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SubjectRe: [patch 16/16] Add hardware breakpoint support for i386
 This patch provides a simple interface for kernel-space watchpoints
using processor's debug registers. Using Kwatch interface users can
monitor kernel global variables and dump the debugging information such
as kernel stack, global variables, processor registers.

int register_kwatch(unsigned long addr, u8 length, u8 type,
kwatch_handler_t handler)

-length of the breakpoint can be 1,2 or 4 bytes long.
-type can be read, write, execute.
0 Break on instruction execution only.
1 Break on data writes only.
3 Break on data reads or writes but not instruction fetches.

-return value is the debug register number allocated/used
for setting up this watch point.

Sample code:

This sample code sets a watchpoint on the pid_max
and registers a call back function if any writes
happen to pid_max.

struct kwatch kp;
void kwatch_handler(struct kwatch *p, struct pt_regs *regs)
{
.......<do-any-thing>........
}

Register watchpoint probe from init_module:

static int debug_regs_num;
int init_module(void)
{
..........<do-any-thing>............

debug_regs_num = register_kwatch(kallsyms_lookup_name(pid_max),
4, 1, kwatch_handler);
..........<do-any-thing>............
}

Test this by changing the value of pid_max in
/proc/sys/kernel/pid_max

echo 1000 > /proc/sys/kernel/pid_max

You see the call back function being called.

Unregister the watchpoint from cleanup_module:

void cleanup_module(void)
{
..........<do-any-thing>............
unregister_kwatch(debug_regs_num);
..........<do-any-thing>............
}

Signed-off-by: Prasanna S Panchamukhi <prasanna@in.ibm.com>
---


---

linux-2.6.13-prasanna/arch/i386/Kconfig.debug | 8 +
linux-2.6.13-prasanna/arch/i386/kernel/Makefile | 1
linux-2.6.13-prasanna/arch/i386/kernel/kwatch.c | 189 ++++++++++++++++++++++++
linux-2.6.13-prasanna/include/asm-i386/kwatch.h | 60 +++++++
4 files changed, 258 insertions(+)

diff -puN arch/i386/Kconfig.debug~kernel-watchpoint arch/i386/Kconfig.debug
--- linux-2.6.13/arch/i386/Kconfig.debug~kernel-watchpoint 2005-08-30 11:44:25.921069488 +0530
+++ linux-2.6.13-prasanna/arch/i386/Kconfig.debug 2005-08-30 11:44:25.932067816 +0530
@@ -32,6 +32,14 @@ config KPROBES
for kernel debugging, non-intrusive instrumentation and testing.
If in doubt, say "N".

+config KWATCH
+ bool "Kwatch points"
+ depends on DEBUG_KERNEL
+ select DEBUGREG
+ help
+ This enables kernel-space watchpoints using processor's debug
+ registers. If in doubt, say "N".
+
config DEBUGREG
bool "Global Debug Registers"
depends on DEBUG_KERNEL
diff -puN /dev/null arch/i386/kernel/kwatch.c
--- /dev/null 2005-08-30 16:04:24.253093808 +0530
+++ linux-2.6.13-prasanna/arch/i386/kernel/kwatch.c 2005-08-30 11:44:25.933067664 +0530
@@ -0,0 +1,189 @@
+/*
+ * Kernel Watchpoint interface.
+ * arch/i386/kernel/kwatch.c
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *
+ * Copyright (C) IBM Corporation, 2002, 2004
+ *
+ * 2002-Oct Created by Vamsi Krishna S <vamsi_krishna@in.ibm.com> for
+ * Kernel Watchpoint implementation.
+ * 2004-Oct Updated by Prasanna S Panchamukhi <prasanna@in.ibm.com> to
+ * to make use of notifiers.
+ */
+#include <linux/config.h>
+#include <linux/kprobes.h>
+#include <linux/ptrace.h>
+#include <linux/spinlock.h>
+#include <linux/module.h>
+#include <linux/init.h>
+#include <asm/kwatch.h>
+#include <asm/kdebug.h>
+#include <asm/debugreg.h>
+#include <asm/bitops.h>
+
+static struct kwatch kwatch_list[DR_MAX];
+static spinlock_t kwatch_lock = SPIN_LOCK_UNLOCKED;
+static unsigned long kwatch_in_progress; /* currently being handled */
+
+struct dr_info {
+ int debugreg;
+ unsigned long addr;
+ int type;
+};
+
+static inline void write_smp_dr(void *info)
+{
+ struct dr_info *dr = (struct dr_info *)info;
+
+ if (cpu_has_de && dr->type == DR_TYPE_IO)
+ set_in_cr4(X86_CR4_DE);
+ write_dr(dr->debugreg, dr->addr);
+}
+
+/* Update the debug register on all CPUs */
+static void sync_dr(int debugreg, unsigned long addr, int type)
+{
+ struct dr_info dr;
+ dr.debugreg = debugreg;
+ dr.addr = addr;
+ dr.type = type;
+ smp_call_function(write_smp_dr, &dr, 0, 0);
+}
+
+/*
+ * Interrupts are disabled on entry as trap1 is an interrupt gate and they
+ * remain disabled thorough out this function.
+ */
+int kwatch_handler(unsigned long condition, struct pt_regs *regs)
+{
+ int debugreg = dr_trap(condition);
+ unsigned long addr = dr_trap_addr(condition);
+ int retval = 0;
+
+ if (!(condition & (DR_TRAP0 | DR_TRAP1 | DR_TRAP2 | DR_TRAP3)))
+ return 0;
+
+ /* We're in an interrupt, but this is clear and BUG()-safe. */
+ preempt_disable();
+
+ /* If we are recursing, we already hold the lock. */
+ if (kwatch_in_progress)
+ goto recursed;
+
+ set_bit(debugreg, &kwatch_in_progress);
+
+ spin_lock(&kwatch_lock);
+ if (kwatch_list[debugreg].addr != addr)
+ goto out;
+
+ if (kwatch_list[debugreg].handler)
+ kwatch_list[debugreg].handler(&kwatch_list[debugreg], regs);
+
+ if (kwatch_list[debugreg].type == DR_TYPE_EXECUTE)
+ regs->eflags |= RF_MASK;
+ out:
+ clear_bit(debugreg, &kwatch_in_progress);
+ spin_unlock(&kwatch_lock);
+ preempt_enable_no_resched();
+ return retval;
+
+ recursed:
+ if (kwatch_list[debugreg].type == DR_TYPE_EXECUTE)
+ regs->eflags |= RF_MASK;
+ preempt_enable_no_resched();
+ return 1;
+}
+
+int register_kwatch(unsigned long addr, u8 length, u8 type,
+ kwatch_handler_t handler)
+{
+ int debugreg;
+ unsigned long dr7, flags;
+
+ debugreg = dr_alloc(DR_ANY, DR_ALLOC_GLOBAL);
+ if (debugreg < 0)
+ return -1;
+
+ spin_lock_irqsave(&kwatch_lock, flags);
+ kwatch_list[debugreg].addr = addr;
+ kwatch_list[debugreg].length = length;
+ kwatch_list[debugreg].type = type;
+ kwatch_list[debugreg].handler = handler;
+ spin_unlock_irqrestore(&kwatch_lock, flags);
+
+ write_dr(debugreg, (unsigned long)addr);
+ sync_dr(debugreg, (unsigned long)addr, type);
+ if (cpu_has_de && type == DR_TYPE_IO)
+ set_in_cr4(X86_CR4_DE);
+
+ dr7 = read_dr(7);
+ SET_DR7(dr7, debugreg, type, length);
+ write_dr(7, dr7);
+ sync_dr(7, dr7, 0);
+ return debugreg;
+}
+
+void unregister_kwatch(int debugreg)
+{
+ unsigned long flags;
+ unsigned long dr7 = read_dr(7);
+
+ RESET_DR7(dr7, debugreg);
+ write_dr(7, dr7);
+ sync_dr(7, dr7, 0);
+ dr_free(debugreg);
+
+ spin_lock_irqsave(&kwatch_lock, flags);
+ kwatch_list[debugreg].addr = 0;
+ kwatch_list[debugreg].handler = NULL;
+ spin_unlock_irqrestore(&kwatch_lock, flags);
+}
+
+/*
+ * Wrapper routine to for handling exceptions.
+ */
+int kwatch_exceptions_notify(struct notifier_block *self, unsigned long val,
+ void *data)
+{
+ struct die_args *args = (struct die_args *)data;
+ switch (val) {
+ case DIE_DEBUG:
+ if (kwatch_handler(args->err, args->regs))
+ return NOTIFY_STOP;
+ break;
+ default:
+ break;
+ }
+ return NOTIFY_DONE;
+}
+
+static struct notifier_block kwatch_exceptions_nb = {
+ .notifier_call = kwatch_exceptions_notify,
+ .priority = 0x7ffffffe /* we need to notified second */
+};
+
+static int __init init_kwatch(void)
+{
+ int err = 0;
+
+ err = register_die_notifier(&kwatch_exceptions_nb);
+ return err;
+}
+
+__initcall(init_kwatch);
+
+EXPORT_SYMBOL_GPL(register_kwatch);
+EXPORT_SYMBOL_GPL(unregister_kwatch);
diff -puN arch/i386/kernel/Makefile~kernel-watchpoint arch/i386/kernel/Makefile
--- linux-2.6.13/arch/i386/kernel/Makefile~kernel-watchpoint 2005-08-30 11:44:25.926068728 +0530
+++ linux-2.6.13-prasanna/arch/i386/kernel/Makefile 2005-08-30 11:44:25.934067512 +0530
@@ -35,6 +35,7 @@ obj-$(CONFIG_HPET_TIMER) += time_hpet.o
obj-$(CONFIG_EFI) += efi.o efi_stub.o
obj-$(CONFIG_EARLY_PRINTK) += early_printk.o
obj-$(CONFIG_DEBUGREG) += debugreg.o
+obj-$(CONFIG_KWATCH) += kwatch.o

EXTRA_AFLAGS := -traditional

diff -puN /dev/null include/asm-i386/kwatch.h
--- /dev/null 2005-08-30 16:04:24.253093808 +0530
+++ linux-2.6.13-prasanna/include/asm-i386/kwatch.h 2005-08-30 11:44:25.935067360 +0530
@@ -0,0 +1,60 @@
+#ifndef _ASM_KWATCH_H
+#define _ASM_KWATCH_H
+/*
+ * Kernel Watchpoint interface.
+ * include/asm-i386/kwatch.h
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *
+ * Copyright (C) IBM Corporation, 2002, 2004
+ *
+ * 2002-Oct Created by Vamsi Krishna S <vamsi_krishna@in.ibm.com> for
+ * Kernel Watchpoint implementation.
+ */
+#include <linux/types.h>
+#include <linux/ptrace.h>
+
+struct kwatch;
+typedef void (*kwatch_handler_t) (struct kwatch *, struct pt_regs *);
+
+struct kwatch {
+ unsigned long addr; /* location of watchpoint */
+ u8 length; /* range of address */
+ u8 type; /* type of watchpoint */
+ kwatch_handler_t handler;
+};
+
+#define RF_MASK 0x00010000
+
+#ifdef CONFIG_KWATCH
+extern int register_kwatch(unsigned long addr, u8 length, u8 type,
+ kwatch_handler_t handler);
+extern void unregister_kwatch(int debugreg);
+extern int kwatch_handler(unsigned long condition, struct pt_regs *regs);
+#else
+static inline int register_kwatch(unsigned long addr, u8 length, u8 type,
+ kwatch_handler_t handler)
+{
+ return -ENOSYS;
+}
+static inline void unregister_kwatch(int debugreg)
+{
+}
+static inline int kwatch_handler(unsigned long condition, struct pt_regs *regs)
+{
+ return 0;
+}
+#endif
+#endif /* _ASM_KWATCH_H */
_
--

Prasanna S Panchamukhi
Linux Technology Center
India Software Labs, IBM Bangalore
Ph: 91-80-25044636
<prasanna@in.ibm.com>
-
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