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Subject: Re: [RFC PATCH v3 1/4] arm64: Introduce stack trace reliability
 checks in the unwinder
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     broonie@kernel.org, mark.rutland@arm.com, jthierry@redhat.com,
        catalin.marinas@arm.com, will@kernel.org, jmorris@namei.org,
        pasha.tatashin@soleen.com, linux-arm-kernel@lists.infradead.org,
        live-patching@vger.kernel.org, linux-kernel@vger.kernel.org
References: <65cf4dfbc439b010b50a0c46ec500432acde86d6>
 <20210503173615.21576-1-madvenka@linux.microsoft.com>
 <20210503173615.21576-2-madvenka@linux.microsoft.com>
 <20210504215248.oi3zay3memgqri33@treble>
 <b000767b-26ca-01a9-a109-c9fc3357f832@linux.microsoft.com>
 <20210505000728.yxg3xbwa3emcu2wi@treble>
From:   "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
Message-ID: <2fc9a77a-ddf5-812a-0681-ece94b433d71@linux.microsoft.com>
Date:   Tue, 4 May 2021 19:21:37 -0500
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On 5/4/21 7:07 PM, Josh Poimboeuf wrote:
> On Tue, May 04, 2021 at 06:13:39PM -0500, Madhavan T. Venkataraman wrote:
>>
>>
>> On 5/4/21 4:52 PM, Josh Poimboeuf wrote:
>>> On Mon, May 03, 2021 at 12:36:12PM -0500, madvenka@linux.microsoft.com wrote:
>>>> @@ -44,6 +44,8 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
>>>>  	unsigned long fp = frame->fp;
>>>>  	struct stack_info info;
>>>>  
>>>> +	frame->reliable = true;
>>>> +
>>>
>>> Why set 'reliable' to true on every invocation of unwind_frame()?
>>> Shouldn't it be remembered across frames?
>>>
>>
>> This is mainly for debug purposes in case a caller wants to print the whole stack and also
>> print which functions are unreliable. For livepatch, it does not make any difference. It will
>> quit as soon as it encounters an unreliable frame.
> 
> Hm, ok.  So 'frame->reliable' refers to the current frame, not the
> entire stack.
> 

Yes.

>>> Also, it looks like there are several error scenarios where it returns
>>> -EINVAL but doesn't set 'reliable' to false.
>>>
>>
>> I wanted to make a distinction between an error situation (like stack corruption where unwinding
>> has to stop) and an unreliable situation (where unwinding can still proceed). E.g., when a
>> stack trace is taken for informational purposes or debug purposes, the unwinding will try to
>> proceed until either the stack trace ends or an error happens.
> 
> Ok, but I don't understand how that relates to my comment.
> 
> Why wouldn't a stack corruption like !on_accessible_stack() set
> 'frame->reliable' to false?
> 

I do see your point. If an error has been hit, then the stack trace is essentially unreliable
regardless of anything else. So, I accept your comment. I will mark the stack trace as unreliable
if any kind of error is encountered.

Thanks!

Madhavan