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Date:   Wed, 15 Feb 2017 13:18:40 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     Jessica Yu <jeyu@...hat.com>, Jiri Kosina <jikos@...nel.org>,
        Petr Mladek <pmladek@...e.com>, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Heiko Carstens <heiko.carstens@...ibm.com>, x86@...nel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        Vojtech Pavlik <vojtech@...e.com>, Jiri Slaby <jslaby@...e.cz>,
        Chris J Arges <chris.j.arges@...onical.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        Balbir Singh <bsingharora@...il.com>
Subject: Re: [PATCH v5 01/15] stacktrace/x86: add function for detecting
 reliable stack traces

On Mon, 13 Feb 2017, Josh Poimboeuf wrote:

> For live patching and possibly other use cases, a stack trace is only
> useful if it can be assured that it's completely reliable.  Add a new
> save_stack_trace_tsk_reliable() function to achieve that.
> 
> Note that if the target task isn't the current task, and the target task
> is allowed to run, then it could be writing the stack while the unwinder
> is reading it, resulting in possible corruption.  So the caller of
> save_stack_trace_tsk_reliable() must ensure that the task is either
> 'current' or inactive.
> 
> save_stack_trace_tsk_reliable() relies on the x86 unwinder's detection
> of pt_regs on the stack.  If the pt_regs are not user-mode registers
> from a syscall, then they indicate an in-kernel interrupt or exception
> (e.g. preemption or a page fault), in which case the stack is considered
> unreliable due to the nature of frame pointers.
> 
> It also relies on the x86 unwinder's detection of other issues, such as:
> 
> - corrupted stack data
> - stack grows the wrong way
> - stack walk doesn't reach the bottom
> - user didn't provide a large enough entries array
> 
> Such issues are reported by checking unwind_error() and !unwind_done().
> 
> Also add CONFIG_HAVE_RELIABLE_STACKTRACE so arch-independent code can
> determine at build time whether the function is implemented.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>

I do not see any difference from 4.1 version, so my

Reviewed-by: Miroslav Benes <mbenes@...e.cz>

stays.

Regards,
Miroslav

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