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/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... :-/
/*
* 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
