Re: [RFC PATCH v1 0/4] arm64: Implement stack trace reliability checks

From: Masami Hiramatsu <mhiramat@kernel.org>
To: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>,
	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
Subject: Re: [RFC PATCH v1 0/4] arm64: Implement stack trace reliability checks
Date: Tue, 6 Apr 2021 08:40:46 +0900	[thread overview]
Message-ID: <20210406084046.4f0b946728dc01da09045338@kernel.org> (raw)
In-Reply-To: <7dda9af3-1ecf-5e6f-1e46-8870a2a5e550@linux.microsoft.com>

On Mon, 5 Apr 2021 09:56:48 -0500
"Madhavan T. Venkataraman" <madvenka@linux.microsoft.com> wrote:

> 
> 
> On 4/5/21 8:24 AM, Masami Hiramatsu wrote:
> > Hi Madhaven,
> > 
> > On Sat, 3 Apr 2021 22:29:12 -0500
> > "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com> wrote:
> > 
> > 
> >>>> Check for kretprobe
> >>>> ===================
> >>>>
> >>>> For functions with a kretprobe set up, probe code executes on entry
> >>>> to the function and replaces the return address in the stack frame with a
> >>>> kretprobe trampoline. Whenever the function returns, control is
> >>>> transferred to the trampoline. The trampoline eventually returns to the
> >>>> original return address.
> >>>>
> >>>> A stack trace taken while executing in the function (or in functions that
> >>>> get called from the function) will not show the original return address.
> >>>> Similarly, a stack trace taken while executing in the trampoline itself
> >>>> (and functions that get called from the trampoline) will not show the
> >>>> original return address. This means that the caller of the probed function
> >>>> will not show. This makes the stack trace unreliable.
> >>>>
> >>>> Add the kretprobe trampoline to special_functions[].
> >>>>
> >>>> FYI, each task contains a task->kretprobe_instances list that can
> >>>> theoretically be consulted to find the orginal return address. But I am
> >>>> not entirely sure how to safely traverse that list for stack traces
> >>>> not on the current process. So, I have taken the easy way out.
> >>>
> >>> For kretprobes, unwinding from the trampoline or kretprobe handler
> >>> shouldn't be a reliability concern for live patching, for similar
> >>> reasons as above.
> >>>
> >>
> >> Please see previous answer.
> >>
> >>> Otherwise, when unwinding from a blocked task which has
> >>> 'kretprobe_trampoline' on the stack, the unwinder needs a way to get the
> >>> original return address.  Masami has been working on an interface to
> >>> make that possible for x86.  I assume something similar could be done
> >>> for arm64.
> >>>
> >>
> >> OK. Until that is available, this case needs to be addressed.
> > 
> > Actually, I've done that on arm64 :) See below patch.
> > (and I also have a similar code for arm32, what I'm considering is how
> > to unify x86/arm/arm64 kretprobe_find_ret_addr(), since those are very
> > similar.)
> > 
> > This is applicable on my x86 series v5
> > 
> > https://lore.kernel.org/bpf/161676170650.330141.6214727134265514123.stgit@devnote2/
> > 
> > Thank you,
> > 
> > 
> 
> I took a brief look at your changes. Looks reasonable.
> 
> However, for now, I am going to include the kretprobe_trampoline in the special_functions[]
> array until your changes are merged. At that point, it is just a matter of deleting
> kretprobe_trampoline from the special_functions[] array. That is all.
> 
> I hope that is fine with everyone.

Agreed, that is reasonable unless my series is merged. 

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>