n_tty: Distribute switch variables for initialization

Variables declared in a switch statement before any case statements
cannot be automatically initialized with compiler instrumentation (as
they are not part of any execution flow). With GCC's proposed automatic
stack variable initialization feature, this triggers a warning (and they
don't get initialized). Clang's automatic stack variable initialization
(via CONFIG_INIT_STACK_ALL=y) doesn't throw a warning, but it also
doesn't initialize such variables[1]. Note that these warnings (or silent
skipping) happen before the dead-store elimination optimization phase,
so even when the automatic initializations are later elided in favor of
direct initializations, the warnings remain.

To avoid these problems, move such variables into the "case" where
they're used or lift them up into the main function body.

drivers/tty/n_tty.c: In function ‘__process_echoes’:
drivers/tty/n_tty.c:657:18: warning: statement will never be executed [-Wswitch-unreachable]
  657 |     unsigned int num_chars, num_bs;
      |                  ^~~~~~~~~

[1] https://bugs.llvm.org/show_bug.cgi?id=44916

Reviewed-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200220062313.69209-1-keescook@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_tty.c

@@ -654,9 +654,9 @@ static size_t __process_echoes(struct tty_struct *tty)
 			op = echo_buf(ldata, tail + 1);
 
 			switch (op) {
+			case ECHO_OP_ERASE_TAB: {
 				unsigned int num_chars, num_bs;
 
-			case ECHO_OP_ERASE_TAB:
 				if (MASK(ldata->echo_commit) == MASK(tail + 2))
 					goto not_yet_stored;
 				num_chars = echo_buf(ldata, tail + 2);
@@ -687,7 +687,7 @@ static size_t __process_echoes(struct tty_struct *tty)
 				}
 				tail += 3;
 				break;
-
+			}
 			case ECHO_OP_SET_CANON_COL:
 				ldata->canon_column = ldata->column;
 				tail += 2;