linux-kernel - Re: [PATCH v2] printk/kdb: Redirect printk messages into kdb in any context

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Message-ID: <20200520160752.mnkpptc2efscolcs@holly.lan>
Date:   Wed, 20 May 2020 17:07:52 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sumit Garg <sumit.garg@...aro.org>,
        Jason Wessel <jason.wessel@...driver.com>,
        Douglas Anderson <dianders@...omium.org>,
        kgdb-bugreport@...ts.sourceforge.net,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>
Subject: Re: [PATCH v2] printk/kdb: Redirect printk messages into kdb in any
 context

On Wed, May 20, 2020 at 12:22:33PM +0200, Petr Mladek wrote:
> kdb has to get messages on consoles even when the system is stopped.
> It uses kdb_printf() internally and calls console drivers on its own.
> 
> It uses a hack to reuse an existing code. It sets "kdb_trap_printk"
> global variable to redirect even the normal printk() into the
> kdb_printf() variant.
> 
> The variable "kdb_trap_printk" is checked in printk_default() and
> it is ignored when printk is redirected to printk_safe in NMI context.
> Solve this by moving the check into printk_func().
> 
> It is obvious that it is not fully safe. But it does not make things
> worse. The console drivers are already called in this context by
> db_printf() direct calls.
> 
> Reported-by: Sumit Garg <sumit.garg@...aro.org>
> Tested-by: Sumit Garg <sumit.garg@...aro.org>
> Signed-off-by: Petr Mladek <pmladek@...e.com>

Reviewed-by: Daniel Thompson <daniel.thompson@...aro.org>


> ---
> Changes in v2:
> 
>    + more detailed commit message
> 
>  kernel/printk/printk.c      | 14 +-------------
>  kernel/printk/printk_safe.c |  7 +++++++
>  2 files changed, 8 insertions(+), 13 deletions(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 9a9b6156270b..63a1aa377cd9 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -35,7 +35,6 @@
>  #include <linux/memblock.h>
>  #include <linux/syscalls.h>
>  #include <linux/crash_core.h>
> -#include <linux/kdb.h>
>  #include <linux/ratelimit.h>
>  #include <linux/kmsg_dump.h>
>  #include <linux/syslog.h>
> @@ -2036,18 +2035,7 @@ EXPORT_SYMBOL(vprintk);
>  
>  int vprintk_default(const char *fmt, va_list args)
>  {
> -	int r;
> -
> -#ifdef CONFIG_KGDB_KDB
> -	/* Allow to pass printk() to kdb but avoid a recursion. */
> -	if (unlikely(kdb_trap_printk && kdb_printf_cpu < 0)) {
> -		r = vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args);
> -		return r;
> -	}
> -#endif
> -	r = vprintk_emit(0, LOGLEVEL_DEFAULT, NULL, 0, fmt, args);
> -
> -	return r;
> +	return vprintk_emit(0, LOGLEVEL_DEFAULT, NULL, 0, fmt, args);
>  }
>  EXPORT_SYMBOL_GPL(vprintk_default);
>  
> diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
> index d9a659a686f3..7ccb821d0bfe 100644
> --- a/kernel/printk/printk_safe.c
> +++ b/kernel/printk/printk_safe.c
> @@ -6,6 +6,7 @@
>  #include <linux/preempt.h>
>  #include <linux/spinlock.h>
>  #include <linux/debug_locks.h>
> +#include <linux/kdb.h>
>  #include <linux/smp.h>
>  #include <linux/cpumask.h>
>  #include <linux/irq_work.h>
> @@ -359,6 +360,12 @@ void __printk_safe_exit(void)
>  
>  __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
>  {
> +#ifdef CONFIG_KGDB_KDB
> +	/* Allow to pass printk() to kdb but avoid a recursion. */
> +	if (unlikely(kdb_trap_printk && kdb_printf_cpu < 0))
> +		return vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args);
> +#endif
> +
>  	/*
>  	 * Try to use the main logbuf even in NMI. But avoid calling console
>  	 * drivers that might have their own locks.
> -- 
> 2.26.1
>