linux-kernel - Re: [PATCH] vsprintf: Use %p4chR instead of %p4cn for reading data in reversed host ordering

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Message-ID:
 <PN3PR01MB959730DE0A0FF281A0DC12FCB8812@PN3PR01MB9597.INDPRD01.PROD.OUTLOOK.COM>
Date: Mon, 28 Apr 2025 18:32:48 +0530
From: Aditya Garg <gargaditya08@...e.com>
To: Petr Mladek <pmladek@...e.com>, alyssa@...enzweig.io,
 Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
 Sven Peter <sven@...npeter.dev>, Thomas Zimmermann <tzimmermann@...e.de>,
 Aun-Ali Zaidi <admin@...eit.net>, Maxime Ripard <mripard@...nel.org>,
 airlied@...hat.com, Simona Vetter <simona@...ll.ch>,
 Steven Rostedt <rostedt@...dmis.org>,
 Rasmus Villemoes <linux@...musvillemoes.dk>,
 Sergey Senozhatsky <senozhatsky@...omium.org>,
 Jonathan Corbet <corbet@....net>, Andrew Morton <akpm@...ux-foundation.org>,
 apw@...onical.com, joe@...ches.com, dwaipayanray1@...il.com,
 lukas.bulwahn@...il.com, Kees Cook <kees@...nel.org>, tamird@...il.com
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
 dri-devel@...ts.freedesktop.org, linux-doc@...r.kernel.org,
 Hector Martin <marcan@...can.st>,
 Asahi Linux Mailing List <asahi@...ts.linux.dev>,
 Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH] vsprintf: Use %p4chR instead of %p4cn for reading data in
 reversed host ordering



On 28-04-2025 06:01 pm, Petr Mladek wrote:
> The generic FourCC format always prints the data using the big endian
> order. It is generic because it allows to read the data using a custom
> ordering.
> 
> The current code uses "n" for reading data in the reverse host ordering.
> It makes the 4 variants [hnbl] consistent with the generic printing
> of IPv4 addresses.
> 
> Unfortunately, it creates confusion on big endian systems. For example,
> it shows the data &(u32)0x67503030 as
> 
> 	%p4cn	00Pg (0x30305067)
> 
> But people expect that the ordering stays the same. The network ordering
> is a big-endian ordering.
> 
> The problem is that the semantic is not the same. The modifiers affect
> the output ordering of IPv4 addresses while they affect the reading order
> in case of FourCC code.
> 
> Avoid the confusion by replacing the "n" modifier with "hR", aka
> reverse host ordering. It is inspired by the existing %p[mM]R printf
> format.
> 
> Reported-by: Geert Uytterhoeven <geert@...ux-m68k.org>
> Closes: https://lore.kernel.org/r/CAMuHMdV9tX=TG7E_CrSF=2PY206tXf+_yYRuacG48EWEtJLo-Q@mail.gmail.com
> Signed-off-by: Petr Mladek <pmladek@...e.com>

Reviewed-by: Aditya Garg <gargaditya08@...e.com>

> ---
> Hi,
> 
> I am sending this as a proper patch. It would be nice to queue it
> together with the other patches adding the generic printf modifiers.