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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Sven Schnelle <svens@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 4.19 195/219] s390/ftrace: fix endless recursion in function_graph tracer
Date:   Sun, 29 Dec 2019 18:19:57 +0100
Message-Id: <20191229162539.369860382@linuxfoundation.org>
In-Reply-To: <20191229162508.458551679@linuxfoundation.org>
References: <20191229162508.458551679@linuxfoundation.org>
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From: Sven Schnelle <svens@linux.ibm.com>

[ Upstream commit 6feeee8efc53035c3195b02068b58ae947538aa4 ]

The following sequence triggers a kernel stack overflow on s390x:

mount -t tracefs tracefs /sys/kernel/tracing
cd /sys/kernel/tracing
echo function_graph > current_tracer
[crash]

This is because preempt_count_{add,sub} are in the list of traced
functions, which can be demonstrated by:

echo preempt_count_add >set_ftrace_filter
echo function_graph > current_tracer
[crash]

The stack overflow happens because get_tod_clock_monotonic() gets called
by ftrace but itself calls preempt_{disable,enable}(), which leads to a
endless recursion. Fix this by using preempt_{disable,enable}_notrace().

Fixes: 011620688a71 ("s390/time: ensure get_clock_monotonic() returns monotonic values")
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/s390/include/asm/timex.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/s390/include/asm/timex.h b/arch/s390/include/asm/timex.h
index 0f12a3f91282..2dc9eb4e1acc 100644
--- a/arch/s390/include/asm/timex.h
+++ b/arch/s390/include/asm/timex.h
@@ -195,9 +195,9 @@ static inline unsigned long long get_tod_clock_monotonic(void)
 {
 	unsigned long long tod;
 
-	preempt_disable();
+	preempt_disable_notrace();
 	tod = get_tod_clock() - *(unsigned long long *) &tod_clock_base[1];
-	preempt_enable();
+	preempt_enable_notrace();
 	return tod;
 }
 
-- 
2.20.1