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Subject[PATCH] sysrq: Use panic() to force a crash
Date
sysrq_handle_crash() currently forces a crash by dereferencing a
NULL pointer, which is undefined behavior in C. Just call panic()
instead, which is simpler and doesn't depend on compiler specific
handling of the undefined behavior.

Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
---
Not sure if it is strictly needed to release the RCU read lock now
that panic() is invoked directly (I couldn't repro the warning
without rcu_read_unlock()), but since this is a forced crash it
seems good practice to keep doing it.

The commit that added rcu_read_unlock() and the comment is:

commit 984cf355aeaa8f2eda3861b50d0e8d3e3f77e83b
Author: Ani Sinha <ani@arista.com>
Date: Thu Dec 17 17:15:10 2015 -0800

sysrq: Fix warning in sysrq generated crash.

Commit 984d74a72076a1 ("sysrq: rcu-ify __handle_sysrq") replaced
spin_lock_irqsave() calls with rcu_read_lock() calls in sysrq. Since
rcu_read_lock() does not disable preemption, faulthandler_disabled() in
__do_page_fault() in x86/fault.c returns false. When the code later calls
might_sleep() in the pagefault handler, we get the following warning:

BUG: sleeping function called from invalid context at ../arch/x86/mm/fault.c:1187
in_atomic(): 0, irqs_disabled(): 0, pid: 4706, name: bash
Preemption disabled at:[<ffffffff81484339>] printk+0x48/0x4a

To fix this, we release the RCU read lock before we crash.

Tested this patch on linux 3.18 by booting off one of our boards.
---
drivers/tty/sysrq.c | 13 +++----------
1 file changed, 3 insertions(+), 10 deletions(-)

diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index 06ed20dd01ba..d779a51499a0 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -134,17 +134,10 @@ static struct sysrq_key_op sysrq_unraw_op = {

static void sysrq_handle_crash(int key)
{
- char *killer = NULL;
-
- /* we need to release the RCU read lock here,
- * otherwise we get an annoying
- * 'BUG: sleeping function called from invalid context'
- * complaint from the kernel before the panic.
- */
+ /* release the RCU read lock before crashing */
rcu_read_unlock();
- panic_on_oops = 1; /* force panic */
- wmb();
- *killer = 1;
+
+ panic("sysrq triggered crash\n");
}
static struct sysrq_key_op sysrq_crash_op = {
.handler = sysrq_handle_crash,
--
2.19.0.397.gdd90340f6a-goog
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