[PATCH v2] x86: Avoid pr_cont() in show_opcodes()

[Tetsuo Handa]

Since syzbot is confused by concurrent printk() messages [1], this patch
changes show_opcodes() to use snprintf(). By this change, the Code: line
will always be printed as one line even if multiple threads concurrently
called printk().

To save on-stack footprint, this patch shares opcodes[] and buf[] because
we sequentially reads from opcodes[] and sequentially writes to buf[].

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: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
---
 arch/x86/kernel/dumpstack.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 666a284..6408123 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -94,25 +94,27 @@ 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;
-	u8 opcodes[OPCODE_BUFSIZE];
 	u8 *ip;
 	int i;
-
-	printk("%sCode: ", loglvl);
+	int pos = 0;
+	char buf[(3 * OPCODE_BUFSIZE + 2) + 1];
+	u8 *opcodes = (u8 *) buf + sizeof(buf) - OPCODE_BUFSIZE;
 
 	ip = (u8 *)rip - code_prologue;
 	if (probe_kernel_read(opcodes, ip, OPCODE_BUFSIZE)) {
-		pr_cont("Bad RIP value.\n");
+		printk("%sCode: Bad RIP value.\n", loglvl);
 		return;
 	}
 
 	for (i = 0; i < OPCODE_BUFSIZE; i++, ip++) {
 		if (ip == rip)
-			pr_cont("<%02x> ", opcodes[i]);
+			pos += snprintf(buf + pos, sizeof(buf) - pos,
+					"<%02x> ", opcodes[i]);
 		else
-			pr_cont("%02x ", opcodes[i]);
+			pos += snprintf(buf + pos, sizeof(buf) - pos,
+					"%02x ", opcodes[i]);
 	}
-	pr_cont("\n");
+	printk("%sCode: %s\n", loglvl, buf);
 }
 
 void show_ip(struct pt_regs *regs, const char *loglvl)
-- 
1.8.3.1