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Date: Tue, 21 Oct 2025 16:06:57 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Doug Anderson <dianders@chromium.org>
Cc: linux-kernel@vger.kernel.org, Andrew Chant <achant@google.com>,
 Alexander Shishkin <alexander.shishkin@linux.intel.com>, Brian Gerst
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 =?ISO-8859-1?Q?Wei=DFschuh?= <thomas.weissschuh@linutronix.de>,
 "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Subject: Re: [PATCH] init/main.c: Wrap long kernel cmdline when printing to
 logs
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On Tue, 21 Oct 2025 15:57:18 -0700 Doug Anderson <dianders@chromium.org> wrote:

> In other words `dmesg` seemed to be trying to read 2047 bytes from
> `/dev/kmsg` and this wasn't enough to hold the output line. You can
> see in the kernel function devkmsg_read() that when this happens the
> kernel returns -EINVAL.
> 
> We could _try_ to improve devkmsg_read() to be able to return partial
> log lines, but that violates the docs. The file
> `Documentation/ABI/testing/dev-kmsg` says:
> 
>   Every read() from the opened device node receives one record
>   of the kernel's printk buffer.
>   ...
>   Messages in the record ring buffer get overwritten as whole,
>   there are never partial messages received by read().

Well that was dumb of us.  POSIX be damned.

> So tl;dr: as far as I can tell, we simply cannot put the whole cmdline
> (which is 2048+ on many architectures) on one line without breaking
> userspace. My userspace reads 2047 bytes and we'd need to return not
> just the cmdline but the prefix "Kernel command line:" as well as the
> data about time/log_level/etc.
> 
> 
> Does any of the above change your mind about my wrapping scheme? ;-)

Yeah.

> Obviously, I'd want to update my commit message with some of the
> research...

Sure.