[kvm-unit-tests PATCH 03/32] powerpc: Fix stack backtrace termination

[Nicholas Piggin]

On Tue Feb 27, 2024 at 6:50 PM AEST, Thomas Huth wrote:
> On 26/02/2024 11.11, Nicholas Piggin wrote:
> > The backtrace handler terminates when it sees a NULL caller address,
> > but the powerpc stack setup does not keep such a NULL caller frame
> > at the start of the stack.
> > 
> > This happens to work on pseries because the memory at 0 is mapped and
> > it contains 0 at the location of the return address pointer if it
> > were a stack frame. But this is fragile, and does not work with powernv
> > where address 0 contains firmware instructions.
> > 
> > Use the existing dummy frame on stack as the NULL caller, and create a
> > new frame on stack for the entry code.
> > 
> > Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
> > ---
> >   powerpc/cstart64.S | 12 ++++++++++--
> >   1 file changed, 10 insertions(+), 2 deletions(-)
>
> Thanks for tackling this! ... however, not doing powerpc work since years 
> anymore, I have some ignorant questions below...
>
> > diff --git a/powerpc/cstart64.S b/powerpc/cstart64.S
> > index e18ae9a22..14ab0c6c8 100644
> > --- a/powerpc/cstart64.S
> > +++ b/powerpc/cstart64.S
> > @@ -46,8 +46,16 @@ start:
> >   	add	r1, r1, r31
> >   	add	r2, r2, r31
> >   
> > +	/* Zero backpointers in initial stack frame so backtrace() stops */
> > +	li	r0,0
> > +	std	r0,0(r1)
>
> 0(r1) is the back chain pointer ...
>
> > +	std	r0,16(r1)
>
> ... but what is 16(r1) ? I suppose that should be the "LR save word" ? But 
> isn't that at 8(r1) instead?? (not sure whether I'm looking at the right ELF 
> abi spec right now...)
>
> Anyway, a comment in the source would be helpful here.
>
> > +
> > +	/* Create entry frame */
> > +	stdu	r1,-INT_FRAME_SIZE(r1)
>
> Since we already create an initial frame via stackptr from powerpc/flat.lds, 
> do we really need to create this additional one here? Or does the one from 
> flat.lds have to be completely empty, i.e. also no DTB pointer in it?

Oh you already figured the above questions. For this, we do have
one frame allocated already statically yes. But if we don't create
another one here then our callee will store LR into it, but the
unwinder only exits when it sees a NULL return address so it would
keep trying to walk.

We could make it terminate on NULL back chain pointer, but that's
a bit more change that also touches non-powerpc code in the generic
unwinder, and still needs some changes here. Maybe we should do
that after this series though. I'll include a comment to look at
redoing it later.

>
> >   	/* save DTB pointer */
> > -	std	r3, 56(r1)
> > +	SAVE_GPR(3,r1)
>
> Isn't SAVE_GPR rather meant for the interrupt frame, not for the normal C 
> calling convention frames?
>
> Sorry for asking dumb questions ... I still have a hard time understanding 
> the changes here... :-/

Ah, that was me being lazy and using an interrupt frame for the new
frame.

Thanks,
Nick

>
> >   	/*
> >   	 * Call relocate. relocate is C code, but careful to not use
> > @@ -101,7 +109,7 @@ start:
> >   	stw	r4, 0(r3)
> >   
> >   	/* complete setup */
> > -1:	ld	r3, 56(r1)
> > +1:	REST_GPR(3, r1)
> >   	bl	setup
> >   
> >   	/* run the test */
>
>   Thomas