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

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Message-ID: <YD5LKqtSz5r1Xp7B@elver.google.com>
Date:   Tue, 2 Mar 2021 15:26:50 +0100
From:   Marco Elver <elver@...gle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.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>,
        Matthew Wilcox <willy@...radead.org>,
        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, Mar 02, 2021 at 09:08AM -0500, Steven Rostedt wrote:
> On Tue, 2 Mar 2021 14:49:42 +0100
> Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> 
> > > So this is basically a kernel tinyfication issue, right? Is that still pursued
> > > today? Are there better config options suitable for this than CONFIG_DEBUG_KERNEL?  
> > 
> > As long as I hear about products running Linux on SoCs with 10 MiB of
> > SRAM, I think the answer is yes.
> > I'm not immediately aware of a better config option.  There are no more
> > TINY options left, and EXPERT selects DEBUG_KERNEL.
> 
> Since the trace_printk() uses the same type of notice, I wonder if we could
> make this into a helper function and just pass in the top part.
> 
> +	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");
> 
> Only the above section is really unique. The rest can be a boiler plate.

Short of procedurally generating some of these strings, I was
experimenting with the below.

Would that be reasonable?

Thanks,
-- Marco

------ >8 ------

From: Marco Elver <elver@...gle.com>
Date: Tue, 2 Mar 2021 15:07:28 +0100
Subject: [PATCH] lib/vsprintf: reduce space taken by no_hash_pointers warning

Move the no_hash_pointers warning string into __initconst section, so
that it is discarded after init. Remove common start/end characters and
remove repeated lines from the array.

Link: https://lkml.kernel.org/r/CAMuHMdULKZCJevVJcp7TxzLdWLjsQPhE8hqxhnztNi9bjT_cEw@mail.gmail.com
Reported-by: Geert Uytterhoeven <geert@...ux-m68k.org>
Signed-off-by: Marco Elver <elver@...gle.com>
---
 lib/vsprintf.c | 30 +++++++++++++++++-------------
 1 file changed, 17 insertions(+), 13 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 41ddc353ebb8..1e63b43955f6 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -2094,23 +2094,27 @@ char *fwnode_string(char *buf, char *end, struct fwnode_handle *fwnode,
 bool no_hash_pointers __ro_after_init;
 EXPORT_SYMBOL_GPL(no_hash_pointers);
 
+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]]);
 
 	return 0;
 }
-- 
2.30.1.766.gb4fecdf3b7-goog