linux-kernel - [PATCH v4 2/3] powerpc/modules: Don't try to restore r2 after a sibling call

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Message-Id: <20171114092910.20399-3-kamalesh@linux.vnet.ibm.com>
Date:   Tue, 14 Nov 2017 04:29:09 -0500
From:   Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        live-patching@...r.kernel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Balbir Singh <bsingharora@...il.com>
Subject: [PATCH v4 2/3] powerpc/modules: Don't try to restore r2 after a sibling call

From: Josh Poimboeuf <jpoimboe@...hat.com>

When attempting to load a livepatch module, I got the following error:

  module_64: patch_module: Expect noop after relocate, got 3c820000

The error was triggered by the following code in
unregister_netdevice_queue():

  14c:   00 00 00 48     b       14c <unregister_netdevice_queue+0x14c>
                         14c: R_PPC64_REL24      net_set_todo
  150:   00 00 82 3c     addis   r4,r2,0

GCC didn't insert a nop after the branch to net_set_todo() because it's
a sibling call, so it never returns.  The nop isn't needed after the
branch in that case.

Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
Signed-off-by: Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
---
 arch/powerpc/kernel/module_64.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
index 39b01fd..9e5391f 100644
--- a/arch/powerpc/kernel/module_64.c
+++ b/arch/powerpc/kernel/module_64.c
@@ -489,6 +489,10 @@ static int restore_r2(u32 *instruction, struct module *me)
 	if (is_early_mcount_callsite(instruction - 1))
 		return 1;
 
+	/* Sibling calls don't return, so they don't need to restore r2 */
+	if (instruction[-1] == PPC_INST_BRANCH)
+		return 1;
+
 	if (*instruction != PPC_INST_NOP) {
 		pr_err("%s: Expect noop after relocate, got %08x\n",
 		       me->name, *instruction);
-- 
2.9.3