Re: [RFC PATCH 0/3] arm64: Implement reliable stack trace

From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	linux-kernel@vger.kernel.org, Mark Brown <broonie@kernel.org>,
	live-patching@vger.kernel.org, Miroslav Benes <mbenes@suse.cz>,
	Will Deacon <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [RFC PATCH 0/3] arm64: Implement reliable stack trace
Date: Tue, 20 Oct 2020 10:58:02 -0500	[thread overview]
Message-ID: <20201020155802.da6ca652hramdlnb@treble> (raw)
In-Reply-To: <20201020100352.GA48360@C02TD0UTHF1T.local>

On Tue, Oct 20, 2020 at 11:03:52AM +0100, Mark Rutland wrote:
> On Fri, Oct 16, 2020 at 12:14:31PM +0100, Mark Rutland wrote:
> > Mark B's reply dropped this, but the next paragraph covered that:
> > 
> > | I was planning to send a mail once I've finished writing a test, but
> > | IIUC there are some windows where ftrace/kretprobes
> > | detection/repainting may not work, e.g. if preempted after
> > | ftrace_return_to_handler() decrements curr_ret_stack, but before the
> > | arch trampoline asm restores the original return addr. So we might
> > | need something like an in_return_trampoline() to detect and report
> > | that reliably.
> > 
> > ... so e.g. for a callchain A->B->C, where C is instrumented there are
> > windows where B might be missing from the trace, but the trace is
> > reported as reliable.
> 
> I'd missed a couple of details, and I think I see how each existing
> architecture prevents this case now.
> 
> Josh, just to confirm the x86 case, am I right in thinking that the ORC
> unwinder will refuse to unwind from the return_to_handler and
> kretprobe_trampoline asm? IIRC objtool shouldn't build unwind info for
> those as return_to_handler is marked with SYM_CODE_{START,END}() and
> kretprobe_trampoline is marked with STACK_FRAME_NON_STANDARD().

Hm, return_to_handler() actually looks like a bug.  UNWIND_HINT_EMPTY
sets end=1, which causes the ORC unwinder to treat it like entry code
(end of the stack).  So while it does stop the unwind, it fails to
report an error.

This would be fixed by the idea I previously mentioned, changing
UNWIND_HINT_EMPTY -> UNWIND_HINT_UNDEFINED (end=0) for the non-entry
cases.  I'll need to work up some patches.

> Both powerpc and s390 refuse to reliably unwind through exceptions, so
> they can rely on function call boundaries to keep the callchain in a
> sane state.

Yes, and also true for x86 frame pointers.

-- 
Josh

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