linux-kernel - [PATCH 06/12] powerpc/module: Rework is_early_mcount

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Date:	Thu, 25 Feb 2016 01:28:29 +1100
From:	Michael Ellerman <mpe@...erman.id.au>
To:	<linuxppc-dev@...abs.org>
Cc:	bsingharora@...il.com, duwe@....de, <linux-kernel@...r.kernel.org>,
	rostedt@...dmis.org, kamalesh@...ux.vnet.ibm.com, pmladek@...e.com,
	jeyu@...hat.com, jkosina@...e.cz, live-patching@...r.kernel.org,
	mbenes@...e.cz
Subject: [PATCH 06/12] powerpc/module: Rework is_early_mcount_callsite()

is_early_mcount_callsite() needs to detect either the two instruction or
the three instruction versions of the _mcount() sequence.

But if we're running a kernel with the two instruction sequence, we need
to be careful not to read instruction - 2, otherwise we might fall off
the front of a page and cause an oops.

While we're here convert to bool to make the return semantics clear.

Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
---
 arch/powerpc/kernel/module_64.c | 30 ++++++++++++++----------------
 1 file changed, 14 insertions(+), 16 deletions(-)

Squash.

diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
index 32c10e0d2aa5..495df4340623 100644
--- a/arch/powerpc/kernel/module_64.c
+++ b/arch/powerpc/kernel/module_64.c
@@ -449,27 +449,25 @@ static unsigned long stub_for_addr(const Elf64_Shdr *sechdrs,
 }
 
 #ifdef CC_USING_MPROFILE_KERNEL
-static int is_early_mcount_callsite(u32 *instruction)
+static bool is_early_mcount_callsite(u32 *instruction)
 {
-	/* -mprofile-kernel sequence starting with
-	 * mflr r0 and maybe std r0, LRSAVE(r1).
+	/*
+	 * Check if this is one of the -mprofile-kernel sequences.
 	 */
-	if ((instruction[-3] == PPC_INST_MFLR &&
-	     instruction[-2] == PPC_INST_STD_LR) ||
-	    instruction[-2] == PPC_INST_MFLR) {
-		/* Nothing to be done here, it's an _mcount
-		 * call location and r2 will have to be
-		 * restored in the _mcount function.
-		 */
-		return 1;
-	}
-	return 0;
+	if (instruction[-1] == PPC_INST_STD_LR &&
+	    instruction[-2] == PPC_INST_MFLR)
+		return true;
+
+	if (instruction[-1] == PPC_INST_MFLR)
+		return true;
+
+	return false;
 }
 #else
 /* without -mprofile-kernel, mcount calls are never early */
-static int is_early_mcount_callsite(u32 *instruction)
+static bool is_early_mcount_callsite(u32 *instruction)
 {
-	return 0;
+	return false;
 }
 #endif
 
@@ -478,7 +476,7 @@ static int is_early_mcount_callsite(u32 *instruction)
 static int restore_r2(u32 *instruction, struct module *me)
 {
 	if (*instruction != PPC_INST_NOP) {
-		if (is_early_mcount_callsite(instruction))
+		if (is_early_mcount_callsite(instruction - 1))
 			return 1;
 		pr_err("%s: Expect noop after relocate, got %08x\n",
 		       me->name, *instruction);
-- 
2.5.0