linux-hardening - Re: [PATCH] printk: add cpu id information to printk() output

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Message-ID: <8734zfx2bo.fsf@jogness.linutronix.de>
Date:   Fri, 15 Sep 2023 10:40:03 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Enlin Mu <enlin.mu@...look.com>, pmladek@...e.com,
        rostedt@...dmis.org, senozhatsky@...omium.org,
        keescook@...omium.org, tony.luck@...el.com, gpiccoli@...lia.com,
        enlin.mu@...soc.com, enlinmu@...il.com
Cc:     linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] printk: add cpu id information to printk() output

On 2023-09-15, Enlin Mu <enlin.mu@...look.com> wrote:
> Sometimes we want to print cpu id of printk() messages to consoles
>
> diff --git a/include/linux/threads.h b/include/linux/threads.h
> index c34173e6c5f1..6700bd9a174f 100644
> --- a/include/linux/threads.h
> +++ b/include/linux/threads.h
> @@ -34,6 +34,9 @@
>  #define PID_MAX_LIMIT (CONFIG_BASE_SMALL ? PAGE_SIZE * 8 : \
>  	(sizeof(long) > 4 ? 4 * 1024 * 1024 : PID_MAX_DEFAULT))
>  
> +#define CPU_ID_SHIFT 23
> +#define CPU_ID_MASK  0xff800000

This only supports 256 CPUs. I think it doesn't make sense to try to
squish CPU and Task IDs into 32 bits.

What about introducing a caller_id option to always only print the CPU
ID? Or do you really need Task _and_ CPU?

John Ogness