tracing: Have traceprobe_probes_write() not access userspace unnecessarily

The code in traceprobe_probes_write() reads up to 4096 bytes from userpace
for each line. If userspace passes in several lines to execute, the code
will do a large read for each line, even though, it is highly likely that
the first read from userspace received all of the lines at once.

I changed the logic to do a single read from userspace, and to only read
from userspace again if not all of the read from userspace made it in.

I tested this by adding printk()s and writing files that would test -1, ==,
and +1 the buffer size, to make sure that there's no overflows and that if a
single line is written with +1 the buffer size, that it fails properly.

Link: http://lkml.kernel.org/r/20170209180458.5c829ab2@gandalf.local.home

Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_probe.c

@@ -647,7 +647,7 @@ ssize_t traceprobe_probes_write(struct file *file, const char __user *buffer,
 				size_t count, loff_t *ppos,
 				int (*createfn)(int, char **))
 {
-	char *kbuf, *tmp;
+	char *kbuf, *buf, *tmp;
 	int ret = 0;
 	size_t done = 0;
 	size_t size;
@@ -667,27 +667,37 @@ ssize_t traceprobe_probes_write(struct file *file, const char __user *buffer,
 			goto out;
 		}
 		kbuf[size] = '\0';
-		tmp = strchr(kbuf, '\n');
+		buf = kbuf;
-
+		do {
+			tmp = strchr(buf, '\n');
 			if (tmp) {
 				*tmp = '\0';
-			size = tmp - kbuf + 1;
+				size = tmp - buf + 1;
-		} else if (done + size < count) {
+			} else {
+				size = strlen(buf);
+				if (done + size < count) {
+					if (buf != kbuf)
+						break;
 					pr_warn("Line length is too long: Should be less than %d\n",
 						WRITE_BUFSIZE);
 					ret = -EINVAL;
 					goto out;
 				}
+			}
 			done += size;
+
 			/* Remove comments */
-		tmp = strchr(kbuf, '#');
+			tmp = strchr(buf, '#');
 
 			if (tmp)
 				*tmp = '\0';
 
-		ret = traceprobe_command(kbuf, createfn);
+			ret = traceprobe_command(buf, createfn);
 			if (ret)
 				goto out;
+			buf += size;
+
+		} while (done < count);
 	}
 	ret = done;