linux-kernel - [PATCH AUTOSEL for 4.15 087/102] powerpc/modules: Don't try to restore r2 after a sibling call

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Date:   Sat, 3 Mar 2018 22:25:00 +0000
From:   Sasha Levin <Alexander.Levin@...rosoft.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
CC:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Sasha Levin <Alexander.Levin@...rosoft.com>
Subject: [PATCH AUTOSEL for 4.15 087/102] powerpc/modules: Don't try to
 restore r2 after a sibling call

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

[ Upstream commit b9eab08d012fa093947b230f9a87257c27fb829b ]

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>
Acked-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
Reviewed-and-tested-by: Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
---
 arch/powerpc/include/asm/code-patching.h |  1 +
 arch/powerpc/kernel/module_64.c          | 12 +++++++++++-
 arch/powerpc/lib/code-patching.c         |  5 +++++
 3 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/code-patching.h b/arch/powerpc/include/asm/code-patching.h
index abef812de7f8..2c895e8d07f7 100644
--- a/arch/powerpc/include/asm/code-patching.h
+++ b/arch/powerpc/include/asm/code-patching.h
@@ -33,6 +33,7 @@ int patch_branch(unsigned int *addr, unsigned long target, int flags);
 int patch_instruction(unsigned int *addr, unsigned int instr);
 
 int instr_is_relative_branch(unsigned int instr);
+int instr_is_relative_link_branch(unsigned int instr);
 int instr_is_branch_to_addr(const unsigned int *instr, unsigned long addr);
 unsigned long branch_target(const unsigned int *instr);
 unsigned int translate_branch(const unsigned int *dest,
diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
index 759104b99f9f..180c16f04063 100644
--- a/arch/powerpc/kernel/module_64.c
+++ b/arch/powerpc/kernel/module_64.c
@@ -487,7 +487,17 @@ static bool is_early_mcount_callsite(u32 *instruction)
    restore r2. */
 static int restore_r2(u32 *instruction, struct module *me)
 {
-	if (is_early_mcount_callsite(instruction - 1))
+	u32 *prev_insn = instruction - 1;
+
+	if (is_early_mcount_callsite(prev_insn))
+		return 1;
+
+	/*
+	 * Make sure the branch isn't a sibling call.  Sibling calls aren't
+	 * "link" branches and they don't return, so they don't need the r2
+	 * restore afterwards.
+	 */
+	if (!instr_is_relative_link_branch(*prev_insn))
 		return 1;
 
 	if (*instruction != PPC_INST_NOP) {
diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
index d469224c4ada..096d4e4d31e6 100644
--- a/arch/powerpc/lib/code-patching.c
+++ b/arch/powerpc/lib/code-patching.c
@@ -302,6 +302,11 @@ int instr_is_relative_branch(unsigned int instr)
 	return instr_is_branch_iform(instr) || instr_is_branch_bform(instr);
 }
 
+int instr_is_relative_link_branch(unsigned int instr)
+{
+	return instr_is_relative_branch(instr) && (instr & BRANCH_SET_LINK);
+}
+
 static unsigned long branch_iform_target(const unsigned int *instr)
 {
 	signed long imm;
-- 
2.14.1