linux-kernel - [RFC][PATCH v5 39/51] objtool: mcount: Verify x86 instruction with memcmp()

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Date:   Thu, 18 Jun 2020 13:38:25 -0700
From:   Matt Helsley <mhelsley@...are.com>
To:     <linux-kernel@...r.kernel.org>
CC:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Julien Thierry <jthierry@...hat.com>,
        Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        Matt Helsley <mhelsley@...are.com>
Subject: [RFC][PATCH v5 39/51] objtool: mcount: Verify x86 instruction with memcmp()

Instead of hard-coding what amounts to a memcmp() use memcmp to
determine if the instruction we wish to replace matches what we
expect. This makes the x86 code more like that of, for instance,
ARM.

Signed-off-by: Matt Helsley <mhelsley@...are.com>
---
 tools/objtool/mcount.c | 19 ++++++-------------
 1 file changed, 6 insertions(+), 13 deletions(-)

diff --git a/tools/objtool/mcount.c b/tools/objtool/mcount.c
index 4d6596a031bf..5c59df0df97b 100644
--- a/tools/objtool/mcount.c
+++ b/tools/objtool/mcount.c
@@ -47,9 +47,10 @@ extern int warn_on_notrace_sect; /* warn when section has mcount not being recor
 
 static struct elf *lf;
 
-static unsigned char ideal_nop5_x86_64[5] = { 0x0f, 0x1f, 0x44, 0x00, 0x00 };
-static unsigned char ideal_nop5_x86_32[5] = { 0x3e, 0x8d, 0x74, 0x26, 0x00 };
-static unsigned char *ideal_nop;
+static const unsigned char ip_relative_call_x86[5] = { 0xe8, 0x00, 0x00, 0x00, 0x00 };
+static const unsigned char ideal_nop5_x86_64[5] = { 0x0f, 0x1f, 0x44, 0x00, 0x00 };
+static const unsigned char ideal_nop5_x86_32[5] = { 0x3e, 0x8d, 0x74, 0x26, 0x00 };
+static const unsigned char *ideal_nop;
 
 static char rel_type_nop;
 
@@ -57,20 +58,12 @@ static int (*make_nop)(struct section *, size_t const offset);
 
 static int make_nop_x86(struct section *txts, size_t const offset)
 {
-	uint32_t *ptr;
-	unsigned char *op;
-	void *map = txts->data->d_buf;
+	unsigned char *op = txts->data->d_buf + offset - 1;
 
 	if (offset < 1)
 		return -1;
 
-	/* Confirm we have 0xe8 0x0 0x0 0x0 0x0 */
-	ptr = map + offset;
-	if (*ptr != 0)
-		return -1;
-
-	op = map + offset - 1;
-	if (*op != 0xe8)
+	if (memcmp(op, ip_relative_call_x86, 5) != 0)
 		return -1;
 
 	/* convert to nop */
-- 
2.20.1