linux-kernel - Re: [PATCH v5 01/15] stacktrace/x86: add function for detecting reliable stack traces

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Message-ID: <20170215144044.yn63piijka7zqqlz@treble>
Date:   Wed, 15 Feb 2017 08:40:44 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Miroslav Benes <mbenes@...e.cz>
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 Wed, Feb 15, 2017 at 01:18:40PM +0100, Miroslav Benes wrote:
> 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>

Sorry, forgot to add your Reviewed-by from last time.  The patch is
indeed the same.  Thanks!

-- 
Josh