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Subject[PATCH 08/11] ARM: kgdb_fiq: Implement knocking into KDB via escape command
Date
As Colin Cross noticed, serial ports could be noisy, so occasional
characters once in a while are possible. So, considering the noise
possibility, entering the debugger on any received byte is unacceptable
for production devices.

This changes KGDB FIQ behaviour in a such way so that we have to type the
GDB-protocol "$3#33" command to actually enter the debugger, the kernel
will print the following prompt:

Type $3#33 to enter the debugger>

This is the exactly the same command we use to escape from KGDB to KDB,
so it should be all pretty familiar.

For convenience, there is a kgdb_fiq.knock kernel command line option,
when set to 0, this turns the special command to just a return key
press, so the kernel will be printing this:

Hit <return> to enter the debugger>

And for the cases when NMI connected to a dedicated button, the knocking
can be disabled altogether by setting kgdb_fiq.knock to -1.

Suggested-by: Colin Cross <ccross@android.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
arch/arm/kernel/kgdb_fiq.c | 58 ++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 58 insertions(+)
diff --git a/arch/arm/kernel/kgdb_fiq.c b/arch/arm/kernel/kgdb_fiq.c
index 72a62c7..2bf4467 100644
--- a/arch/arm/kernel/kgdb_fiq.c
+++ b/arch/arm/kernel/kgdb_fiq.c
@@ -18,6 +18,7 @@
#include <linux/slab.h>
#include <linux/errno.h>
#include <linux/hardirq.h>
+#include <linux/kdb.h>
#include <linux/kgdb.h>
#include <asm/fiq.h>
#include <asm/exception.h>
@@ -26,6 +27,60 @@ static int kgdb_fiq_enabled;
module_param_named(enable, kgdb_fiq_enabled, int, 0600);
MODULE_PARM_DESC(enable, "set to 1 to enable FIQ KGDB");

+static int kgdb_fiq_knock = 1;
+module_param_named(knock, kgdb_fiq_knock, int, 0600);
+MODULE_PARM_DESC(knock, "if set to 1 (default), the special '$3#33' command "
+ "must be used to enter the debugger; when set to 0, "
+ "hitting return key is enough to enter the debugger; "
+ "when set to -1, the debugger is entered immediately "
+ "upon NMI");
+
+/*
+ * "Serial ports are often noisy, especially when muxed over another port (we
+ * often use serial over the headset connector). Noise on the async command
+ * line just causes characters that are ignored, on a command line that blocked
+ * execution noise would be catastrophic." -- Colin Cross
+ *
+ * So, this small function implements KGDB/KDB knocking on the serial line: we
+ * won't enter the debugger until we receive a known magic phrase (which is
+ * actually "$3#33", known as "escape to KDB" command. If knocking is disabled,
+ * just pressing the return key is enough to enter the debugger.
+ */
+static bool kgdb_fiq_poll_knock(void)
+{
+ static int n;
+ int c = -1;
+ get_char_func *getc;
+ char magic[] = "$3#33";
+ size_t m = strlen(magic);
+
+ if (kgdb_fiq_knock < 0)
+ return 1;
+
+ for (getc = &kdb_poll_funcs[0]; *getc; ++getc) {
+ c = (*getc)();
+ if (c >= 0)
+ break;
+ }
+
+ if (!kgdb_fiq_knock && (c == '\r' || c == '\n')) {
+ return 1;
+ } else if (c == magic[n]) {
+ kdb_printf("%c", c);
+ n = (n + 1) % m;
+ if (!n)
+ return 1;
+ } else {
+ n = 0;
+ kdb_printf("\r%s %s to enter the debugger> %*s",
+ kgdb_fiq_knock ? "Type" : "Hit",
+ kgdb_fiq_knock ? magic : "<return>", m, "");
+ memset(magic, '\b', m);
+ kdb_printf("%s", magic);
+ }
+ return 0;
+}
+
static unsigned int kgdb_fiq;
static void (*kgdb_enable_fiq)(unsigned int irq, bool on);
static bool (*is_kgdb_fiq)(unsigned int irq);
@@ -36,6 +91,9 @@ asmlinkage void __exception_irq_entry kgdb_fiq_do_handle(struct pt_regs *regs)
return;
dbg_io_ops->clear_irqs();

+ if (!kgdb_fiq_poll_knock())
+ return;
+
nmi_enter();
kgdb_handle_exception(1, 0, 0, regs);
nmi_exit();
--
1.7.10.4


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