linux-kernel - Re: [PATCH] init/main.c: Print all command line when boot

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Message-ID: <79e649a5a73b2137bc576a75271854dc4024ae24.camel@perches.com>
Date:   Mon, 18 May 2020 22:09:34 -0700
From:   Joe Perches <joe@...ches.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        王程刚 <wangchenggang@...o.com>
Cc:     'Masami Hiramatsu' <mhiramat@...nel.org>,
        "'Steven Rostedt (VMware'" <rostedt@...dmis.org>,
        'Kees Cook' <keescook@...omium.org>,
        'Thomas Gleixner' <tglx@...utronix.de>,
        'Dominik Brodowski' <linux@...inikbrodowski.net>,
        'Arvind Sankar' <nivedita@...m.mit.edu>,
        'Mike Rapoport' <rppt@...ux.ibm.com>,
        'Alexander Potapenko' <glider@...gle.com>,
        linux-kernel@...r.kernel.org, kernel@...o.com
Subject: Re: [PATCH] init/main.c: Print all command line when boot

On Mon, 2020-05-18 at 20:44 -0700, Andrew Morton wrote:
> On Tue, 19 May 2020 11:29:46 +0800 王程刚 <wangchenggang@...o.com> wrote:
> 
> > Function pr_notice print max length maybe less than the command line length,
> > need more times to print all.
> > For example, arm64 has 2048 bytes command line length, but printk maximum
> > length is only 1024 bytes.
> 
> I can see why that might be a problem!
> 
> > --- a/init/main.c
> > +++ b/init/main.c
> > @@ -825,6 +825,16 @@ void __init __weak arch_call_rest_init(void)
> >  	rest_init();
> >  }
> >  
> > +static void __init print_cmdline(void)
> > +{
> > +	const char *prefix = "Kernel command line: ";
> 
> const char prefix[] = "...";
> 
> might generate slightly more efficient code.
> 
> > +	int len = -strlen(prefix);
> 
> hm, tricky.  What the heck does printk() actually return to the caller?
> Seems that we forgot to document this, and there are so many different
> paths which a printk call can take internally that I'm not confident
> that they all got it right!

There is no use of the return value of any pr_<level> or
dev_<level> or netdev_<level) in the kernel.

All the pr_<level> mechanisms (as functions) should return void.
https://lore.kernel.org/lkml/1466739971-30399-1-git-send-email-joe@perches.com/

> > +	len += pr_notice("%s%s\n", prefix, boot_command_line);
> > +	while (boot_command_line[len])
> > +		len += pr_notice("%s\n", &boot_command_line[len]);
> > +}

More likely it'd be better to use a strlen(boot_command_line)
and perhaps do something like print multiple lines with args
using strchr(, ' ') at some largish value, say 132 or 256 chars
maximum per line.