kdb: Remove special case logic from kdb_read()

kdb_read() contains special case logic to force it exit after reading
a single character. We can remove all the special case logic by directly
calling the function to read a single character instead. This also
allows us to tidy up the function prototype which, because it now matches
getchar(), we can also rename in order to make its role clearer.

This does involve some extra code to handle btaprompt properly but we
don't mind the new lines of code here because the old code had some
interesting problems (bad newline handling, treating unexpected
characters like <cr>).
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20191025073328.643-4-daniel.thompson@linaro.org

kernel/debug/kdb/kdb_bt.c

@@ -75,9 +75,10 @@ static void kdb_show_stack(struct task_struct *p, void *addr)
 static int
 kdb_bt1(struct task_struct *p, unsigned long mask, bool btaprompt)
 {
-	char buffer[2];
-	if (kdb_getarea(buffer[0], (unsigned long)p) ||
-	    kdb_getarea(buffer[0], (unsigned long)(p+1)-1))
+	char ch;
+
+	if (kdb_getarea(ch, (unsigned long)p) ||
+	    kdb_getarea(ch, (unsigned long)(p+1)-1))
 		return KDB_BADADDR;
 	if (!kdb_task_state(p, mask))
 		return 0;
@@ -85,13 +86,18 @@ kdb_bt1(struct task_struct *p, unsigned long mask, bool btaprompt)
 	kdb_ps1(p);
 	kdb_show_stack(p, NULL);
 	if (btaprompt) {
-		kdb_getstr(buffer, sizeof(buffer),
-			   "Enter <q> to end, <cr> to continue:");
-		if (buffer[0] == 'q') {
+		kdb_printf("Enter <q> to end, <cr> or <space> to continue:");
+		do {
+			ch = kdb_getchar();
+		} while (!strchr("\r\n q", ch));
 		kdb_printf("\n");
+
+		/* reset the pager */
+		kdb_nextline = 1;
+
+		if (ch == 'q')
 			return 1;
 	}
-	}
 	touch_nmi_watchdog();
 	return 0;
 }

kernel/debug/kdb/kdb_io.c

@@ -108,7 +108,22 @@ static int kdb_handle_escape(char *buf, size_t sz)
 	return -1;
 }
 
-static int kdb_read_get_key(char *buffer, size_t bufsize)
+/**
+ * kdb_getchar() - Read a single character from a kdb console (or consoles).
+ *
+ * Other than polling the various consoles that are currently enabled,
+ * most of the work done in this function is dealing with escape sequences.
+ *
+ * An escape key could be the start of a vt100 control sequence such as \e[D
+ * (left arrow) or it could be a character in its own right.  The standard
+ * method for detecting the difference is to wait for 2 seconds to see if there
+ * are any other characters.  kdb is complicated by the lack of a timer service
+ * (interrupts are off), by multiple input sources. Escape sequence processing
+ * has to be done as states in the polling loop.
+ *
+ * Return: The key pressed or a control code derived from an escape sequence.
+ */
+char kdb_getchar(void)
 {
 #define ESCAPE_UDELAY 1000
 #define ESCAPE_DELAY (2*1000000/ESCAPE_UDELAY) /* 2 seconds worth of udelays */
@@ -126,7 +141,6 @@ static int kdb_read_get_key(char *buffer, size_t bufsize)
 		}
 
 		key = (*f)();
-
 		if (key == -1) {
 			if (escape_delay) {
 				udelay(ESCAPE_UDELAY);
@@ -136,14 +150,6 @@ static int kdb_read_get_key(char *buffer, size_t bufsize)
 			continue;
 		}
 
-		if (bufsize <= 2) {
-			if (key == '\r')
-				key = '\n';
-			*buffer++ = key;
-			*buffer = '\0';
-			return -1;
-		}
-
 		if (escape_delay == 0 && key == '\e') {
 			escape_delay = ESCAPE_DELAY;
 			ped = escape_data;
@@ -184,17 +190,7 @@ static int kdb_read_get_key(char *buffer, size_t bufsize)
  *	function.  It is not reentrant - it relies on the fact
  *	that while kdb is running on only one "master debug" cpu.
  * Remarks:
- *
- * The buffer size must be >= 2.  A buffer size of 2 means that the caller only
- * wants a single key.
- *
- * An escape key could be the start of a vt100 control sequence such as \e[D
- * (left arrow) or it could be a character in its own right.  The standard
- * method for detecting the difference is to wait for 2 seconds to see if there
- * are any other characters.  kdb is complicated by the lack of a timer service
- * (interrupts are off), by multiple input sources and by the need to sometimes
- * return after just one key.  Escape sequence processing has to be done as
- * states in the polling loop.
+ *	The buffer size must be >= 2.
  */
 
 static char *kdb_read(char *buffer, size_t bufsize)
@@ -229,9 +225,7 @@ static char *kdb_read(char *buffer, size_t bufsize)
 	*cp = '\0';
 	kdb_printf("%s", buffer);
 poll_again:
-	key = kdb_read_get_key(buffer, bufsize);
-	if (key == -1)
-		return buffer;
+	key = kdb_getchar();
 	if (key != 9)
 		tab = 0;
 	switch (key) {
@@ -742,7 +736,7 @@ int vkdb_printf(enum kdb_msgsrc src, const char *fmt, va_list ap)
 
 	/* check for having reached the LINES number of printed lines */
 	if (kdb_nextline >= linecount) {
-		char buf1[16] = "";
+		char ch;
 
 		/* Watch out for recursion here.  Any routine that calls
 		 * kdb_printf will come back through here.  And kdb_read
@@ -777,39 +771,38 @@ int vkdb_printf(enum kdb_msgsrc src, const char *fmt, va_list ap)
 		if (logging)
 			printk("%s", moreprompt);
 
-		kdb_read(buf1, 2); /* '2' indicates to return
-				    * immediately after getting one key. */
+		ch = kdb_getchar();
 		kdb_nextline = 1;	/* Really set output line 1 */
 
 		/* empty and reset the buffer: */
 		kdb_buffer[0] = '\0';
 		next_avail = kdb_buffer;
 		size_avail = sizeof(kdb_buffer);
-		if ((buf1[0] == 'q') || (buf1[0] == 'Q')) {
+		if ((ch == 'q') || (ch == 'Q')) {
 			/* user hit q or Q */
 			KDB_FLAG_SET(CMD_INTERRUPT); /* command interrupted */
 			KDB_STATE_CLEAR(PAGER);
 			/* end of command output; back to normal mode */
 			kdb_grepping_flag = 0;
 			kdb_printf("\n");
-		} else if (buf1[0] == ' ') {
+		} else if (ch == ' ') {
 			kdb_printf("\r");
 			suspend_grep = 1; /* for this recursion */
-		} else if (buf1[0] == '\n') {
+		} else if (ch == '\n' || ch == '\r') {
 			kdb_nextline = linecount - 1;
 			kdb_printf("\r");
 			suspend_grep = 1; /* for this recursion */
-		} else if (buf1[0] == '/' && !kdb_grepping_flag) {
+		} else if (ch == '/' && !kdb_grepping_flag) {
 			kdb_printf("\r");
 			kdb_getstr(kdb_grep_string, KDB_GREP_STRLEN,
 				   kdbgetenv("SEARCHPROMPT") ?: "search> ");
 			*strchrnul(kdb_grep_string, '\n') = '\0';
 			kdb_grepping_flag += KDB_GREPPING_FLAG_SEARCH;
 			suspend_grep = 1; /* for this recursion */
-		} else if (buf1[0] && buf1[0] != '\n') {
-			/* user hit something other than enter */
+		} else if (ch) {
+			/* user hit something unexpected */
 			suspend_grep = 1; /* for this recursion */
-			if (buf1[0] != '/')
+			if (ch != '/')
 				kdb_printf(
 				    "\nOnly 'q', 'Q' or '/' are processed at "
 				    "more prompt, input ignored\n");

kernel/debug/kdb/kdb_private.h

@@ -210,6 +210,7 @@ extern void kdb_ps1(const struct task_struct *p);
 extern void kdb_print_nameval(const char *name, unsigned long val);
 extern void kdb_send_sig(struct task_struct *p, int sig);
 extern void kdb_meminfo_proc_show(void);
+extern char kdb_getchar(void);
 extern char *kdb_getstr(char *, size_t, const char *);
 extern void kdb_gdb_state_pass(char *buf);