From: Josh Poimboeuf <jpoimboe@redhat.com>
To: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
broonie@kernel.org, jthierry@redhat.com, catalin.marinas@arm.com,
will@kernel.org, linux-arm-kernel@lists.infradead.org,
live-patching@vger.kernel.org, linux-kernel@vger.kernel.org,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [RFC PATCH v2 0/4] arm64: Implement stack trace reliability checks
Date: Fri, 9 Apr 2021 17:32:27 -0500 [thread overview]
Message-ID: <20210409223227.rvf6tfhvgnpzmabn@treble> (raw)
In-Reply-To: <8c30ec5f-b51e-494f-5f6c-d2f012135f69@linux.microsoft.com>
On Fri, Apr 09, 2021 at 05:05:58PM -0500, Madhavan T. Venkataraman wrote:
> > FWIW, over the years we've had zero issues with encoding the frame
> > pointer on x86. After you save pt_regs, you encode the frame pointer to
> > point to it. Ideally in the same macro so it's hard to overlook.
> >
>
> I had the same opinion. In fact, in my encoding scheme, I have additional
> checks to make absolutely sure that it is a true encoding and not stack
> corruption. The chances of all of those values accidentally matching are,
> well, null.
Right, stack corruption -- which is already exceedingly rare -- would
have to be combined with a miracle or two in order to come out of the
whole thing marked as 'reliable' :-)
And really, we already take a similar risk today by "trusting" the frame
pointer value on the stack to a certain extent.
> >> I think there's a lot more code that we cannot unwind, e.g. KVM
> >> exception code, or almost anything marked with SYM_CODE_END().
> >
> > Just a reminder that livepatch only unwinds blocked tasks (plus the
> > 'current' task which calls into livepatch). So practically speaking, it
> > doesn't matter whether the 'unreliable' detection has full coverage.
> > The only exceptions which really matter are those which end up calling
> > schedule(), e.g. preemption or page faults.
> >
> > Being able to consistently detect *all* possible unreliable paths would
> > be nice in theory, but it's unnecessary and may not be worth the extra
> > complexity.
> >
>
> You do have a point. I tried to think of arch_stack_walk_reliable() as
> something that should be implemented independent of livepatching. But
> I could not really come up with a single example of where else it would
> really be useful.
>
> So, if we assume that the reliable stack trace is solely for the purpose
> of livepatching, I agree with your earlier comments as well.
One thought: if folks really view this as a problem, it might help to
just rename things to reduce confusion.
For example, instead of calling it 'reliable', we could call it
something more precise, like 'klp_reliable', to indicate that its
reliable enough for live patching.
Then have a comment above 'klp_reliable' and/or
stack_trace_save_tsk_klp_reliable() which describes what that means.
Hm, for that matter, even without renaming things, a comment above
stack_trace_save_tsk_reliable() describing the meaning of "reliable"
would be a good idea.
--
Josh
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next prev parent reply other threads:[~2021-04-09 22:44 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <705993ccb34a611c75cdae0a8cb1b40f9b218ebd>
2021-04-05 20:43 ` [RFC PATCH v2 0/4] arm64: Implement stack trace reliability checks madvenka
2021-04-05 20:43 ` [RFC PATCH v2 1/4] arm64: Implement infrastructure for " madvenka
2021-04-08 15:15 ` Mark Brown
2021-04-08 17:17 ` Mark Brown
2021-04-08 19:30 ` Madhavan T. Venkataraman
2021-04-08 23:30 ` Madhavan T. Venkataraman
2021-04-09 11:57 ` Mark Brown
2021-04-05 20:43 ` [RFC PATCH v2 2/4] arm64: Mark a stack trace unreliable if an EL1 exception frame is detected madvenka
2021-04-05 20:43 ` [RFC PATCH v2 3/4] arm64: Detect FTRACE cases that make the stack trace unreliable madvenka
2021-04-08 16:58 ` Mark Brown
2021-04-08 19:23 ` Madhavan T. Venkataraman
2021-04-09 11:31 ` Mark Brown
2021-04-09 14:02 ` Madhavan T. Venkataraman
2021-04-09 12:27 ` Mark Rutland
2021-04-09 17:23 ` Madhavan T. Venkataraman
2021-04-05 20:43 ` [RFC PATCH v2 4/4] arm64: Mark stack trace as unreliable if kretprobed functions are present madvenka
2021-04-09 12:09 ` [RFC PATCH v2 0/4] arm64: Implement stack trace reliability checks Mark Rutland
2021-04-09 17:16 ` Madhavan T. Venkataraman
2021-04-09 21:37 ` Josh Poimboeuf
2021-04-09 22:05 ` Madhavan T. Venkataraman
2021-04-09 22:32 ` Josh Poimboeuf [this message]
2021-04-09 22:53 ` Josh Poimboeuf
2021-04-11 17:54 ` Madhavan T. Venkataraman
2021-04-12 16:59 ` Mark Brown
2021-04-13 22:53 ` Josh Poimboeuf
2021-04-14 12:24 ` Mark Brown
2021-04-12 17:36 ` Mark Brown
2021-04-12 19:55 ` Madhavan T. Venkataraman
2021-04-13 11:02 ` Mark Brown
2021-04-14 10:23 ` Madhavan T. Venkataraman
2021-04-14 12:35 ` Mark Brown
2021-04-16 14:43 ` Madhavan T. Venkataraman
2021-04-16 15:36 ` Mark Brown
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