linux-kernel - Re: [PATCH 1/3] printk: Fix kdb_trap

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Date:   Tue, 29 Nov 2016 14:54:33 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Tejun Heo <tj@...nel.org>,
        Calvin Owens <calvinowens@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Jason Wessel <jason.wessel@...driver.com>
Subject: Re: [PATCH 1/3] printk: Fix kdb_trap_printk placement

On Tue 2016-10-18 19:08:31, Peter Zijlstra wrote:
> Some people figured vprintk_emit() makes for a nice API and exported
> it, bypassing the kdb trap.

I think that nobody saw this problem because kdb_trap_printk was
used only for a limited number of printk's. It is just a trick
how to use generic functions to print messages in the kdb context,
e.g. for getting backtraces.

But the patch makes sense.


> This still leaves vprintk_nmi() outside of the kbd reach, should that
> be fixed too?

It is perfectly fine. Messages from NMI context are not meant for
kdb anyway.

> Cc: Jason Wessel <jason.wessel@...driver.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  kernel/printk/printk.c |   19 ++++++++-----------
>  1 file changed, 8 insertions(+), 11 deletions(-)
> 
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1750,6 +1750,13 @@ asmlinkage int vprintk_emit(int facility
>  	/* cpu currently holding logbuf_lock in this function */
>  	static unsigned int logbuf_cpu = UINT_MAX;
>  
> +#ifdef CONFIG_KGDB_KDB
> +	if (unlikely(kdb_trap_printk)) {
> +		r = vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args);
> +		return r;

r is not defined. It is fixed in the next patch but it breaks
bisectability.

Please, find below an updated patch that also includes
my Reviewed-by.


>From 5adf18de45ba986ea3ae611446828238f4d65fe0 Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@...radead.org>
Date: Tue, 18 Oct 2016 19:08:31 +0200
Subject: [PATCH 1/3] printk: Fix kdb_trap_printk placement

Some people figured vprintk_emit() makes for a nice API and exported
it, bypassing the kdb trap.

This still leaves vprintk_nmi() outside of the kbd reach, should that
be fixed too?

Cc: Jason Wessel <jason.wessel@...driver.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Reviewed-by: Petr Mladek <pmladek@...e.com>
---
 kernel/printk/printk.c | 17 ++++++-----------
 1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f7a55e9ff2f7..541ce7705353 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1781,6 +1781,11 @@ asmlinkage int vprintk_emit(int facility, int level,
 	/* cpu currently holding logbuf_lock in this function */
 	static unsigned int logbuf_cpu = UINT_MAX;
 
+#ifdef CONFIG_KGDB_KDB
+	if (unlikely(kdb_trap_printk))
+		return vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args);
+#endif
+
 	if (level == LOGLEVEL_SCHED) {
 		level = LOGLEVEL_DEFAULT;
 		in_sched = true;
@@ -1923,17 +1928,7 @@ asmlinkage int printk_emit(int facility, int level,
 
 int vprintk_default(const char *fmt, va_list args)
 {
-	int r;
-
-#ifdef CONFIG_KGDB_KDB
-	if (unlikely(kdb_trap_printk)) {
-		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);
 
-- 
1.8.5.6