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Date:   Tue, 6 Nov 2018 12:29:33 +0000
From:   Mark Rutland <mark.rutland@arm.com>
To:     Dave P Martin <Dave.Martin@arm.com>
Cc:     Daniel Thompson <daniel.thompson@linaro.org>,
        Zhaoyang Huang <huangzhaoyang@gmail.com>,
        Catalin Marinas <Catalin.Marinas@arm.com>,
        Will Deacon <Will.Deacon@arm.com>,
        Michael Weiser <michael.weiser@gmx.de>,
        James Morse <James.Morse@arm.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] arch/arm64 : fix error in dump_backtrace
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On Tue, Nov 06, 2018 at 11:32:50AM +0000, Dave P Martin wrote:
> On Tue, Nov 06, 2018 at 11:00:19AM +0000, Mark Rutland wrote:
> > On Tue, Nov 06, 2018 at 08:57:51AM +0000, Daniel Thompson wrote:
> > > On Tue, Nov 06, 2018 at 08:39:01AM +0000, Mark Rutland wrote:
> > > > On Tue, Nov 06, 2018 at 03:19:35PM +0800, Zhaoyang Huang wrote:
> > > > > From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> > > > > 
> > > > > In some cases, the instruction of "bl foo1" will be the last one of the
> > > > > foo2[1], which will cause the lr be the first instruction of the adjacent
> > > > > foo3[2]. Hence, the backtrace will show the weird result as bellow[3].
> > > > > The patch will fix it by miner 4 of the lr when dump_backtrace
> > > > 
> > > > This has come up in the past (and a similar patch has been applied, then
> > > > reverted).
> > > > 
> > > > In general, we don't know that a function call was made via BL, and therefore
> > > > cannot know that LR - 4 is the address of the caller. The caller could set up
> > > > the LR as it likes, then B or BR to the callee, and depending on how the basic
> > > > blocks get laid out in memory, LR - 4 might point at something completely
> > > > different.
> > > > 
> > > > More ideally, the compiler wouldn't end a function with a BL. When does that
> > > > happen, and is there some way we could arrange for that to not happen? e.g.
> > > > somehow pad a NOP after the BL.
> > > 
> > > It's a consequence of having __noreturn isn't it? __noreturn frees the
> > > compiler from the burden of having to produce a valid return stack... so
> > > it doesn't and unwinding becomes hard.
> > 
> > In that case, the compiler could equally just use B rather than BL,
> > which this patch doesn't solve.
> > 
> > The documentation for the GCC noreturn attribute [1] says:
> > 
> > | In order to preserve backtraces, GCC will never turn calls to noreturn
> > | functions into tail calls. 
> > 
> > ... so clearly it's not intended to mess up backtracing.
> 
> Which is a bit odd, since every call to a noreturn function is a tail-
> call by definition, and other tail-calls are typically optimised to a B
> (thus interfering with backtracing in all except the noreturn case).
> 
> Avoiding this would require a change to the compiler, and since there's
> no obvious correct answer anyway, I guess we shouldn't hold our breath.
> 
> > IIUC we mostly use noreturn to prevent warings about uninitialised
> > variables and such after a call to a noreturn function. I think
> > optimization is a secondary concern.
> 
> Agreed.
> 
> > We could ask the GCC folk if they can ensure that a noreturn function
> > call leave thes LR pointing into the caller, e.g. by padding with a NOP:
> > 
> > 	BL	<noreturn function>
> > 	NOP
> > 
> > That seems cheap enough, and would keep backtraces reliable.
> 
> -fpatchable-function-entry=1,1 does almost the right thing, by
> inserting 1 NOP at the start of each function, and putting the function
> entry point after that (1) NOP.

Neat hack, but unfortunately insufficient in general since:

* The next function may be notrace or asm, and hence will not have a
  preceding NOP.

* There may not be a next function (e.g. for the final instruction in
  .text), and the LR value may point at some data symbol.

* This relies on the preceding NOP being accounted as part of the
  previous function, which feels like a bug given we should have the
  function size somewhere.

Generally, I think that trying to bodge around the exiting behaviour is
going to cause just as many problems as it solves, and worse, makes it
harder to consistently analyse a backtrace.

IMO, we shouldn't change the kernel here.

Thanks,
Mark.