[linux-yocto] [yocto-4.12] [PATCH] x86/debug: Handle warnings before the notifier chain, to fix KGDB crash

[Hongzhi.Song]

From: Alexander Shishkin <alexander.shishkin at linux.intel.com>

commit b8347c2196492f4e1cccde3d92fda1cc2cc7de7e upstream

Commit:

  9a93848fe787 ("x86/debug: Implement __WARN() using UD0")

turned warnings into UD0, but the fixup code only runs after the
notify_die() chain. This is a problem, in particular, with kgdb,
which kicks in as if it was a BUG().

Fix this by running the fixup code before the notifier chain in
the invalid op handler path.

Signed-off-by: Alexander Shishkin <alexander.shishkin at linux.intel.com>
Tested-by: Ilya Dryomov <idryomov at gmail.com>
Acked-by: Daniel Thompson <daniel.thompson at linaro.org>
Acked-by: Thomas Gleixner <tglx at linutronix.de>
Cc: Jason Wessel <jason.wessel at windriver.com>
Cc: Arjan van de Ven <arjan at linux.intel.com>
Cc: Borislav Petkov <bp at alien8.de>
Cc: Linus Torvalds <torvalds at linux-foundation.org>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Richard Weinberger <richard.weinberger at gmail.com>
Cc: <stable at vger.kernel.org> # v4.12+
Link: http://lkml.kernel.org/r/20170724100428.19173-1-alexander.shishkin@linux.intel.com
Signed-off-by: Ingo Molnar <mingo at kernel.org>

This patch is to resolve the problem shown as follows, when testing KGDB with 'echo kgdbts=V1 > /sys/module/kgdbts/parameters/kgdbts':

[  196.275931] Kernel panic - not syncing: Recursive entry to debugger
[  196.276658] CPU: 0 PID: 1495 Comm: sh Not tainted 4.12.16-yocto-standard #2
[  196.277648] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.10.2-0-g5f4c7b1-prebuilt.qemu-project.org 04/01/2014
[  196.278634] Call Trace:
[  196.278891]  <IRQ>
[  196.279124]  dump_stack+0x65/0x8d
[  196.279477]  ? kgdb_breakpoint+0x13/0x20
[  196.279968]  kgdb_handle_exception+0x16e/0x1a0
[  196.280405]  __kgdb_notify+0x6d/0xd0
[  196.280761]  kgdb_notify+0x23/0x40
[  196.281189]  notifier_call_chain+0x4a/0x70
[  196.281589]  atomic_notifier_call_chain+0x33/0x50
[  196.282033]  notify_die+0x2e/0x30
[  196.282469]  do_int3+0x70/0x100
[  196.282792]  int3+0x35/0x70
[  196.283080] RIP: 0010:kgdb_breakpoint+0x14/0x20
[  196.283596] RSP: 0000:ffff9f2a0fc01d00 EFLAGS: 00000002
[  196.284078] RAX: 0000000000000000 RBX: 00000000ffffffff RCX: 00000000ffffffff
[  196.284751] RDX: ffffffffa90cd700 RSI: 0000000000000000 RDI: ffffffffa8e4ca10
[  196.285356] RBP: ffff9f2a0fc01d00 R08: 0000000000000000 R09: ffffffffa8e4ca10
[  196.286002] R10: ffff9f2a0fc01e00 R11: ffffffffa90d40ed R12: ffffffffa8e4d2e0
[  196.286614] R13: 0000000000000000 R14: ffffffffa90cd700 R15: 0000000000000000

Signed-off-by: Hongzhi.Song <hongzhi.song at windriver.com>
---
 arch/x86/kernel/traps.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 67db4f4..5a6b8f8 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -209,9 +209,6 @@ do_trap_no_signal(struct task_struct *tsk, int trapnr, char *str,
 		if (fixup_exception(regs, trapnr))
 			return 0;
 
-		if (fixup_bug(regs, trapnr))
-			return 0;
-
 		tsk->thread.error_code = error_code;
 		tsk->thread.trap_nr = trapnr;
 		die(str, regs, error_code);
@@ -292,6 +289,13 @@ static void do_error_trap(struct pt_regs *regs, long error_code, char *str,
 
 	RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
 
+	/*
+	 * WARN*()s end up here; fix them up before we call the
+	 * notifier chain.
+	 */
+	if (!user_mode(regs) && fixup_bug(regs, trapnr))
+		return;
+
 	if (notify_die(DIE_TRAP, str, regs, error_code, trapnr, signr) !=
 			NOTIFY_STOP) {
 		cond_local_irq_enable(regs);
-- 
2.8.1