[PATCH v6] x86: Avoid pr_cont() in show_opcodes().

[PATCH v6] x86: Avoid pr_cont() in show_opcodes().

[Tetsuo Handa]

From: Rasmus Villemoes <linux@rasmusvillemoes.dk>

Since syzbot is confused by concurrent printk() messages [1],
this patch changes show_opcodes() to use %ph format string.

When we start adding prefix to each line of printk() output,
we will be able to handle concurrent printk() messages.

[1] https://syzkaller.appspot.com/text?tag=CrashReport&x=139d342c400000

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Borislav Petkov <bp@suse.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
---
 arch/x86/kernel/dumpstack.c | 29 +++++++++--------------------
 1 file changed, 9 insertions(+), 20 deletions(-)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 666a284..0821da3 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -22,8 +22,6 @@
 #include <asm/stacktrace.h>
 #include <asm/unwind.h>
 
-#define OPCODE_BUFSIZE 64
-
 int panic_on_unrecovered_nmi;
 int panic_on_io_nmi;
 static int die_counter;
@@ -93,26 +91,17 @@ static void printk_stack_address(unsigned long address, int reliable,
  */
 void show_opcodes(u8 *rip, const char *loglvl)
 {
-	unsigned int code_prologue = OPCODE_BUFSIZE * 2 / 3;
+#define PROLOGUE_SIZE 42
+#define EPILOGUE_SIZE 21
+#define OPCODE_BUFSIZE (PROLOGUE_SIZE + 1 + EPILOGUE_SIZE)
 	u8 opcodes[OPCODE_BUFSIZE];
-	u8 *ip;
-	int i;
-
-	printk("%sCode: ", loglvl);
 
-	ip = (u8 *)rip - code_prologue;
-	if (probe_kernel_read(opcodes, ip, OPCODE_BUFSIZE)) {
-		pr_cont("Bad RIP value.\n");
-		return;
-	}
-
-	for (i = 0; i < OPCODE_BUFSIZE; i++, ip++) {
-		if (ip == rip)
-			pr_cont("<%02x> ", opcodes[i]);
-		else
-			pr_cont("%02x ", opcodes[i]);
-	}
-	pr_cont("\n");
+	if (probe_kernel_read(opcodes, rip - PROLOGUE_SIZE, OPCODE_BUFSIZE))
+		printk("%sCode: Bad RIP value.\n", loglvl);
+	else
+		printk("%sCode: %" __stringify(PROLOGUE_SIZE) "ph <%02x> %"
+		       __stringify(EPILOGUE_SIZE) "ph\n", loglvl, opcodes,
+		       opcodes[PROLOGUE_SIZE], opcodes + PROLOGUE_SIZE + 1);
 }
 
 void show_ip(struct pt_regs *regs, const char *loglvl)
-- 
1.8.3.1

Re: [PATCH v6] x86: Avoid pr_cont() in show_opcodes().

[Peter Zijlstra]

On Thu, Jul 19, 2018 at 10:53:03PM +0900, Tetsuo Handa wrote:
> +	if (probe_kernel_read(opcodes, rip - PROLOGUE_SIZE, OPCODE_BUFSIZE))
> +		printk("%sCode: Bad RIP value.\n", loglvl);
> +	else
> +		printk("%sCode: %" __stringify(PROLOGUE_SIZE) "ph <%02x> %"
> +		       __stringify(EPILOGUE_SIZE) "ph\n", loglvl, opcodes,
> +		       opcodes[PROLOGUE_SIZE], opcodes + PROLOGUE_SIZE + 1);
>  }

Kernel coding style wants { } on the else branch because multi-line, and
if we have { } on one leg of an if-else it should also be on the other.