[kvm-unit-tests PATCH v8 09/35] powerpc: Fix stack backtrace termination

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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 | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/powerpc/cstart64.S b/powerpc/cstart64.S
index e18ae9a22..80baabe8f 100644
--- a/powerpc/cstart64.S
+++ b/powerpc/cstart64.S
@@ -46,6 +46,21 @@ 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)
+	std	r0,16(r1)
+	/*
+	 * Create entry frame of 64-bytes, same as the initial frame. A callee
+	 * may use the caller frame to store LR, and backtrace() termination
+	 * looks for return address == NULL, so the initial stack frame can't
+	 * be used to call C or else it could overwrite the zeroed LR save slot
+	 * and break backtrace termination.  This frame would be unnecessary if
+	 * backtrace looked for a zeroed frame address.
+	 */
+	stdu	r1,-64(r1)
 	/* save DTB pointer */
 	std	r3, 56(r1)

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