kdb,vt_console: Fix missed data due to pager overruns

It is possible to miss data when using the kdb pager.  The kdb pager
does not pay attention to the maximum column constraint of the screen
or serial terminal.  This result is not incrementing the shown lines
correctly and the pager will print more lines that fit on the screen.
Obviously that is less than useful when using a VGA console where you
cannot scroll back.

The pager will now look at the kdb_buffer string to see how many
characters are printed.  It might not be perfect considering you can
output ASCII that might move the cursor position, but it is a
substantially better approximation for viewing dmesg and trace logs.

This also means that the vt screen needs to set the kdb COLUMNS
variable.

Cc: <stable@vger.kernel.org>
Signed-off-by: NJason Wessel <jason.wessel@windriver.com>

drivers/tty/vt/vt.c

@@ -3475,6 +3475,19 @@ int con_debug_enter(struct vc_data *vc)
 			kdb_set(2, setargs);
 		}
 	}
+	if (vc->vc_cols < 999) {
+		int colcount;
+		char cols[4];
+		const char *setargs[3] = {
+			"set",
+			"COLUMNS",
+			cols,
+		};
+		if (kdbgetintenv(setargs[0], &colcount)) {
+			snprintf(cols, 4, "%i", vc->vc_cols);
+			kdb_set(2, setargs);
+		}
+	}
 #endif /* CONFIG_KGDB_KDB */
 	return ret;
 }

kernel/debug/kdb/kdb_io.c

@@ -552,6 +552,7 @@ int vkdb_printf(const char *fmt, va_list ap)
 {
 	int diag;
 	int linecount;
+	int colcount;
 	int logging, saved_loglevel = 0;
 	int saved_trap_printk;
 	int got_printf_lock = 0;
@@ -584,6 +585,10 @@ int vkdb_printf(const char *fmt, va_list ap)
 	if (diag || linecount <= 1)
 		linecount = 24;
 
+	diag = kdbgetintenv("COLUMNS", &colcount);
+	if (diag || colcount <= 1)
+		colcount = 80;
+
 	diag = kdbgetintenv("LOGGING", &logging);
 	if (diag)
 		logging = 0;
@@ -690,7 +695,7 @@ int vkdb_printf(const char *fmt, va_list ap)
 		gdbstub_msg_write(kdb_buffer, retlen);
 	} else {
 		if (dbg_io_ops && !dbg_io_ops->is_console) {
-			len = strlen(kdb_buffer);
+			len = retlen;
 			cp = kdb_buffer;
 			while (len--) {
 				dbg_io_ops->write_char(*cp);
@@ -709,11 +714,29 @@ int vkdb_printf(const char *fmt, va_list ap)
 		printk(KERN_INFO "%s", kdb_buffer);
 	}
 
-	if (KDB_STATE(PAGER) && strchr(kdb_buffer, '\n'))
-		kdb_nextline++;
+	if (KDB_STATE(PAGER)) {
+		/*
+		 * Check printed string to decide how to bump the
+		 * kdb_nextline to control when the more prompt should
+		 * show up.
+		 */
+		int got = 0;
+		len = retlen;
+		while (len--) {
+			if (kdb_buffer[len] == '\n') {
+				kdb_nextline++;
+				got = 0;
+			} else if (kdb_buffer[len] == '\r') {
+				got = 0;
+			} else {
+				got++;
+			}
+		}
+		kdb_nextline += got / (colcount + 1);
+	}
 
 	/* check for having reached the LINES number of printed lines */
-	if (kdb_nextline == linecount) {
+	if (kdb_nextline >= linecount) {
 		char buf1[16] = "";
 
 		/* Watch out for recursion here.  Any routine that calls
@@ -765,7 +788,7 @@ int vkdb_printf(const char *fmt, va_list ap)
 			kdb_grepping_flag = 0;
 			kdb_printf("\n");
 		} else if (buf1[0] == ' ') {
-			kdb_printf("\n");
+			kdb_printf("\r");
 			suspend_grep = 1; /* for this recursion */
 		} else if (buf1[0] == '\n') {
 			kdb_nextline = linecount - 1;