On 27/02/2024 09.50, 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@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...)
Ok, I was looking at the wrong ELF spec, indeed (it was an ancient 32-bit
spec, not the 64-bit ABI). Sorry for the confusion. Having a proper #define
or a comment for the 16 here would still be helpful, though.
Thomas
