kdb: Fix console handling when editing and tab-completing commands

Currently, if the cursor position is not at the end of the command buffer
and the user uses the Tab-complete functions, then the console does not
leave the cursor in the correct position.

For example consider the following buffer with the cursor positioned
at the ^:

md kdb_pro 10
          ^

Pressing tab should result in:

md kdb_prompt_str 10
                 ^

However this does not happen. Instead the cursor is placed at the end
(after then 10) and further cursor movement redraws incorrectly. The
same problem exists when we double-Tab but in a different part of the
code.

Fix this by sending a carriage return and then redisplaying the text to
the left of the cursor.

Cc: stable@vger.kernel.org
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/20240424-kgdb_read_refactor-v3-3-f236dbe9828d@linaro.orgSigned-off-by: Daniel Thompson <daniel.thompson@linaro.org>

kernel/debug/kdb/kdb_io.c

@@ -383,6 +383,8 @@ static char *kdb_read(char *buffer, size_t bufsize)
 			kdb_printf("\n");
 			kdb_printf(kdb_prompt_str);
 			kdb_printf("%s", buffer);
+			if (cp != lastchar)
+				kdb_position_cursor(kdb_prompt_str, buffer, cp);
 		} else if (tab != 2 && count > 0) {
 			/* How many new characters do we want from tmpbuffer? */
 			len_tmp = strlen(p_tmp) - len;
@@ -396,6 +398,9 @@ static char *kdb_read(char *buffer, size_t bufsize)
 				kdb_printf("%s", cp);
 				cp += len_tmp;
 				lastchar += len_tmp;
+				if (cp != lastchar)
+					kdb_position_cursor(kdb_prompt_str,
+							    buffer, cp);
 			}
 		}
 		kdb_nextline = 1; /* reset output line number */