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

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Message-ID: <1488869455.4285.10.camel@gmail.com>
Date:   Tue, 07 Mar 2017 17:50:55 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>, Jessica Yu <jeyu@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>, Petr Mladek <pmladek@...e.com>
Cc:     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>
Subject: Re: [PATCH v5 01/15] stacktrace/x86: add function for detecting
 reliable stack traces

On Mon, 2017-02-13 at 19:42 -0600, 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>
> ---

Could you comment on why we need a reliable trace for live-patching? Are
we in any way reliant on the stack trace to patch something broken?

Thanks,
Balbir Singh.