linux-kernel - Re: [RFC PATCH v3 1/4] arm64: Introduce stack trace reliability checks in the unwinder

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Message-ID: <b000767b-26ca-01a9-a109-c9fc3357f832@linux.microsoft.com>
Date:   Tue, 4 May 2021 18:13:39 -0500
From:   "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     broonie@...nel.org, mark.rutland@....com, jthierry@...hat.com,
        catalin.marinas@....com, will@...nel.org, jmorris@...ei.org,
        pasha.tatashin@...een.com, linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v3 1/4] arm64: Introduce stack trace reliability
 checks in the unwinder

On 5/4/21 4:52 PM, Josh Poimboeuf wrote:
> On Mon, May 03, 2021 at 12:36:12PM -0500, madvenka@...ux.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.

> 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.

Madhavan