linux-kernel - Re: ppc64le reliable stack unwinder and scheduled tasks

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Date:   Sun, 13 Jan 2019 23:33:56 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Joe Lawrence <joe.lawrence@...hat.com>,
        Jiri Kosina <jkosina@...e.cz>, linux-kernel@...r.kernel.org,
        Torsten Duwe <duwe@....de>, live-patching@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: ppc64le reliable stack unwinder and scheduled tasks

On Sat, Jan 12, 2019 at 02:45:41AM -0600, Segher Boessenkool wrote:
> On Sat, Jan 12, 2019 at 12:09:14PM +1100, Balbir Singh wrote:
> > Could you please define interesting frame on top a bit more? Usually
> > the topmost return address is in LR
> 
> There is no reliable way (other than DWARF unwind info) to find out where
> the value of LR at function entry currently lives (if anywhere). It may or
> may not be still available in LR, it may or may not be saved to the return
> stack slot.  It can also live in some GPR, or in some other stack slot.
> 
> (The same is true for all other registers).
> 
> The only thing the ABI guarantees you is that you can find all stack frames
> via the back chain.  If you want more you can use some heuristics and do
> some heroics (like GDB does), but this is not fully reliable.  Using DWARF
> unwind info is, but that requires big tables.
>

Thanks, so are you suggesting that a reliable stack is not possible on
ppc64le? Even with the restricted scope of the kernel?

Balbir Singh.