Re: [RFC][PATCH] Kprobes: The ON/OFF knob thru debugfs - updated

From: Ananth N Mavinakayanahalli
Date: Thu Apr 05 2007 - 02:22:13 EST

On Wed, Apr 04, 2007 at 11:13:30AM -0700, Keshavamurthy, Anil S wrote:
> On Wed, Apr 04, 2007 at 05:43:49PM +0530, Ananth N Mavinakayanahalli wrote:
> > This patch provides a debugfs knob to turn kprobes on/off
> Nice feature. Thanks. Review inline.

Thanks for the review Anil...

> > +static void __kprobes disable_all_kprobes(void)
> > +{
> > + struct hlist_head *head;
> > + struct hlist_node *node;
> > + struct kprobe *p;
> > + unsigned int i;
> > +
> > + mutex_lock(&kprobe_mutex);
> > +
> > + /* If kprobes are already disabled, just return */
> > + if (kprobe_enabled == 0)
> > + goto already_disabled;
> > +
> > + kprobe_enabled = 0;
> > + for (i = 0; i < KPROBE_TABLE_SIZE; i++) {
> > + head = &kprobe_table[i];
> > + hlist_for_each_entry_rcu(p, node, head, hlist)
> > + arch_disarm_kprobe(p);
> Humm... you are disarming all probes including the probe on
> kretprobe_trampoline. I think you should not disarm the probe on
> the kretprobe_trampoline ( which is registerd in arch_init_kprobe() function)
> as any return probe which has its return address modified to this
> kretprobe_trampoline address on its stack should be able to recover.
>
> Do something like this...
> hlist_for_each_entry_rcu(p, node, head, hlist)
> if (!trampoline_probe(p)) /* implement this in arch files */
> arch_disarm_kprobe(p);

Good catch! Its taken care of now.

> > + }
> > +
> > + mutex_unlock(&kprobe_mutex);
> > + /* Allow all currently running kprobes to complete */
> > + synchronize_sched();
> > +
>
> [....]
>
> > +
> > +/debug/kprobes/list: Lists all registered probes on the system
> > +
> > +c015d71a k vfs_read+0x0 1
> > +c011a316 j do_fork+0x0 1
> > +c03dedc5 r tcp_v4_rcv+0x0 1
> Do you really need to display the last column? As this is global information
> and not per probe info and user can always read from /debug/kprobes/enabled file.

Agreed.

Updated patch below

This patch provides a debugfs knob to turn kprobes on/off

o A new file /debug/kprobes/enabled indicates if kprobes is enabled or
not (default enabled)
o Echoing 0 to this file will disarm all installed probes
o Any new probe registration when disabled will register the probe but
not arm it. A message will be printed out in such a case.
o When a value 1 is echoed to the file, all probes (including ones
registered in the intervening period) will be enabled
o Unregistration will happen irrespective of whether probes are globally
enabled or not.
o Update Documentation/kprobes.txt to reflect these changes. While there
also update the doc to make it current.

Patch against 2.6.21-rc5-mm4

We are also looking at providing sysrq key support to tie to the
disabling feature provided by this patch.

Signed-off-by: Ananth N Mavinakayanahalli <ananth@xxxxxxxxxx>
Signed-off-by: Srinivasa DS <srinivasa@xxxxxxxxxx>

---
Documentation/kprobes.txt | 34 ++++++++-
arch/i386/kernel/kprobes.c | 5 +
arch/ia64/kernel/kprobes.c | 9 ++
arch/powerpc/kernel/kprobes.c | 8 ++
arch/x86_64/kernel/kprobes.c | 8 ++
include/linux/kprobes.h | 5 +
kernel/kprobes.c | 154 ++++++++++++++++++++++++++++++++++++++++--
7 files changed, 214 insertions(+), 9 deletions(-)

Index: linux-2.6.21-rc5/kernel/kprobes.c
===================================================================
--- linux-2.6.21-rc5.orig/kernel/kprobes.c
+++ linux-2.6.21-rc5/kernel/kprobes.c
@@ -46,6 +46,7 @@
#include <asm-generic/sections.h>
#include <asm/cacheflush.h>
#include <asm/errno.h>
+#include <asm/uaccess.h>

#define KPROBE_HASH_BITS 6
#define KPROBE_TABLE_SIZE (1 << KPROBE_HASH_BITS)
@@ -64,6 +65,9 @@ static struct hlist_head kprobe_table[KP
static struct hlist_head kretprobe_inst_table[KPROBE_TABLE_SIZE];
static atomic_t kprobe_count;

+/* NOTE: change this value only with kprobe_mutex held */
+static bool kprobe_enabled;
+
DEFINE_MUTEX(kprobe_mutex); /* Protects kprobe_table */
DEFINE_SPINLOCK(kretprobe_lock); /* Protects kretprobe_inst_table */
static DEFINE_PER_CPU(struct kprobe *, kprobe_instance) = NULL;
@@ -564,11 +568,15 @@ static int __kprobes __register_kprobe(s
hlist_add_head_rcu(&p->hlist,
&kprobe_table[hash_ptr(p->addr, KPROBE_HASH_BITS)]);

- if (atomic_add_return(1, &kprobe_count) == \
+ if (kprobe_enabled == 1) {
+ if (atomic_add_return(1, &kprobe_count) == \
(ARCH_INACTIVE_KPROBE_COUNT + 1))
- register_page_fault_notifier(&kprobe_page_fault_nb);
+ register_page_fault_notifier(&kprobe_page_fault_nb);

- arch_arm_kprobe(p);
+ arch_arm_kprobe(p);
+ } else
+ printk("Kprobes are globally disabled. This kprobe [@ %p] "
+ "will be enabled with all other probes\n", p->addr);

out:
mutex_unlock(&kprobe_mutex);
@@ -607,8 +615,13 @@ valid_p:
if (old_p == p ||
(old_p->pre_handler == aggr_pre_handler &&
p->list.next == &old_p->list && p->list.prev == &old_p->list)) {
- /* Only probe on the hash list */
- arch_disarm_kprobe(p);
+ /*
+ * Only probe on the hash list. Disarm only if kprobes are
+ * enabled - otherwise, the breakpoint would already have
+ * been removed. We save on flushing icache.
+ */
+ if (kprobe_enabled == 1)
+ arch_disarm_kprobe(p);
hlist_del_rcu(&old_p->hlist);
cleanup_p = 1;
} else {
@@ -797,6 +810,9 @@ static int __init init_kprobes(void)
}
atomic_set(&kprobe_count, 0);

+ /* By default, kprobes are enabled */
+ kprobe_enabled = 1;
+
err = arch_init_kprobes();
if (!err)
err = register_die_notifier(&kprobe_exceptions_nb);
@@ -806,7 +822,7 @@ static int __init init_kprobes(void)

#ifdef CONFIG_DEBUG_FS
static void __kprobes report_probe(struct seq_file *pi, struct kprobe *p,
- const char *sym, int offset,char *modname)
+ const char *sym, int offset,char *modname)
{
char *kprobe_type;

@@ -885,9 +901,128 @@ static struct file_operations debugfs_kp
.release = seq_release,
};

+static void __kprobes enable_all_kprobes(void)
+{
+ struct hlist_head *head;
+ struct hlist_node *node;
+ struct kprobe *p;
+ unsigned int i;
+
+ mutex_lock(&kprobe_mutex);
+
+ /* If kprobes are already enabled, just return */
+ if (kprobe_enabled == 1)
+ goto already_enabled;
+
+ /*
+ * Re-register the page fault notifier only if there are any
+ * active probes at the time of enabling kprobes globally
+ */
+ if (atomic_read(&kprobe_count) > ARCH_INACTIVE_KPROBE_COUNT)
+ register_page_fault_notifier(&kprobe_page_fault_nb);
+
+ for (i = 0; i < KPROBE_TABLE_SIZE; i++) {
+ head = &kprobe_table[i];
+ hlist_for_each_entry_rcu(p, node, head, hlist)
+ arch_arm_kprobe(p);
+ }
+
+ kprobe_enabled = 1;
+
+already_enabled:
+ mutex_unlock(&kprobe_mutex);
+ return;
+}
+
+static void __kprobes disable_all_kprobes(void)
+{
+ struct hlist_head *head;
+ struct hlist_node *node;
+ struct kprobe *p;
+ unsigned int i;
+
+ mutex_lock(&kprobe_mutex);
+
+ /* If kprobes are already disabled, just return */
+ if (kprobe_enabled == 0)
+ goto already_disabled;
+
+ kprobe_enabled = 0;
+ for (i = 0; i < KPROBE_TABLE_SIZE; i++) {
+ head = &kprobe_table[i];
+ hlist_for_each_entry_rcu(p, node, head, hlist) {
+ if (!arch_trampoline_kprobe(p))
+ arch_disarm_kprobe(p);
+ }
+ }
+
+ mutex_unlock(&kprobe_mutex);
+ /* Allow all currently running kprobes to complete */
+ synchronize_sched();
+
+ mutex_lock(&kprobe_mutex);
+ /* Unconditionally unregister the page_fault notifier */
+ unregister_page_fault_notifier(&kprobe_page_fault_nb);
+
+already_disabled:
+ mutex_unlock(&kprobe_mutex);
+ return;
+}
+
+/*
+ * XXX: The debugfs bool file interface doesn't allow for callbacks
+ * when the bool state is switched. We can reuse that facility when
+ * available
+ */
+static ssize_t read_enabled_file_bool(struct file *file,
+ char __user *user_buf, size_t count, loff_t *ppos)
+{
+ char buf[3];
+
+ if (kprobe_enabled)
+ buf[0] = '1';
+ else
+ buf[0] = '0';
+ buf[1] = '\n';
+ buf[2] = 0x00;
+ return simple_read_from_buffer(user_buf, count, ppos, buf, 2);
+}
+
+static ssize_t write_enabled_file_bool(struct file *file,
+ const char __user *user_buf, size_t count, loff_t *ppos)
+{
+ char buf[32];
+ int buf_size;
+
+ buf_size = min(count, (sizeof(buf)-1));
+ if (copy_from_user(buf, user_buf, buf_size))
+ return -EFAULT;
+
+ switch (buf[0]) {
+ case 'y':
+ case 'Y':
+ case '1':
+ enable_all_kprobes();
+ break;
+ case 'n':
+ case 'N':
+ case '0':
+ disable_all_kprobes();
+ break;
+ }
+
+ return count;
+}
+
+static struct file_operations fops_kp = {
+ .read = read_enabled_file_bool,
+ .write = write_enabled_file_bool,
+};
+
static int __kprobes debugfs_kprobe_init(void)
{
struct dentry *dir, *file;
+ unsigned int value = 1;

dir = debugfs_create_dir("kprobes", NULL);
if (!dir)
@@ -900,6 +1035,13 @@ static int __kprobes debugfs_kprobe_init
return -ENOMEM;
}

+ file = debugfs_create_file("enabled", 0600, dir,
+ &value, &fops_kp);
+ if (!file) {
+ debugfs_remove(dir);
+ return -ENOMEM;
+ }
+
return 0;
}

Index: linux-2.6.21-rc5/Documentation/kprobes.txt
===================================================================
--- linux-2.6.21-rc5.orig/Documentation/kprobes.txt
+++ linux-2.6.21-rc5/Documentation/kprobes.txt
@@ -14,6 +14,7 @@ CONTENTS
8. Kprobes Example
9. Jprobes Example
10. Kretprobes Example
+Appendix A: The kprobes debugfs interface

1. Concepts: Kprobes, Jprobes, Return Probes

@@ -349,9 +350,12 @@ for instrumentation and error reporting.

If the number of times a function is called does not match the number
of times it returns, registering a return probe on that function may
-produce undesirable results. We have the do_exit() case covered.
-do_execve() and do_fork() are not an issue. We're unaware of other
-specific cases where this could be a problem.
+produce undesirable results. In such a case, a line:
+kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
+gets printed. With this information, one will be able to correlate the
+exact instance of the kretprobe that caused the problem. We have the
+do_exit() case covered. do_execve() and do_fork() are not an issue.
+We're unaware of other specific cases where this could be a problem.

If, upon entry to or exit from a function, the CPU is running on
a stack other than that of the current task, registering a return
@@ -614,3 +618,27 @@ http://www-106.ibm.com/developerworks/li
http://www.redhat.com/magazine/005mar05/features/kprobes/
http://www-users.cs.umn.edu/~boutcher/kprobes/
http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115)
+
+
+Appendix A: The kprobes debugfs interface
+
+With recent kernels (> 2.6.20) the list of registered kprobes is visible
+under the /debug/kprobes/ directory (assuming debugfs is mounted at /debug).
+
+/debug/kprobes/list: Lists all registered probes on the system
+
+c015d71a k vfs_read+0x0
+c011a316 j do_fork+0x0
+c03dedc5 r tcp_v4_rcv+0x0
+
+The first column provides the kernel address where the probe is inserted.
+The second column identifies the type of probe (k - kprobe, r - kretprobe
+and j - jprobe), while the third column specifies the symbol+offset of
+the probe. If the probed function belongs to a module, the module name
+is also specified.
+
+/debug/kprobes/enabled: Turn kprobes ON/OFF
+
+Provides a knob to globally turn registered kprobes ON or OFF. By default,
+all kprobes are enabled. By echoing "0" to this file, all registered probes
+will be disarmed, till such time a "1" is echoed to this file.
Index: linux-2.6.21-rc5/arch/i386/kernel/kprobes.c
===================================================================
--- linux-2.6.21-rc5.orig/arch/i386/kernel/kprobes.c
+++ linux-2.6.21-rc5/arch/i386/kernel/kprobes.c
@@ -748,6 +748,11 @@ int __kprobes longjmp_break_handler(stru
return 0;
}

+int __kprobes arch_trampoline_kprobe(struct kprobe *p)
+{
+ return 0;
+}
+
int __init arch_init_kprobes(void)
{
return 0;
Index: linux-2.6.21-rc5/arch/ia64/kernel/kprobes.c
===================================================================
--- linux-2.6.21-rc5.orig/arch/ia64/kernel/kprobes.c
+++ linux-2.6.21-rc5/arch/ia64/kernel/kprobes.c
@@ -1016,3 +1016,12 @@ int __init arch_init_kprobes(void)
(kprobe_opcode_t *)((struct fnptr *)kretprobe_trampoline)->ip;
return register_kprobe(&trampoline_p);
}
+
+int __kprobes arch_trampoline_kprobe(struct kprobe *p)
+{
+ if (p->addr ==
+ (kprobe_opcode_t *)((struct fnptr *)kretprobe_trampoline)->ip)
+ return 1;
+
+ return 0;
+}
Index: linux-2.6.21-rc5/arch/powerpc/kernel/kprobes.c
===================================================================
--- linux-2.6.21-rc5.orig/arch/powerpc/kernel/kprobes.c
+++ linux-2.6.21-rc5/arch/powerpc/kernel/kprobes.c
@@ -521,3 +521,11 @@ int __init arch_init_kprobes(void)
{
return register_kprobe(&trampoline_p);
}
+
+int __kprobes arch_trampoline_kprobe(struct kprobe *p)
+{
+ if (p->addr == (kprobe_opcode_t *)&kretprobe_trampoline)
+ return 1;
+
+ return 0;
+}
Index: linux-2.6.21-rc5/arch/x86_64/kernel/kprobes.c
===================================================================
--- linux-2.6.21-rc5.orig/arch/x86_64/kernel/kprobes.c
+++ linux-2.6.21-rc5/arch/x86_64/kernel/kprobes.c
@@ -748,3 +748,11 @@ int __init arch_init_kprobes(void)
{
return register_kprobe(&trampoline_p);
}
+
+int __kprobes arch_trampoline_kprobe(struct kprobe *p)
+{
+ if (p->addr == (kprobe_opcode_t *)&kretprobe_trampoline)
+ return 1;
+
+ return 0;
+}
Index: linux-2.6.21-rc5/include/linux/kprobes.h
===================================================================
--- linux-2.6.21-rc5.orig/include/linux/kprobes.h
+++ linux-2.6.21-rc5/include/linux/kprobes.h
@@ -125,11 +125,16 @@ DECLARE_PER_CPU(struct kprobe_ctlblk, kp
#ifdef ARCH_SUPPORTS_KRETPROBES
extern void arch_prepare_kretprobe(struct kretprobe_instance *ri,
struct pt_regs *regs);
+extern int arch_trampoline_kprobe(struct kprobe *p);
#else /* ARCH_SUPPORTS_KRETPROBES */
static inline void arch_prepare_kretprobe(struct kretprobe *rp,
struct pt_regs *regs)
{
}
+static inline int arch_trampoline_kprobe(struct kprobe *p)
+{
+ return 0;
+}
#endif /* ARCH_SUPPORTS_KRETPROBES */
/*
* Function-return probe -
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/