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

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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@xxxxxxxxx>
  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/, do we really need to create this additional one here? Or does the one from 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... :-/

  	 * 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 */


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