linux-kernel - Re: [PATCH 3/3] [v4] lib/vsprintf: no_hash_pointers prints all addresses as unhashed

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Message-ID: <CANpmjNP2Z7O-2GhsjiHtTFzHGEryQPGdjHN6EeGweLWZHq-u2Q@mail.gmail.com>
Date:   Tue, 2 Mar 2021 15:57:01 +0100
From:   Marco Elver <elver@...gle.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Petr Mladek <pmladek@...e.com>, Timur Tabi <timur@...nel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        roman.fietze@...na.com, Kees Cook <keescook@...omium.org>,
        John Ogness <john.ogness@...utronix.de>,
        Akinobu Mita <akinobu.mita@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Pavel Machek <pavel@....cz>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH 3/3] [v4] lib/vsprintf: no_hash_pointers prints all
 addresses as unhashed

On Tue, 2 Mar 2021 at 15:55, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>
> Hi Marco,
>
> On Tue, Mar 2, 2021 at 3:40 PM Marco Elver <elver@...gle.com> wrote:
> > On Tue, 2 Mar 2021 at 15:35, Matthew Wilcox <willy@...radead.org> wrote:
> > > On Tue, Mar 02, 2021 at 03:26:50PM +0100, Marco Elver wrote:
> > > > +static const char no_hash_pointers_warning[9][55] __initconst = {
> > > > +     "******************************************************",
> > > > +     "   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   ",
> > > > +     "                                                      ",
> > > > +     " This system shows unhashed kernel memory addresses   ",
> > > > +     " via the console, logs, and other interfaces. This    ",
> > > > +     " might reduce the security of your system.            ",
> > > > +     " If you see this message and you are not debugging    ",
> > > > +     " the kernel, report this immediately to your system   ",
> > > > +     " administrator!                                       ",
> > > > +};
> > > > +
> > > >  static int __init no_hash_pointers_enable(char *str)
> > > >  {
> > > > +     const int lines[] = { 0, 1, 2, 3, 4, 5, 2, 6, 7, 8, 2, 1, 0 };
> > > > +     int i;
> > > > +
> > > >       no_hash_pointers = true;
> > > >
> > > > -     pr_warn("**********************************************************\n");
> > > > -     pr_warn("**   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **\n");
> > > > -     pr_warn("**                                                      **\n");
> > > > -     pr_warn("** This system shows unhashed kernel memory addresses   **\n");
> > > > -     pr_warn("** via the console, logs, and other interfaces. This    **\n");
> > > > -     pr_warn("** might reduce the security of your system.            **\n");
> > > > -     pr_warn("**                                                      **\n");
> > > > -     pr_warn("** If you see this message and you are not debugging    **\n");
> > > > -     pr_warn("** the kernel, report this immediately to your system   **\n");
> > > > -     pr_warn("** administrator!                                       **\n");
> > > > -     pr_warn("**                                                      **\n");
> > > > -     pr_warn("**   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **\n");
> > > > -     pr_warn("**********************************************************\n");
> > > > +     for (i = 0; i < ARRAY_SIZE(lines); i++)
> > > > +             pr_warn("**%s**\n", no_hash_pointers_warning[lines[i]]);
> > >
> > > +       for (i = 0; i < 3; i++)
> > > +               pr_warn("**%s**\n", no_hash_pointers_warning[lines[2 - i]]);
> >
> > Yeah, I had that before, but then wanted to deal with the blank line
> > in the middle of the thing. So I just went with the lines array above,
> > which seemed cleanest for dealing with the middle blank line and
> > footer. Or maybe there's something even nicer I missed? :-)
>
> Gcc deduplicates the identical strings, so you don't have to go through
> a double indirection at all?

In this case I think we do, because we're asking the compiler to
create a giant array char[9][55]. If we had char*[9], then you're
right, but in that case we would not benefit from __initconst for the
majority of the data.

Thanks,
-- Marco