[RFC PATCH 1/7] serial: Emulate break using control characters
From: Daniel Thompson
Date: Wed Mar 18 2015 - 10:21:17 EST
Currently the magic SysRq functions can accessed by sending a break on
the serial port. Unfortunately some networked serial proxies make it
difficult to send a break meaning SysRq functions cannot be used. This
patch provides a workaround by allowing the (fairly unlikely) sequence
of ^B^R^K characters to emulate a real break.
This approach is very nearly as robust as normal sysrq/break handling
because all trigger recognition happens during interrupt handling. Only
major difference is that to emulate a break we must enter the ISR four
times (instead of twice) and manage an extra byte of state.
No means is provided to escape the trigger sequence (and pass ^B^R^K to
the underlying process) however the sequence is proved reasonably pretty
collision resistant in practice. The most significant consequence is
that ^B and ^B^R are delayed until a new character is observed.
The most significant collision I am aware of is with emacs-like
backward-char bindings (^B) because the character movement will become
lumpy (two characters every two key presses rather than one character
per key press). Arrow keys or ^B^B^F provide workarounds.
Special note for tmux users:
tmux defaults to using ^B as its escape character but does not have a
default binding for ^B^R. Likewise tmux had no visual indicator
showing the beginning of break sequence meaning delayed the delivery
of ^B is not observable. Thus serial break emulation does not interfere
with the use of tmux's default key bindings.
Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
---
include/linux/serial_core.h | 83 +++++++++++++++++++++++++++++++++++----------
lib/Kconfig.debug | 15 ++++++++
2 files changed, 80 insertions(+), 18 deletions(-)
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index cf9c2ce9479d..56f8e3daf42c 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -160,6 +160,9 @@ struct uart_port {
struct console *cons; /* struct console, if any */
#if defined(CONFIG_SERIAL_CORE_CONSOLE) || defined(SUPPORT_SYSRQ)
unsigned long sysrq; /* sysrq timeout */
+#ifdef CONFIG_MAGIC_SYSRQ_BREAK_EMULATION
+ char sysrq_emul; /* emulation state */
+#endif
#endif
/* flags must be updated while holding port mutex */
@@ -420,24 +423,6 @@ extern void uart_handle_cts_change(struct uart_port *uport,
extern void uart_insert_char(struct uart_port *port, unsigned int status,
unsigned int overrun, unsigned int ch, unsigned int flag);
-#ifdef SUPPORT_SYSRQ
-static inline int
-uart_handle_sysrq_char(struct uart_port *port, unsigned int ch)
-{
- if (port->sysrq) {
- if (ch && time_before(jiffies, port->sysrq)) {
- handle_sysrq(ch);
- port->sysrq = 0;
- return 1;
- }
- port->sysrq = 0;
- }
- return 0;
-}
-#else
-#define uart_handle_sysrq_char(port,ch) ({ (void)port; 0; })
-#endif
-
/*
* We do the SysRQ and SAK checking like this...
*/
@@ -462,6 +447,68 @@ static inline int uart_handle_break(struct uart_port *port)
return 0;
}
+#if defined(SUPPORT_SYSRQ) && defined(CONFIG_MAGIC_SYSRQ_BREAK_EMULATION)
+/*
+ * Emulate a break if we are the serial console and receive ^B, ^R, ^K.
+ */
+static inline int
+uart_handle_sysrq_break_emulation(struct uart_port *port, unsigned int ch)
+{
+ const unsigned int ctrlb = 'B' & 31;
+ const unsigned int ctrlr = 'R' & 31;
+ const unsigned int ctrlk = 'K' & 31;
+
+ if (uart_console(port)) {
+ if ((port->sysrq_emul == 0 && ch == ctrlb) ||
+ (port->sysrq_emul == ctrlb && ch == ctrlr)) {
+ /* for either of the first two trigger characters
+ * update the state variable and move on.
+ */
+ port->sysrq_emul = ch;
+ return 1;
+ } else if (port->sysrq_emul == ctrlr && ch == ctrlk &&
+ uart_handle_break(port)) {
+ /* the break has already been emulated whilst
+ * evaluating the condition, tidy up and move on
+ */
+ port->sysrq_emul = 0;
+ return 1;
+ }
+ }
+
+ if (port->sysrq_emul) {
+ /* received a partial (false) trigger, tidy up and move on */
+ uart_insert_char(port, 0, 0, ctrlb, TTY_NORMAL);
+ if (port->sysrq_emul == ctrlr)
+ uart_insert_char(port, 0, 0, ctrlr, TTY_NORMAL);
+ port->sysrq_emul = 0;
+ }
+
+ return 0;
+}
+#else
+#define uart_handle_sysrq_break_emulation(port, ch) ({ (void)port; 0; })
+#endif
+
+#ifdef SUPPORT_SYSRQ
+static inline int
+uart_handle_sysrq_char(struct uart_port *port, unsigned int ch)
+{
+ if (port->sysrq) {
+ if (ch && time_before(jiffies, port->sysrq)) {
+ handle_sysrq(ch);
+ port->sysrq = 0;
+ return 1;
+ }
+ port->sysrq = 0;
+ }
+
+ return uart_handle_sysrq_break_emulation(port, ch);
+}
+#else
+#define uart_handle_sysrq_char(port, ch) ({ (void)port; 0; })
+#endif
+
/*
* UART_ENABLE_MS - determine if port should enable modem status irqs
*/
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index c2d51af327bc..3f54e85c27d2 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -372,6 +372,21 @@ config MAGIC_SYSRQ_DEFAULT_ENABLE
This may be set to 1 or 0 to enable or disable them all, or
to a bitmask as described in Documentation/sysrq.txt.
+config MAGIC_SYSRQ_BREAK_EMULATION
+ bool "Enable magic SysRq serial break emulation"
+ depends on MAGIC_SYSRQ && SERIAL_CORE_CONSOLE
+ default n
+ help
+ If you say Y here, then you can use the character sequence ^B^R^K
+ to simulate a BREAK on the serial console. This is useful if for
+ some reason you cannot send a BREAK to your console's serial port.
+ For example, if you have a serial device server that cannot
+ send a BREAK. Enabling this feature can delay the delivery of
+ characters to the TTY because the ^B and a subsequent ^R will be
+ delayed until we know what the next character is.
+
+ If unsure, say N.
+
config DEBUG_KERNEL
bool "Kernel debugging"
help
--
2.1.0
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