linux-kernel - Re: [PATCH v4 5/6] powerpc: Add show_user

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Message-ID: <69cf990b-d4aa-97e7-be3b-7936caa91688@c-s.fr>
Date:   Fri, 3 Aug 2018 08:38:26 +0200
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Murilo Opsfelder Araujo <muriloo@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org,
        Alastair D'Silva <alastair@...ilva.org>,
        Andrew Donnellan <andrew.donnellan@....ibm.com>,
        Balbir Singh <bsingharora@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Cyril Bur <cyrilbur@...il.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Joe Perches <joe@...ches.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michael Neuling <mikey@...ling.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Paul Mackerras <paulus@...ba.org>,
        Simon Guo <wei.guo.simon@...il.com>,
        Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
        "Tobin C . Harding" <me@...in.cc>, linuxppc-dev@...ts.ozlabs.org,
        Segher Boessenkool <segher@...nel.crashing.org>
Subject: Re: [PATCH v4 5/6] powerpc: Add show_user_instructions()

Hi Murilo,

Le 03/08/2018 à 02:42, Murilo Opsfelder Araujo a écrit :
> Hi, Christophe.
> 
> On Thu, Aug 02, 2018 at 07:26:20AM +0200, Christophe LEROY wrote:
>>
>>
>> Le 01/08/2018 à 23:33, Murilo Opsfelder Araujo a écrit :
>>> show_user_instructions() is a slightly modified version of
>>> show_instructions() that allows userspace instruction dump.
>>>
>>> This will be useful within show_signal_msg() to dump userspace
>>> instructions of the faulty location.
>>>
>>> Here is a sample of what show_user_instructions() outputs:
>>>
>>>     pandafault[10850]: code: 4bfffeec 4bfffee8 3c401002 38427f00 fbe1fff8 f821ffc1 7c3f0b78 3d22fffe
>>>     pandafault[10850]: code: 392988d0 f93f0020 e93f0020 39400048 <99490000> 39200000 7d234b78 383f0040
>>>
>>> The current->comm and current->pid printed can serve as a glue that
>>> links the instructions dump to its originator, allowing messages to be
>>> interleaved in the logs.
>>>
>>> Signed-off-by: Murilo Opsfelder Araujo <muriloo@...ux.ibm.com>
>>> ---
>>>    arch/powerpc/include/asm/stacktrace.h | 13 +++++++++
>>>    arch/powerpc/kernel/process.c         | 40 +++++++++++++++++++++++++++
>>>    2 files changed, 53 insertions(+)
>>>    create mode 100644 arch/powerpc/include/asm/stacktrace.h
>>>
>>> diff --git a/arch/powerpc/include/asm/stacktrace.h b/arch/powerpc/include/asm/stacktrace.h
>>> new file mode 100644
>>> index 000000000000..6149b53b3bc8
>>> --- /dev/null
>>> +++ b/arch/powerpc/include/asm/stacktrace.h
>>> @@ -0,0 +1,13 @@
>>> +/* SPDX-License-Identifier: GPL-2.0 */
>>> +/*
>>> + * Stack trace functions.
>>> + *
>>> + * Copyright 2018, Murilo Opsfelder Araujo, IBM Corporation.
>>> + */
>>> +
>>> +#ifndef _ASM_POWERPC_STACKTRACE_H
>>> +#define _ASM_POWERPC_STACKTRACE_H
>>> +
>>> +void show_user_instructions(struct pt_regs *regs);
>>> +
>>> +#endif /* _ASM_POWERPC_STACKTRACE_H */
>>> diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
>>> index e9533b4d2f08..364645ac732c 100644
>>> --- a/arch/powerpc/kernel/process.c
>>> +++ b/arch/powerpc/kernel/process.c
>>> @@ -1299,6 +1299,46 @@ static void show_instructions(struct pt_regs *regs)
>>>    	pr_cont("\n");
>>>    }
>>> +void show_user_instructions(struct pt_regs *regs)
>>> +{
>>> +	int i;
>>> +	const char *prefix = KERN_INFO "%s[%d]: code: ";
>>> +	unsigned long pc = regs->nip - (instructions_to_print * 3 / 4 *
>>> +					sizeof(int));
>>> +
>>> +	printk(prefix, current->comm, current->pid);
>>
>> Why not use pr_info() and remove KERN_INFO from *prefix ?
> 
> Because it doesn't compile:
> 
>    arch/powerpc/kernel/process.c:1317:10: error: expected ‘)’ before ‘prefix’
>      pr_info(prefix, current->comm, current->pid);
>              ^
>    ./include/linux/printk.h:288:21: note: in definition of macro ‘pr_fmt’
>     #define pr_fmt(fmt) fmt
>                       ^
> 
> `pr_info(prefix, ...)` expands to `printk("\001" "6" prefix, ...)`,
> which is an invalid string concatenation.
> 
> `pr_info("%s", ...)` expands to `printk("\001" "6" "%s", ...)`, which is
> valid.

Then what about using directly:

pr_info("%s[%d]: code: ", ...);

> 
>>> +
>>> +	for (i = 0; i < instructions_to_print; i++) {
>>> +		int instr;
>>> +
>>> +		if (!(i % 8) && (i > 0)) {
>>> +			pr_cont("\n");
>>> +			printk(prefix, current->comm, current->pid);
>>> +		}
>>> +
>>> +#if !defined(CONFIG_BOOKE)
>>> +		/* If executing with the IMMU off, adjust pc rather
>>> +		 * than print XXXXXXXX.
>>> +		 */
>>> +		if (!(regs->msr & MSR_IR))
>>> +			pc = (unsigned long)phys_to_virt(pc);
>>
>> Shouldn't this be done outside of the loop, only once ?
> 
> I don't think so.
> 
> pc gets incremented at the bottom of the loop:
> 
>    pc += sizeof(int);
> 
> Adjusting pc is necessary at each iteration.  Leaving this block inside
> the loop seems correct.

This looks pretty strange.
The first time, pc is a physical address, that you change to a virtual 
address. Then when you increment it it is still a virtual address.
So when you call phys_to_virt(pc) for the second time, pc is already a 
virt address, so what happens indeed ?

Christophe

> 
> Cheers
> Murilo
>