libbpf: Explicitly call write to append content to file

Write data to fd by calling "vdprintf", in most implementations
of the standard library, the data is finally written by the writev syscall.
But "uprobe_events/kprobe_events" does not allow segmented writes,
so switch the "append_to_file" function to explicit write() call.

Signed-off-by: Liu Pan <patteliu@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230320030720.650-1-patteliu@gmail.com

tools/lib/bpf/libbpf.c

@@ -9918,16 +9918,20 @@ static int append_to_file(const char *file, const char *fmt, ...)
 {
 	int fd, n, err = 0;
 	va_list ap;
+	char buf[1024];
+
+	va_start(ap, fmt);
+	n = vsnprintf(buf, sizeof(buf), fmt, ap);
+	va_end(ap);
+
+	if (n < 0 || n >= sizeof(buf))
+		return -EINVAL;
 
 	fd = open(file, O_WRONLY | O_APPEND | O_CLOEXEC, 0);
 	if (fd < 0)
 		return -errno;
 
-	va_start(ap, fmt);
-	n = vdprintf(fd, fmt, ap);
-	va_end(ap);
-
-	if (n < 0)
+	if (write(fd, buf, n) < 0)
 		err = -errno;
 
 	close(fd);