linux-kernel - [PATCH 1/1] Fix kdb erroneously reporting an INT3 breakpoint is a catastrophic error

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Date:	Fri, 11 Dec 2015 23:04:39 -0700
From:	Jeff Merkey <linux.mdb@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Jason Wessel <jason.wessel@...driver.com>
Subject: [PATCH 1/1] Fix kdb erroneously reporting an INT3 breakpoint is a
 catastrophic error

The default kdb setting for entry assumes that a system oops was
triggered rather than a request for debugger entry.  When an
application codes an int3 breakpoint the intent is to break into a
debugger -- that's what the instruction is for.

I noticed that kdb and kgdb both use an int3 breakpoint to trigger
debugger entry from the sysrq-trigger interface.  So it seems odd that
it would treat another int3 as a catastrophic error.

I would also like to discuss the possibility of a simple registration
interface in kgdboc for the mdb debugger and addition of the text
string "mdb" to select which debugger is active so the system
debuggers don't step all over each other that will allow other
debuggers to register and for us to share the "g" sysrq-trigger key.
At present, I disable kdb/kgdb on module load and would like to put
together something more friendly for folks using it.

I really like the interface in kgdboc for selecting which debugger is
active.  You are always welcome to be involved in mdb is you wish or
are bored.

Since it has tagged and deref disassembly and just runs on the machine
as a module, its easier to setup and use than kgdb and is not headless
like kdb.  There are a lot of folks using it based on emails I get and
the small but constant downloads over the last 15 years I have
distributed it.

This patch changes the default so when kdb sees another int3
breakpoint, it enters the debugger (which is does anyway) but without
the enter multiple times message to continue and thinking a
catastrophic error has occurred.  I have tested kdb/kgdb with mdb both
active and have it setup so they don't crash each other but its better
to not ping pong between consoles.

At any rate, I offer the peace pipe ...

Signed-off-by:  Jeff V. Merkey <jeffmerkey@...il.com>
diff --git a/kernel/debug/kdb/kdb_debugger.c b/kernel/debug/kdb/kdb_debugger.c
index 15e1a7a..d9b944d 100644
--- a/kernel/debug/kdb/kdb_debugger.c
+++ b/kernel/debug/kdb/kdb_debugger.c
@@ -55,7 +55,7 @@ int kdb_stub(struct kgdb_state *ks)
 	int error = 0;
 	kdb_bp_t *bp;
 	unsigned long addr = kgdb_arch_pc(ks->ex_vector, ks->linux_regs);
-	kdb_reason_t reason = KDB_REASON_OOPS;
+	kdb_reason_t reason = KDB_REASON_KEYBOARD;
 	kdb_dbtrap_t db_result = KDB_DB_NOBPT;
 	int i;
--
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