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@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...

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?

  	/* 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... :-/

  Thomas