[PATCH 19/19] [INCOMPLETE] ARM: make return_address available for ARM_UNWIND

On Mon, Jan 28, 2013 at 9:50 PM, Dave Martin [off-list ref] wrote:

On Mon, Jan 28, 2013 at 11:33:11AM +0900, Keun-O Park wrote:

quoted

Hello guys,

Could you please review the patch of fixing bug first of returning
wrong address when using frame pointer?
I am wondering if the first patch is not delivered to the mailing.

I posted a similar patch to alkml a couple of months ago, but I got
no response and it looks like I forgot about it.

http://lists.infradead.org/pipermail/linux-arm-kernel/2012-November/129381.html

Yes, same except initialization of data.addr. :)
This means there might be no one interested in using
ftrace-irqsoff/premptoff in ARM during a couple of months?

[...]

quoted

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From 3a60b536d22a2043d735c890a9aac9e7cb72de8f Mon Sep 17 00:00:00 2001
From: sahara <redacted>
Date: Thu, 3 Jan 2013 17:12:37 +0900
Subject: [PATCH 1/2] arm: fix returning wrong CALLER_ADDRx

This makes return_address return correct value for ftrace feature.
unwind_frame does not update frame->lr but frame->pc for backtrace.
And, the initialization for data.addr was missing so that wrong value
returned when unwind_frame failed.

Signed-off-by: sahara <redacted>
---
 arch/arm/kernel/return_address.c |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/arm/kernel/return_address.c b/arch/arm/kernel/return_address.c
index 8085417..fafedd8 100644
--- a/arch/arm/kernel/return_address.c
+++ b/arch/arm/kernel/return_address.c

@@ -26,7 +26,7 @@ static int save_return_addr(struct stackframe *frame, void *d)
        struct return_address_data *data = d;

        if (!data->level) {
-               data->addr = (void *)frame->lr;
+               data->addr = (void *)frame->pc;

                return 1;
        } else {

@@ -41,7 +41,8 @@ void *return_address(unsigned int level)
        struct stackframe frame;
        register unsigned long current_sp asm ("sp");

-       data.level = level + 1;
+       data.level = level + 2;
+       data.addr = NULL;

Can you explain why this is needed?  I think I concluded it wasn't
necessary, but you may be right -- I think if walk_stackframe()
fails to unwind the next frame just after data.level reaches zero,
then data.addr can remain unset and return_address() may return
uninitialised garbage.

That's correct.
Here is the examples of reproducing the problem.
I added one line printk for test in wakeup_flusher_threads() in
fs/fs-writeback.c.
And then after boot up, I synced.

[TEST#1 : print CALLER_ADDR0, 1 and 2]
Without initialization of data.addr:
~ # sync
TEST: CALLER_ADDR0=(sys_sync+0x34/0xac),
CALLER_ADDR1=(ret_fast_syscall+0x0/0x48),
CALLER_ADDR2=(ret_fast_syscall+0x0/0x48)
With initialization of data.addr:
~ # sync
TEST: CALLER_ADDR0=(sys_sync+0x34/0xac),
CALLER_ADDR1=(ret_fast_syscall+0x0/0x48), CALLER_ADDR2=(  (null))

[TEST#2 : print CALLER_ADDR0 and 2]
Without initialization of data.addr:
~ # sync
TEST: CALLER_ADDR0=(sys_sync+0x34/0xac), CALLER_ADDR2=(0x872fffb0)
With initialization of data.addr:
~ # sync
TEST: CALLER_ADDR0=(sys_sync+0x34/0xac), CALLER_ADDR2=((null))

As you see, when unwind_fame() fails right after data.level reaches zero,
the routine returns data.addr which has uninitialized garbage value.

-- kpark