Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20181221175618.968519903@goodmis.org> <20181221175659.208858193@goodmis.org>
 <CAHk-=wg_sR-UEC1ggmkZpypOUYanL5CMX4R7ceuaV4QMf5jBtg@mail.gmail.com>
 <20181221144054.20bdeb33@gandalf.local.home> <CAHk-=wgSvRhj6KNRdn9gbK9RJVAswdtKSNjAxdf9O9vn04T5XQ@mail.gmail.com>
 <20181221153526.5e6055ca@gandalf.local.home> <CAHk-=wjtkvFUuRNZU67KccuUKYHw=pYoDMQJ_9OVDFxOwmK9zQ@mail.gmail.com>
 <20181221155513.11450ca6@gandalf.local.home> <CAHk-=wjaNWzXSW9XT-Z=9OF=5+2ft6UqjoUgaWeEWx8PUmS9xg@mail.gmail.com>
 <20181221174816.0ed7af47@gandalf.local.home>
In-Reply-To: <20181221174816.0ed7af47@gandalf.local.home>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 21 Dec 2018 14:57:13 -0800
Message-ID: <CAHk-=wgSovEz3gy56q0bhOMVt9Rx5ooq97KmY=XX37_JdE+4pQ@mail.gmail.com>
Subject: Re: [for-next][PATCH 23/24] string.h: Add strncmp_prefix() helper macro
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Joe Perches <joe@perches.com>,
        Tom Zanussi <zanussi@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Dec 21, 2018 at 2:48 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > Your patch actually had them, but in the body of your email you did
> >
> > > #define have_prefix(str, prefix) ({ \
> > >       const char *__pfx = (const char *)prefix; \
> >
> > which is just completely wrong.
> >
> > Considering your _old_ patch had the exact same bug, I really think
> > you need to start internalizing the whole "macro arguments *have* to
> > be properly protected" thing.
>
> Well, there's less with assignments that can go wrong than with other
> code. That is, there's little that can happen with "int x = arg;" where
> arg is the macro paramater to cause something really nasty.

What's wrong, Steven?

The assignment is entirely irrelevant.

The problem is the cast.

A type cast has a very high priority, and so if you do

    (const char *)prefix

it breaks completely if you might have something like"a+6" as the argument.

Think about what if "a" is of type "unsigned long", for example?

                   Linus