linux-kernel - [PATCH] nios2: time: Read timer in get

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Message-Id: <1505187926-19399-1-git-send-email-linux@roeck-us.net>
Date:   Mon, 11 Sep 2017 20:45:26 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Ley Foon Tan <lftan@...era.com>
Cc:     nios2-dev@...ts.rocketboards.org, linux-kernel@...r.kernel.org,
        Guenter Roeck <linux@...ck-us.net>,
        Laura Abbott <labbott@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Daniel Micay <danielmicay@...il.com>
Subject: [PATCH] nios2: time: Read timer in get_cycles only if initialized

Mainline crashes as follows when running nios2 images.

On node 0 totalpages: 65536
free_area_init_node: node 0, pgdat c8408fa0, node_mem_map c8726000
  Normal zone: 512 pages used for memmap
  Normal zone: 0 pages reserved
  Normal zone: 65536 pages, LIFO batch:15
Unable to handle kernel NULL pointer dereference at virtual address 00000000
ea = c8003cb0, ra = c81cbf40, cause = 15
Kernel panic - not syncing: Oops

Problem is seen because get_cycles() is called before the timer it depends
on is initialized. Returning 0 in that situation fixes the problem.

Fixes: 33d72f3822d7 ("init/main.c: extract early boot entropy from the ..")
Cc: Laura Abbott <labbott@...hat.com>
Cc: Kees Cook <keescook@...omium.org>
Cc: Daniel Micay <danielmicay@...il.com>
Signed-off-by: Guenter Roeck <linux@...ck-us.net>
---
Taking a stab at the problem. Fix works for my qemu test.

 arch/nios2/kernel/time.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/nios2/kernel/time.c b/arch/nios2/kernel/time.c
index 645129aaa9a0..20e86209ef2e 100644
--- a/arch/nios2/kernel/time.c
+++ b/arch/nios2/kernel/time.c
@@ -107,7 +107,10 @@ static struct nios2_clocksource nios2_cs = {
 
 cycles_t get_cycles(void)
 {
-	return nios2_timer_read(&nios2_cs.cs);
+	/* Only read timer if it has been initialized */
+	if (nios2_cs.timer.base)
+		return nios2_timer_read(&nios2_cs.cs);
+	return 0;
 }
 EXPORT_SYMBOL(get_cycles);
 
-- 
2.7.4