:2a00:1450:4864:20::629])
	by sourceware.org (Postfix) with ESMTPS id CAE7D383F238
	for <gcc@gcc.gnu.org>; Tue, 13 Dec 2022 20:45:56 +0000 (GMT)
DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org CAE7D383F238
Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=gmail.com
Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=gmail.com
Received: by mail-ej1-x629.google.com with SMTP id kw15so39605683ejc.10
        for <gcc@gcc.gnu.org>; Tue, 13 Dec 2022 12:45:56 -0800 (PST)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
        d=gmail.com; s=20210112;
        h=cc:to:subject:message-id:date:from:in-reply-to:references
         :mime-version:from:to:cc:subject:date:message-id:reply-to;
        bh=C3esI8Zmz9jvSzS0CosWttN5dX/AJB9tGPvzmZCTq+Y=;
        b=V/M9Q28DihJ2h2LBfsjQt1094VeCp4psJfuhjKGgXozLMDZTlWckqSnHFkd2v1cV5w
         uE1vxER/LhZax+R6JuetUTt4jJBBh7HiLoJII4yvTtIV/EpacyN8ia2kyMZGtZCu6Dmy
         DcYtwqsk0CNDbHO0c5k/10/3xQgG9oBD/e6fpn3Si0QP/YM3k3OblIN4I7iknuxzjZFN
         LxU/8mKMSW4WsUg89fMYwSDgDmY0cxYVpc3no8DjJkjQf1Idcj/dEyNWuGI0LSbItjYW
         skJjMaVkq2N+TpQDZBp/yb6KSXLcoAPFti4HM15vk4+X3pPphADKAlF02Fq98h0p/EDi
         2qrw==
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
        d=1e100.net; s=20210112;
        h=cc:to:subject:message-id:date:from:in-reply-to:references
         :mime-version:x-gm-message-state:from:to:cc:subject:date:message-id
         :reply-to;
        bh=C3esI8Zmz9jvSzS0CosWttN5dX/AJB9tGPvzmZCTq+Y=;
        b=R4JbskXCHHMOgN1vHTOlUSPNU02N85RjozYC/A11xbDoe5LpCKCjAF2jhw3jWGV4wu
         y7QJStBLUFe7nlLEDPb9d/KMRFeGUcsAxVLg2ByiGF5SPDXXi2bCOt9J8LsiY+ri8bvZ
         7OsyaiCPw6hQm3NtT3prR3D/kjNDgHrUD2XBIHnWC8zE3MSvYFz+HS97Q2gZs2FB+BcT
         YT4KIUTOsKyTgD+n24iIPybL5uLfzCo2cCrYSNcvnqOR9lWuQUXRFnb4RPfK0RnYw2Sr
         rqV5q8dekBoK67D4pxktG6yk1mdZ7k0+YAd5JHSIUMjKNxhe/P2J1n251mZn1K8NIPB5
         Hkhg==
X-Gm-Message-State: ANoB5pkIQNec+f3nNtes0JnAyIGAfzA6eL1Dxfhc7WCQVQMe7m8EO0Pg
	xzdp6AueQvNe9p/hCc7GF5w2FeG7fwt2AoWxjYk=
X-Google-Smtp-Source: AA0mqf4ed5BRMGQVJHvg3hBcSTrTnQkXOadkUwTgd7O6IQZ8TyAq/zo2aphgjnhuMPxQnLoYigvZxAmB+mB2mtOHey8=
X-Received: by 2002:a17:906:518c:b0:7c1:98f:c16a with SMTP id
 y12-20020a170906518c00b007c1098fc16amr10477449ejk.215.1670964355442; Tue, 13
 Dec 2022 12:45:55 -0800 (PST)
MIME-Version: 1.0
References: <e4e4281f-c837-8427-9843-2a28336fee86@gmail.com> <B9E38906-88DC-45CE-94AB-72DEF6A590B2@comcast.net>
In-Reply-To: <B9E38906-88DC-45CE-94AB-72DEF6A590B2@comcast.net>
From: Jonathan Wakely <jwakely.gcc@gmail.com>
Date: Tue, 13 Dec 2022 20:45:44 +0000
Message-ID: <CAH6eHdSNmtS84H+UDne-QtCrCnYZrTbFKB-qjjShBSuy_vLkAQ@mail.gmail.com>
Subject: Re: [BUG] missing warning for pointer arithmetic out of bounds
To: Paul Koning <paulkoning@comcast.net>
Cc: Alejandro Colomar <alx.manpages@gmail.com>, "gcc@gcc.gnu.org" <gcc@gcc.gnu.org>
Content-Type: text/plain; charset="UTF-8"
X-Spam-Status: No, score=-0.6 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FREEMAIL_FROM,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_PASS,TXREP autolearn=ham autolearn_force=no version=3.4.6
X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org
List-Id: <gcc.gcc.gnu.org>

On Tue, 13 Dec 2022, 19:23 Paul Koning via Gcc, <gcc@gcc.gnu.org> wrote:
>
>
>
> > On Dec 13, 2022, at 2:08 PM, Alejandro Colomar via Gcc <gcc@gcc.gnu.org> wrote:
> >
> > Hi!
> >
> > For the following program:
> >
> >
> >    $ cat buf.c
> >    #include <stdio.h>
> >
> >    int main(void)
> >    {
> >        char *p, buf[5];
> >
> >        p = buf + 6;
> >        printf("%p\n", p);
> >    }
> >
> >
> > There are no warnings in gcc, as I would expect:
> >
> >    $ gcc -Wall -Wextra buf.c -O0
> >
> > Clang does warn, however:
> >
> >    $ clang -Weverything -Wall -Wextra buf.c -O0
> >    buf.c:8:17: warning: format specifies type 'void *' but the argument has type 'char *' [-Wformat-pedantic]
> >        printf("%p\n", p);
> >                ~~     ^
> >                %s
> >    buf.c:7:6: warning: the pointer incremented by 6 refers past the end of the array (that contains 5 elements) [-Warray-bounds-pointer-arithmetic]
> >        p = buf + 6;
> >            ^     ~
>
> I thought void * is a generic pointer that accepts any pointer argument.

Yes, but printf doesn't take a void* argument.

>
>   So a warning about char* being passed in seems to be flat out wrong.

I wouldn't say flat out wrong. It's wrong because the C standard
guarantees that void* and char* have the same representation and
alignment requirements, so no conversion is needed to pass a char*
through a varargs ellipsis where a void* is expected. But if it was
int* or struct X* then the warning would be correct (but not a problem
in practice on common arches, which is why Clang only enabels it with
-Wformat-pedantic).


>
> >    buf.c:5:2: note: array 'buf' declared here
> >        char *p, buf[5];
> >        ^
> >    2 warnings generated.
>
> That was discussed just days ago: C says that a pointer one past the end of the array is legal.  So here too it looks like Clang is wrong and GCC is right.


No, clang is totally right here.