linux-kernel - Re: [PATCH v2] init/main.c: Wrap long kernel cmdline when printing to logs

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Message-ID: <CAMuHMdWAbMoQdeM5-F1YYOfEdcPrkTxdbsU8+TAYO=WuMzCqCw@mail.gmail.com>
Date: Wed, 22 Oct 2025 11:16:06 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Douglas Anderson <dianders@...omium.org>
Cc: linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>, 
	linux-s390 <linux-s390@...r.kernel.org>, Andrew Chant <achant@...gle.com>, 
	Sven Schnelle <svens@...ux.ibm.com>, Randy Dunlap <rdunlap@...radead.org>, 
	Heiko Carstens <hca@...ux.ibm.com>, Alexander Shishkin <alexander.shishkin@...ux.intel.com>, 
	Brian Gerst <brgerst@...il.com>, Christian Brauner <brauner@...nel.org>, 
	Francesco Valla <francesco@...la.it>, Guo Weikang <guoweikang.kernel@...il.com>, 
	Huacai Chen <chenhuacai@...nel.org>, Ingo Molnar <mingo@...nel.org>, 
	Jan Hendrik Farr <kernel@...rr.cc>, Jeff Xu <jeffxu@...omium.org>, Kees Cook <kees@...nel.org>, 
	Masahiro Yamada <masahiroy@...nel.org>, "Masami Hiramatsu (Google)" <mhiramat@...nel.org>, 
	Michal Koutný <mkoutny@...e.com>, 
	Miguel Ojeda <ojeda@...nel.org>, "Mike Rapoport (Microsoft)" <rppt@...nel.org>, 
	Nathan Chancellor <nathan@...nel.org>, "Paul E. McKenney" <paulmck@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Shakeel Butt <shakeel.butt@...ux.dev>, Tejun Heo <tj@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Subject: Re: [PATCH v2] init/main.c: Wrap long kernel cmdline when printing to logs

Hi Douglas,

On Wed, 22 Oct 2025 at 02:41, Douglas Anderson <dianders@...omium.org> wrote:
> The kernel cmdline length is allowed to be longer than what printk can
> handle. When this happens the cmdline that's printed to the kernel
> ring buffer at bootup is cutoff and some kernel cmdline options are
> "hidden" from the logs. This undercuts the usefulness of the log
> message.
>
> Specifically, grepping for COMMAND_LINE_SIZE shows that 2048 is common
> and some architectures even define it as 4096. s390 allows a
> CONFIG-based maximum up to 1MB (though it's not expected that anyone
> will go over the default max of 4096 [1]).

Of course (unless the kernel crashes) you can also look at /proc/cmdline
later.  However, that seems to be limited to a single page, too,
as fs/proc/cmdline.c:cmdline_proc_show() uses seq_puts*().

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds