[PATCH v3 2/5] powerpc: ftrace: restore LR from pt_regs

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

Pass the real LR to the ftrace handler. This is needed for
KPROBES_ON_FTRACE for the pre handlers.

Also, with KPROBES_ON_FTRACE, the link register may be updated by the
pre handlers or by a registed kretprobe. Honor updated LR by restoring
it from pt_regs, rather than from the stack save area.

Live patch and function graph continue to work fine with this change.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
---
 arch/powerpc/kernel/entry_64.S | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index 8fd8718722a1..744b2f91444a 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -1248,9 +1248,10 @@ _GLOBAL(ftrace_caller)
 
 	/* Get the _mcount() call site out of LR */
 	mflr	r7
-	/* Save it as pt_regs->nip & pt_regs->link */
+	/* Save it as pt_regs->nip */
 	std     r7, _NIP(r1)
-	std     r7, _LINK(r1)
+	/* Save the read LR in pt_regs->link */
+	std     r0, _LINK(r1)
 
 	/* Save callee's TOC in the ABI compliant location */
 	std	r2, 24(r1)
@@ -1297,16 +1298,16 @@ ftrace_call:
 	REST_8GPRS(16,r1)
 	REST_8GPRS(24,r1)
 
+	/* Restore possibly modified LR */
+	ld	r0, _LINK(r1)
+	mtlr	r0
+
 	/* Restore callee's TOC */
 	ld	r2, 24(r1)
 
 	/* Pop our stack frame */
 	addi r1, r1, SWITCH_FRAME_SIZE
 
-	/* Restore original LR for return to B */
-	ld	r0, LRSAVE(r1)
-	mtlr	r0
-
 #ifdef CONFIG_LIVEPATCH
         /* Based on the cmpd above, if the NIP was altered handle livepatch */
 	bne-	livepatch_handler
-- 
2.12.1