From: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
To: Jiri Slaby <jslaby@suse.cz>, Matthias Kaehlcke <mka@chromium.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Douglas Anderson <dianders@chromium.org>,
Evan Green <evgreen@chromium.org>,
Manoj Gupta <manojgupta@chromium.org>,
Stephen Boyd <swboyd@chromium.org>,
Nick Desaulniers <ndesaulniers@google.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] tty/sysrq: Make local variable 'killer' in sysrq_handle_crash() global
Date: Tue, 18 Sep 2018 12:28:39 +0530 [thread overview]
Message-ID: <ebea1697-cf33-675b-172c-3e80303a9ee9@codeaurora.org> (raw)
In-Reply-To: <a75e8a5d-25d0-83b8-6f7c-b80e69985b60@suse.cz>
On 9/18/2018 11:41 AM, Jiri Slaby wrote:
> On 09/17/2018, 11:33 PM, Matthias Kaehlcke wrote:
>> sysrq_handle_crash() dereferences a NULL pointer on purpose to force
>> an exception, the local variable 'killer' is assigned to NULL and
>> dereferenced later. Clang detects the NULL pointer dereference at compile
>> time and emits a BRK instruction (on arm64) instead of the expected NULL
>> pointer exception. Change 'killer' to a global variable (and rename it
>> to 'sysrq_killer' to avoid possible clashes) to prevent Clang from
>> detecting the condition. By default global variables are initialized
>> with zero/NULL in C, therefore an explicit initialization is not needed.
>>
>> Reported-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
>> Suggested-by: Evan Green <evgreen@chromium.org>
>> Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
>> ---
>> drivers/tty/sysrq.c | 6 +++---
>> 1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
>> index 06ed20dd01ba..49fa8e758690 100644
>> --- a/drivers/tty/sysrq.c
>> +++ b/drivers/tty/sysrq.c
>> @@ -132,10 +132,10 @@ static struct sysrq_key_op sysrq_unraw_op = {
>> #define sysrq_unraw_op (*(struct sysrq_key_op *)NULL)
>> #endif /* CONFIG_VT */
>>
>> +char *sysrq_killer;
>> +
>> 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'
>> @@ -144,7 +144,7 @@ static void sysrq_handle_crash(int key)
>> rcu_read_unlock();
>> panic_on_oops = 1; /* force panic */
>> wmb();
>> - *killer = 1;
>> + *sysrq_killer = 1;
>
> Just because a static analyzer is wrong? Oh wait, even compiler is
> wrong. At least make it a static global. Or what about OPTIMIZER_HIDE_VAR?
>
static global does not work, clang still inserts brk. As for
OPTIMIZE_HIDE_VAR, it seems to work.
But, I dont think it is defined for clang in which case it defaults to
using barrier(). There is already one wmb(), so will it be right?
--
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member
of Code Aurora Forum, hosted by The Linux Foundation
next prev parent reply other threads:[~2018-09-18 6:58 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-17 21:33 [PATCH] tty/sysrq: Make local variable 'killer' in sysrq_handle_crash() global Matthias Kaehlcke
2018-09-18 6:09 ` Sai Prakash Ranjan
2018-09-18 6:11 ` Jiri Slaby
2018-09-18 6:58 ` Sai Prakash Ranjan [this message]
2018-09-18 7:20 ` Greg Kroah-Hartman
2018-09-18 9:05 ` Sai Prakash Ranjan
2018-09-18 9:17 ` Greg Kroah-Hartman
2018-09-18 9:53 ` Sai Prakash Ranjan
[not found] ` <32bf1760-4967-a37a-6a17-3f7d5f6e071e@suse.cz>
2018-09-18 17:32 ` Matthias Kaehlcke
2018-09-18 17:24 ` Matthias Kaehlcke
2018-09-18 11:46 ` David Laight
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