[PATCH 1/2] ppc64le live patch: clear out storage location(s) in mini stack frame

From: Torsten Duwe
Date: Wed Mar 09 2016 - 12:28:34 EST

This can be applied on top of Petr Mladek's v4 rework of the ppc64le
live patching. Inspired by Balbir Singh's v5, information about the
callee's r2 is stored in a "reserved" 32 bit location in the caller's
stack frame, instead of 64 bits in the newly created mini frame 24(r1).

It only needs to work for a local call, when caller's TOC == callee's
TOC, and along with the return address (LR) it's all within a 4GiB
range (+-31 bits). If the original call already was global, we are
allowed to restore any nonsense into r2, because the global caller
will restore its TOC anyway from the ABI compliant location 24(r1)
right after return.

Signed-off-by: Torsten Duwe <duwe@xxxxxxx>

This is only the preparation for dumping the mini stack frame.
It shouldn't break anything, bisecting-wise.

--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -1284,8 +1284,9 @@ ftrace_call:
* lr and LRSAVE(r1) contain the address of klp_return_helper.
* We loaded ctr with the address of the patched function earlier
+ subf r0, r0, r2 /* Calculate offset from current TOC to LR */
+ stw r0, 12(r1) /* and save it in CR+4 */
stdu r1, -32(r1) /* open new mini stack frame */
- std r2, 24(r1) /* save TOC now, unconditionally. */
bl 5f
5: mflr r12
addi r12, r12, (klp_return_helper + 4 - .)@l
@@ -1323,9 +1324,10 @@ _GLOBAL(ftrace_stub)
* maybe inserting a klp_return_helper frame or not.
- ld r2, 24(r1) /* restore TOC (saved by ftrace_caller) */
addi r1, r1, 32 /* destroy mini stack frame */
+ lwa r2, 12(r1) /* Load from CR+4, offset of TOC w.r.t LR */
ld r0, LRSAVE(r1) /* get the real return address */
+ add r2, r2, r0 /* Add the current LR to offset */
mtlr r0