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Subject: Re: [PATCH] of: unittest: Don't dereference args.np after test errors
To:     Guenter Roeck <linux@roeck-us.net>,
        Rob Herring <robh+dt@kernel.org>
Cc:     devicetree@vger.kernel.org, linux-kernel@vger.kernel.org
References: <1537720412-29919-1-git-send-email-linux@roeck-us.net>
From:   Frank Rowand <frowand.list@gmail.com>
Message-ID: <837495de-36b9-3bff-e3da-9ee79b9ef125@gmail.com>
Date:   Mon, 24 Sep 2018 21:19:55 -0700
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Hi Guenter,

On 09/23/18 09:33, Guenter Roeck wrote:
> If a devicetree parse function fails, it is quite likely that args.np
> is invalid. Trying to dereference it will then cause the system to crash.
> 
> This was seen when trying to run devicetree unittests on a g3beige
> qemu system. This system has the OF_IMAP_OLDWORLD_MAC flag set in
> of_irq_workarounds and expects an 'old style' structure of irq
> nodes. Trying to parse the test nodes fails and results in the
> following crash.
> 
> OF: /testcase-data/phandle-tests/consumer-b: arguments longer than property
> Unable to handle kernel paging request for data at address 0x00bc616e
> Faulting instruction address: 0xc092437c
> Oops: Kernel access of bad area, sig: 11 [#1]
> BE PREEMPT PowerMac
> Modules linked in:
> CPU: 0 PID: 1 Comm: swapper Not tainted 4.19.0-rc4-yocto-standard+ #1
> NIP:  c092437c LR: c0925098 CTR: c0925088
> REGS: cf8dfb40 TRAP: 0300   Not tainted  (4.19.0-rc4-yocto-standard+)
> MSR:  00001032 <ME,IR,DR,RI>  CR: 82004044  XER: 00000000
> DAR: 00bc616e DSISR: 40000000
> GPR00: c0925098 cf8dfbf0 cf8e0000 c14098c7 c14098c7 c1409c50 00000066 00000002
> GPR08: 00000063 00bc614e c0b483e9 000affff 82004048 00000000 00000008 c0b36d80
> GPR16: c0ac0000 c0b4233c c14098c7 c0b13c14 05ffffff 000affff c0b483e4 c0b00a30
> GPR24: cecfe324 cecfe324 c0acc434 c0ac8420 c1409c50 05ffffff c1409c50 c14098c7
> NIP [c092437c] device_node_gen_full_name+0x30/0x15c
> LR [c0925098] device_node_string+0x190/0x3c8
> Call Trace:
> [cf8dfbf0] [c0733704] of_find_property+0x5c/0x74 (unreliable)
> [cf8dfc30] [c0925098] device_node_string+0x190/0x3c8
> [cf8dfca0] [c092690c] pointer+0x274/0x4d4
> [cf8dfcd0] [c0926e20] vsnprintf+0x2b4/0x5ec
> [cf8dfd30] [c0927170] vscnprintf+0x18/0x48
> [cf8dfd40] [c008eb70] vprintk_store+0x4c/0x22c
> [cf8dfd70] [c008f1c4] vprintk_emit+0x94/0x270
> [cf8dfda0] [c008fb60] printk+0x5c/0x6c
> [cf8dfde0] [c0bd1ec0] of_unittest+0x2670/0x2b48
> [cf8dfe80] [c0004ba8] do_one_initcall+0xac/0x320
> [cf8dfee0] [c0b8975c] kernel_init_freeable+0x328/0x3f0
> [cf8dff30] [c00050c4] kernel_init+0x24/0x118
> [cf8dff40] [c0014278] ret_from_kernel_thread+0x14/0x1c
> 
> To avoid this and similar problems, don't try to dereference args.np
> after devicetree parse failures, and display the name of the parsed node
> instead. With this, we get error messages such as
> 
> dt-test ### FAIL of_unittest_parse_interrupts():791 index 0 -
> 	data error on node /testcase-data/interrupts/interrupts0 rc=-22
> 
> This may not display the intended node name, but it is better than a crash.

Thanks for finding and debugging the root cause of the problem.

As the patch comment notes, the changes do not fix the root cause, but
instead avoid the crash.  I'd like to deal with the root cause instead.

I've never encountered OF_IMAP_OLDWORLD_MAC and need to dig deeper to
understand exactly how having it set leads to the error returns from
of_parse_phandle_with_args().  Thus my off-the-top-of-my-head first
observation is likely to be incorrect.  But I'd like to point out
what I suspect is likely to be a more useful direction for the fix.

I'll use a bit of artful logic to claim that the root cause is that
of_parse_phandle_with_args() does not behave as unittests expect when
OF_IMAP_OLDWORLD_MAC is set.

If the of_parse_phandle_with_args() calls are not going to perform the
test that unittest expects to be performing, then it is pointless to
do the tests.  The fix then is to not do those tests.  For example,
at the top of of_unittest_parse_phandle_with_args(), simply do:

        if (of_irq_workarounds & OF_IMAP_OLDWORLD_MAC)
                return;

I did not look to see whether the other test areas you found can be
as easily avoided, without avoiding tests that are still valid when
OF_IMAP_OLDWORLD_MAC is set, but I am hoping so.

While looking at the patch, I noticed that the current
of_unittest_parse_phandle_with_args() also does not call of_node_put()
in the cases where of_count_phandle_with_args() does not return an
error.  I'll add fixing that to my todo list.

And as you pointed out, of_unittest_parse_phandle_with_args() should
not be blindly using the contents of args when an error occurred.  I'll
also put fixing that on my todo list.

-Frank


> 
> Fixes: 53a42093d96ef ("of: Add device tree selftests")
> Signed-off-by: Guenter Roeck <linux@roeck-us.net>
> ---
>  drivers/of/unittest.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/of/unittest.c b/drivers/of/unittest.c
> index 722537e14848..5942ddce1b9f 100644
> --- a/drivers/of/unittest.c
> +++ b/drivers/of/unittest.c
> @@ -424,7 +424,7 @@ static void __init of_unittest_parse_phandle_with_args(void)
>  		}
>  
>  		unittest(passed, "index %i - data error on node %pOF rc=%i\n",
> -			 i, args.np, rc);
> +			 i, np, rc);
>  	}
>  
>  	/* Check for missing list property */
> @@ -554,8 +554,8 @@ static void __init of_unittest_parse_phandle_with_args_map(void)
>  			passed = false;
>  		}
>  
> -		unittest(passed, "index %i - data error on node %s rc=%i\n",
> -			 i, args.np->full_name, rc);
> +		unittest(passed, "index %i - data error on node %pOF rc=%i\n",
> +			 i, np, rc);
>  	}
>  
>  	/* Check for missing list property */
> @@ -788,7 +788,7 @@ static void __init of_unittest_parse_interrupts(void)
>  		passed &= (args.args[0] == (i + 1));
>  
>  		unittest(passed, "index %i - data error on node %pOF rc=%i\n",
> -			 i, args.np, rc);
> +			 i, np, rc);
>  	}
>  	of_node_put(np);
>  
> @@ -834,7 +834,7 @@ static void __init of_unittest_parse_interrupts(void)
>  			passed = false;
>  		}
>  		unittest(passed, "index %i - data error on node %pOF rc=%i\n",
> -			 i, args.np, rc);
> +			 i, np, rc);
>  	}
>  	of_node_put(np);
>  }
> @@ -904,7 +904,7 @@ static void __init of_unittest_parse_interrupts_extended(void)
>  		}
>  
>  		unittest(passed, "index %i - data error on node %pOF rc=%i\n",
> -			 i, args.np, rc);
> +			 i, np, rc);
>  	}
>  	of_node_put(np);
>  }
>