Re: [PATCH] kgdb: Fix broken handling of printk() in NMI context

[Petr Mladek <pmladek@suse.com>]

On Wed 2020-05-13 19:04:48, Sumit Garg wrote:
> On Tue, 12 May 2020 at 19:55, Daniel Thompson
> <daniel.thompson@linaro.org> wrote:
> >
> > On Tue, May 12, 2020 at 02:18:34PM +0530, Sumit Garg wrote:
> > > Since commit 42a0bb3f7138 ("printk/nmi: generic solution for safe printk
> > > in NMI"), kgdb entry in NMI context defaults to use safe NMI printk()
> >
> > I didn't see the author on Cc: nor any of the folks whose hands it
> > passed through. It would definitely be good to involve them in this
> > discussion.
> >
> 
> Thanks for updating the Cc: list.
> 
> >
> > > which involves CPU specific buffers and deferred printk() until exit from
> > > NMI context.
> > >
> > > But kgdb being a stop-the-world debugger, we don't want to defer printk()
> > > especially backtrace on corresponding CPUs. So instead switch to normal
> > > printk() mode in kgdb_cpu_enter() if entry is in NMI context.
> >
> > So, firstly I should *definitely* take a mea cupla for not shouting
> > about this at the time (I was on Cc:... twice). Only thing I can say
> > confidently is that the test suite didn't yell about this and so I
> > didn't look at this as closely as I should have done (and that it
> > didn't yell is mostly because I'm still building out the test suite
> > coverage).
> >
> > Anyhow...
> >
> > This feels a little like we are smearing the printk() interception logic
> > across the kernel in ways that make things hard to read. If we accepted
> > this patch we then have, the new NMI interception logic, the old kdb
> > interception logic and some hacks in the kgdb trap handler to defang the
> > NMI interception logic and force the kdb logic to kick in.
> >
> > Wouldn't it be better to migrate kdb interception logic up a couple of
> > levels so that it continues to function even when we are in nmi printk
> > mode. That way *all* the printk() interception code would end up in
> > one place.
> >
> 
> Yes it would be better to have all printk() interception code at one
> place. Let me see if I can come up with an integrated logic.

It might be enough to move the kdb_check from vprintk_default()
to vprintk_func().

I have never used kdb. I did not know that it was able to stop
kernel in any context.

Would this work? It is only compile tested!

From 14ae6c9f0cbd1479cb898c864c7ab46e20f3cf6f Mon Sep 17 00:00:00 2001
From: Petr Mladek <pmladek@suse.com>
Date: Thu, 14 May 2020 10:37:44 +0200
Subject: [PATCH] printk/kdb: Redirect printk messages into kdb in any context

kdb is able to stop kernel even in NMI context where printk() is redirected
to the printk_safe() lockless variant. Move the check and redirect to kdb
even in this case.

Signed-off-by: Petr Mladek <pmladek@suse.com>
---
 kernel/printk/printk.c      | 14 +-------------
 kernel/printk/printk_safe.c |  8 ++++++++
 2 files changed, 9 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..81734497c625 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 +360,13 @@ 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