powerpc/process: Fix interleaved output in show_user_instructions()

When two processes crash at the same time, we sometimes encounter
interleaving in the middle of a line:

  init[1]: segfault (11) at 0 nip 0 lr 0 code 1
  init[1]: code: XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
  init[74]: segfault (11) at 10a74 nip 1000c198 lr 100078c8 code 1 in sh[10000000+14000]
  XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
  init[1]: code: XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
  init[74]: code: 90010024 bf61000c 91490a7c 3fa01002 3be00000 7d3e4b78 3bbd0c20 3b600000
  init[74]: code: 3b9d0040 7c7fe02e 2f830000 419e0028 <89230000> 2f890000 41be001c 4b7f6e79

This patch fixes it by preparing complete lines in a buffer and
printing it at once.

Fixes: 88b0fe17 ("powerpc: Add show_user_instructions()")
Reviewed-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[mpe: Use seq_buf_printf() not seq_buf_puts() which doesn't NULL terminate]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/process.c

@@ -43,6 +43,7 @@
 #include <linux/uaccess.h>
 #include <linux/elf-randomize.h>
 #include <linux/pkeys.h>
+#include <linux/seq_buf.h>
 
 #include <asm/pgtable.h>
 #include <asm/io.h>
@@ -1300,7 +1301,9 @@ static void show_instructions(struct pt_regs *regs)
 void show_user_instructions(struct pt_regs *regs)
 {
 	unsigned long pc;
-	int i;
+	int n = instructions_to_print;
+	struct seq_buf s;
+	char buf[96]; /* enough for 8 times 9 + 2 chars */
 
 	pc = regs->nip - (instructions_to_print * 3 / 4 * sizeof(int));
 
@@ -1314,29 +1317,27 @@ void show_user_instructions(struct pt_regs *regs)
 		return;
 	}
 
-	pr_info("%s[%d]: code: ", current->comm, current->pid);
+	seq_buf_init(&s, buf, sizeof(buf));
 
-	for (i = 0; i < instructions_to_print; i++) {
-		int instr;
+	while (n) {
+		int i;
 
-		if (!(i % 8) && (i > 0)) {
-			pr_cont("\n");
-			pr_info("%s[%d]: code: ", current->comm, current->pid);
-		}
+		seq_buf_clear(&s);
 
-		if (probe_kernel_address((const void *)pc, instr)) {
-			pr_cont("XXXXXXXX ");
-		} else {
-			if (regs->nip == pc)
-				pr_cont("<%08x> ", instr);
-			else
-				pr_cont("%08x ", instr);
+		for (i = 0; i < 8 && n; i++, n--, pc += sizeof(int)) {
+			int instr;
+
+			if (probe_kernel_address((const void *)pc, instr)) {
+				seq_buf_printf(&s, "XXXXXXXX ");
+				continue;
+			}
+			seq_buf_printf(&s, regs->nip == pc ? "<%08x> " : "%08x ", instr);
 		}
 
-		pc += sizeof(int);
+		if (!seq_buf_has_overflowed(&s))
+			pr_info("%s[%d]: code: %s\n", current->comm,
+				current->pid, s.buffer);
 	}
-
-	pr_cont("\n");
 }
 
 struct regbit {