vc_screen: extract vcs_read_buf_header

The attribute header handling is terrible in vcs_read_buf. Separate it
to a new function and simply do memmove (of up to 4 bytes) to the start
of the con_buf -- if user seeked.
Signed-off-by: NJiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200818085706.12163-15-jslaby@suse.czSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/vt/vc_screen.c

@@ -297,6 +297,22 @@ static void vcs_read_buf_noattr(const struct vc_data *vc, char *con_buf,
 	}
 }
 
+static unsigned int vcs_read_buf_header(const struct vc_data *vc, char *con_buf,
+		unsigned int pos, unsigned int count)
+{
+	count = min(HEADER_SIZE - pos, count);
+
+	/* clamp header values if they don't fit */
+	con_buf[0] = min(vc->vc_rows, 0xFFu);
+	con_buf[1] = min(vc->vc_cols, 0xFFu);
+	getconsxy(vc, con_buf + 2);
+
+	if (pos)
+		memmove(con_buf, con_buf + pos, count);
+
+	return count;
+}
+
 static unsigned int vcs_read_buf(const struct vc_data *vc, char *con_buf,
 		unsigned int pos, unsigned int count, bool viewed,
 		unsigned int *skip)
@@ -306,22 +322,11 @@ static unsigned int vcs_read_buf(const struct vc_data *vc, char *con_buf,
 	unsigned int filled = count;
 
 	if (pos < HEADER_SIZE) {
-		/* clamp header values if they don't fit */
-		con_buf[0] = min(vc->vc_rows, 0xFFu);
-		con_buf[1] = min(vc->vc_cols, 0xFFu);
-		getconsxy(vc, con_buf + 2);
-
-		*skip += pos;
-		count += pos;
-		if (count > CON_BUF_SIZE) {
-			count = CON_BUF_SIZE;
-			filled = count - pos;
-		}
+		count -= vcs_read_buf_header(vc, con_buf, pos, count);
 
-		/* Advance state pointers and move on. */
-		count -= min(HEADER_SIZE, count);
 		pos = HEADER_SIZE;
 		con_buf += HEADER_SIZE;
+
 		/* If count >= 0, then pos is even... */
 	} else if (pos & 1) {
 		/*