hw-breakpoints: ftrace plugin for kernel symbol tracing using HW Breakpoint interfaces

This patch adds an ftrace plugin to detect and profile memory access over kernel
variables. It uses HW Breakpoint interfaces to 'watch memory addresses.
Signed-off-by: K.Prasad <prasad@linux.vnet.ibm.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>

kernel/trace/Kconfig

@@ -314,6 +314,27 @@ config POWER_TRACER
 	  power management decisions, specifically the C-state and P-state
 	  behavior.
 
+config KSYM_TRACER
+	bool "Trace read and write access on kernel memory locations"
+	depends on HAVE_HW_BREAKPOINT
+	select TRACING
+	help
+	  This tracer helps find read and write operations on any given kernel
+	  symbol i.e. /proc/kallsyms.
+
+config PROFILE_KSYM_TRACER
+	bool "Profile all kernel memory accesses on 'watched' variables"
+	depends on KSYM_TRACER
+	help
+	  This tracer profiles kernel accesses on variables watched through the
+	  ksym tracer ftrace plugin. Depending upon the hardware, all read
+	  and write operations on kernel variables can be monitored for
+	  accesses.
+
+	  The results will be displayed in:
+	  /debugfs/tracing/profile_ksym
+
+	  Say N if unsure.
 
 config STACK_TRACER
 	bool "Trace max stack"

kernel/trace/Makefile

@@ -51,5 +51,6 @@ obj-$(CONFIG_EVENT_TRACING) += trace_export.o
 obj-$(CONFIG_FTRACE_SYSCALLS) += trace_syscalls.o
 obj-$(CONFIG_EVENT_PROFILE) += trace_event_profile.o
 obj-$(CONFIG_EVENT_TRACING) += trace_events_filter.o
+obj-$(CONFIG_KSYM_TRACER) += trace_ksym.o
 
 libftrace-y := ftrace.o

kernel/trace/trace.h

@@ -15,6 +15,10 @@
 #include <linux/trace_seq.h>
 #include <linux/ftrace_event.h>
 
+#ifdef CONFIG_KSYM_TRACER
+#include <asm/hw_breakpoint.h>
+#endif
+
 enum trace_type {
 	__TRACE_FIRST_TYPE = 0,
 
@@ -40,6 +44,7 @@ enum trace_type {
 	TRACE_KMEM_FREE,
 	TRACE_POWER,
 	TRACE_BLK,
+	TRACE_KSYM,
 
 	__TRACE_LAST_TYPE,
 };
@@ -207,6 +212,21 @@ struct syscall_trace_exit {
 	unsigned long		ret;
 };
 
+#define KSYM_SELFTEST_ENTRY "ksym_selftest_dummy"
+extern int process_new_ksym_entry(char *ksymname, int op, unsigned long addr);
+
+struct trace_ksym {
+	struct trace_entry	ent;
+	struct hw_breakpoint	*ksym_hbp;
+	unsigned long		ksym_addr;
+	unsigned long		ip;
+#ifdef CONFIG_PROFILE_KSYM_TRACER
+	unsigned long		counter;
+#endif
+	struct hlist_node	ksym_hlist;
+	char			ksym_name[KSYM_NAME_LEN];
+	char			p_name[TASK_COMM_LEN];
+};
 
 /*
  * trace_flag_type is an enumeration that holds different
@@ -323,6 +343,7 @@ extern void __ftrace_bad_type(void);
 			  TRACE_SYSCALL_ENTER);				\
 		IF_ASSIGN(var, ent, struct syscall_trace_exit,		\
 			  TRACE_SYSCALL_EXIT);				\
+		IF_ASSIGN(var, ent, struct trace_ksym, TRACE_KSYM);	\
 		__ftrace_bad_type();					\
 	} while (0)
 
@@ -540,6 +561,8 @@ extern int trace_selftest_startup_branch(struct tracer *trace,
 					 struct trace_array *tr);
 extern int trace_selftest_startup_hw_branches(struct tracer *trace,
 					      struct trace_array *tr);
+extern int trace_selftest_startup_ksym(struct tracer *trace,
+					 struct trace_array *tr);
 #endif /* CONFIG_FTRACE_STARTUP_TEST */
 
 extern void *head_page(struct trace_array_cpu *data);

kernel/trace/trace_ksym.c

+/*
+ * trace_ksym.c - Kernel Symbol Tracer
+ *
+ * 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, 2009
+ */
+
+#include <linux/kallsyms.h>
+#include <linux/uaccess.h>
+#include <linux/debugfs.h>
+#include <linux/ftrace.h>
+#include <linux/module.h>
+#include <linux/fs.h>
+
+#include "trace_output.h"
+#include "trace_stat.h"
+#include "trace.h"
+
+/* For now, let us restrict the no. of symbols traced simultaneously to number
+ * of available hardware breakpoint registers.
+ */
+#define KSYM_TRACER_MAX HBP_NUM
+
+#define KSYM_TRACER_OP_LEN 3 /* rw- */
+#define KSYM_FILTER_ENTRY_LEN (KSYM_NAME_LEN + KSYM_TRACER_OP_LEN + 1)
+
+static struct trace_array *ksym_trace_array;
+
+static unsigned int ksym_filter_entry_count;
+static unsigned int ksym_tracing_enabled;
+
+static HLIST_HEAD(ksym_filter_head);
+
+#ifdef CONFIG_PROFILE_KSYM_TRACER
+
+#define MAX_UL_INT 0xffffffff
+
+static DEFINE_MUTEX(ksym_tracer_mutex);
+
+void ksym_collect_stats(unsigned long hbp_hit_addr)
+{
+	struct hlist_node *node;
+	struct trace_ksym *entry;
+
+	rcu_read_lock();
+	hlist_for_each_entry_rcu(entry, node, &ksym_filter_head, ksym_hlist) {
+		if ((entry->ksym_addr == hbp_hit_addr) &&
+		    (entry->counter <= MAX_UL_INT)) {
+			entry->counter++;
+			break;
+		}
+	}
+	rcu_read_unlock();
+}
+#endif /* CONFIG_PROFILE_KSYM_TRACER */
+
+void ksym_hbp_handler(struct hw_breakpoint *hbp, struct pt_regs *regs)
+{
+	struct ring_buffer_event *event;
+	struct trace_array *tr;
+	struct trace_ksym *entry;
+	int pc;
+
+	if (!ksym_tracing_enabled)
+		return;
+
+	tr = ksym_trace_array;
+	pc = preempt_count();
+
+	event = trace_buffer_lock_reserve(tr, TRACE_KSYM,
+							sizeof(*entry), 0, pc);
+	if (!event)
+		return;
+
+	entry = ring_buffer_event_data(event);
+	strlcpy(entry->ksym_name, hbp->info.name, KSYM_SYMBOL_LEN);
+	entry->ksym_hbp = hbp;
+	entry->ip = instruction_pointer(regs);
+	strlcpy(entry->p_name, current->comm, TASK_COMM_LEN);
+#ifdef CONFIG_PROFILE_KSYM_TRACER
+	ksym_collect_stats(hbp->info.address);
+#endif /* CONFIG_PROFILE_KSYM_TRACER */
+
+	trace_buffer_unlock_commit(tr, event, 0, pc);
+}
+
+/* Valid access types are represented as
+ *
+ * rw- : Set Read/Write Access Breakpoint
+ * -w- : Set Write Access Breakpoint
+ * --- : Clear Breakpoints
+ * --x : Set Execution Break points (Not available yet)
+ *
+ */
+static int ksym_trace_get_access_type(char *access_str)
+{
+	int pos, access = 0;
+
+	for (pos = 0; pos < KSYM_TRACER_OP_LEN; pos++) {
+		switch (access_str[pos]) {
+		case 'r':
+			access += (pos == 0) ? 4 : -1;
+			break;
+		case 'w':
+			access += (pos == 1) ? 2 : -1;
+			break;
+		case '-':
+			break;
+		default:
+			return -EINVAL;
+		}
+	}
+
+	switch (access) {
+	case 6:
+		access = HW_BREAKPOINT_RW;
+		break;
+	case 2:
+		access = HW_BREAKPOINT_WRITE;
+		break;
+	case 0:
+		access = 0;
+	}
+
+	return access;
+}
+
+/*
+ * There can be several possible malformed requests and we attempt to capture
+ * all of them. We enumerate some of the rules
+ * 1. We will not allow kernel symbols with ':' since it is used as a delimiter.
+ *    i.e. multiple ':' symbols disallowed. Possible uses are of the form
+ *    <module>:<ksym_name>:<op>.
+ * 2. No delimiter symbol ':' in the input string
+ * 3. Spurious operator symbols or symbols not in their respective positions
+ * 4. <ksym_name>:--- i.e. clear breakpoint request when ksym_name not in file
+ * 5. Kernel symbol not a part of /proc/kallsyms
+ * 6. Duplicate requests
+ */
+static int parse_ksym_trace_str(char *input_string, char **ksymname,
+							unsigned long *addr)
+{
+	char *delimiter = ":";
+	int ret;
+
+	ret = -EINVAL;
+	*ksymname = strsep(&input_string, delimiter);
+	*addr = kallsyms_lookup_name(*ksymname);
+
+	/* Check for malformed request: (2), (1) and (5) */
+	if ((!input_string) ||
+		(strlen(input_string) != (KSYM_TRACER_OP_LEN + 1)) ||
+			(*addr == 0))
+		goto return_code;
+	ret = ksym_trace_get_access_type(input_string);
+
+return_code:
+	return ret;
+}
+
+int process_new_ksym_entry(char *ksymname, int op, unsigned long addr)
+{
+	struct trace_ksym *entry;
+	int ret;
+
+	if (ksym_filter_entry_count >= KSYM_TRACER_MAX) {
+		printk(KERN_ERR "ksym_tracer: Maximum limit:(%d) reached. No"
+		" new requests for tracing can be accepted now.\n",
+			KSYM_TRACER_MAX);
+		return -ENOSPC;
+	}
+
+	entry = kzalloc(sizeof(struct trace_ksym), GFP_KERNEL);
+	if (!entry)
+		return -ENOMEM;
+
+	entry->ksym_hbp = kzalloc(sizeof(struct hw_breakpoint), GFP_KERNEL);
+	if (!entry->ksym_hbp) {
+		kfree(entry);
+		return -ENOMEM;
+	}
+
+	entry->ksym_hbp->info.name = ksymname;
+	entry->ksym_hbp->info.type = op;
+	entry->ksym_addr = entry->ksym_hbp->info.address = addr;
+#ifdef CONFIG_X86
+	entry->ksym_hbp->info.len = HW_BREAKPOINT_LEN_4;
+#endif
+	entry->ksym_hbp->triggered = (void *)ksym_hbp_handler;
+
+	ret = register_kernel_hw_breakpoint(entry->ksym_hbp);
+	if (ret < 0) {
+		printk(KERN_INFO "ksym_tracer request failed. Try again"
+					" later!!\n");
+		kfree(entry->ksym_hbp);
+		kfree(entry);
+		return -EAGAIN;
+	}
+	hlist_add_head_rcu(&(entry->ksym_hlist), &ksym_filter_head);
+	ksym_filter_entry_count++;
+
+	return 0;
+}
+
+static ssize_t ksym_trace_filter_read(struct file *filp, char __user *ubuf,
+						size_t count, loff_t *ppos)
+{
+	struct trace_ksym *entry;
+	struct hlist_node *node;
+	char buf[KSYM_FILTER_ENTRY_LEN * KSYM_TRACER_MAX];
+	ssize_t ret, cnt = 0;
+
+	mutex_lock(&ksym_tracer_mutex);
+
+	hlist_for_each_entry(entry, node, &ksym_filter_head, ksym_hlist) {
+		cnt += snprintf(&buf[cnt], KSYM_FILTER_ENTRY_LEN - cnt, "%s:",
+				entry->ksym_hbp->info.name);
+		if (entry->ksym_hbp->info.type == HW_BREAKPOINT_WRITE)
+			cnt += snprintf(&buf[cnt], KSYM_FILTER_ENTRY_LEN - cnt,
+								"-w-\n");
+		else if (entry->ksym_hbp->info.type == HW_BREAKPOINT_RW)
+			cnt += snprintf(&buf[cnt], KSYM_FILTER_ENTRY_LEN - cnt,
+								"rw-\n");
+	}
+	ret = simple_read_from_buffer(ubuf, count, ppos, buf, strlen(buf));
+	mutex_unlock(&ksym_tracer_mutex);
+
+	return ret;
+}
+
+static ssize_t ksym_trace_filter_write(struct file *file,
+					const char __user *buffer,
+						size_t count, loff_t *ppos)
+{
+	struct trace_ksym *entry;
+	struct hlist_node *node;
+	char *input_string, *ksymname = NULL;
+	unsigned long ksym_addr = 0;
+	int ret, op, changed = 0;
+
+	/* Ignore echo "" > ksym_trace_filter */
+	if (count == 0)
+		return 0;
+
+	input_string = kzalloc(count, GFP_KERNEL);
+	if (!input_string)
+		return -ENOMEM;
+
+	if (copy_from_user(input_string, buffer, count)) {
+		kfree(input_string);
+		return -EFAULT;
+	}
+
+	ret = op = parse_ksym_trace_str(input_string, &ksymname, &ksym_addr);
+	if (ret < 0) {
+		kfree(input_string);
+		return ret;
+	}
+
+	mutex_lock(&ksym_tracer_mutex);
+
+	ret = -EINVAL;
+	hlist_for_each_entry(entry, node, &ksym_filter_head, ksym_hlist) {
+		if (entry->ksym_addr == ksym_addr) {
+			/* Check for malformed request: (6) */
+			if (entry->ksym_hbp->info.type != op)
+				changed = 1;
+			else
+				goto err_ret;
+			break;
+		}
+	}
+	if (changed) {
+		unregister_kernel_hw_breakpoint(entry->ksym_hbp);
+		entry->ksym_hbp->info.type = op;
+		if (op > 0) {
+			ret = register_kernel_hw_breakpoint(entry->ksym_hbp);
+			if (ret == 0) {
+				ret = count;
+				goto unlock_ret_path;
+			}
+		}
+		ksym_filter_entry_count--;
+		hlist_del_rcu(&(entry->ksym_hlist));
+		synchronize_rcu();
+		kfree(entry->ksym_hbp);
+		kfree(entry);
+		ret = count;
+		goto err_ret;
+	} else {
+		/* Check for malformed request: (4) */
+		if (op == 0)
+			goto err_ret;
+		ret = process_new_ksym_entry(ksymname, op, ksym_addr);
+		if (ret)
+			goto err_ret;
+	}
+	ret = count;
+	goto unlock_ret_path;
+
+err_ret:
+	kfree(input_string);
+
+unlock_ret_path:
+	mutex_unlock(&ksym_tracer_mutex);
+	return ret;
+}
+
+static const struct file_operations ksym_tracing_fops = {
+	.open		= tracing_open_generic,
+	.read		= ksym_trace_filter_read,
+	.write		= ksym_trace_filter_write,
+};
+
+static void ksym_trace_reset(struct trace_array *tr)
+{
+	struct trace_ksym *entry;
+	struct hlist_node *node, *node1;
+
+	ksym_tracing_enabled = 0;
+
+	mutex_lock(&ksym_tracer_mutex);
+	hlist_for_each_entry_safe(entry, node, node1, &ksym_filter_head,
+								ksym_hlist) {
+		unregister_kernel_hw_breakpoint(entry->ksym_hbp);
+		ksym_filter_entry_count--;
+		hlist_del_rcu(&(entry->ksym_hlist));
+		synchronize_rcu();
+		/* Free the 'input_string' only if reset
+		 * after startup self-test
+		 */
+#ifdef CONFIG_FTRACE_SELFTEST
+		if (strncmp(entry->ksym_hbp->info.name, KSYM_SELFTEST_ENTRY,
+					strlen(KSYM_SELFTEST_ENTRY)) != 0)
+#endif /* CONFIG_FTRACE_SELFTEST*/
+			kfree(entry->ksym_hbp->info.name);
+		kfree(entry->ksym_hbp);
+		kfree(entry);
+	}
+	mutex_unlock(&ksym_tracer_mutex);
+}
+
+static int ksym_trace_init(struct trace_array *tr)
+{
+	int cpu, ret = 0;
+
+	for_each_online_cpu(cpu)
+		tracing_reset(tr, cpu);
+	ksym_tracing_enabled = 1;
+	ksym_trace_array = tr;
+
+	return ret;
+}
+
+static void ksym_trace_print_header(struct seq_file *m)
+{
+
+	seq_puts(m,
+		 "#       TASK-PID      CPU#      Symbol         Type    "
+		 "Function         \n");
+	seq_puts(m,
+		 "#          |           |          |              |         "
+		 "|            \n");
+}
+
+static enum print_line_t ksym_trace_output(struct trace_iterator *iter)
+{
+	struct trace_entry *entry = iter->ent;
+	struct trace_seq *s = &iter->seq;
+	struct trace_ksym *field;
+	char str[KSYM_SYMBOL_LEN];
+	int ret;
+
+	if (entry->type != TRACE_KSYM)
+		return TRACE_TYPE_UNHANDLED;
+
+	trace_assign_type(field, entry);
+
+	ret = trace_seq_printf(s, "%-15s %-5d %-3d %-20s ", field->p_name,
+				entry->pid, iter->cpu, field->ksym_name);
+	if (!ret)
+		return TRACE_TYPE_PARTIAL_LINE;
+
+	switch (field->ksym_hbp->info.type) {
+	case HW_BREAKPOINT_WRITE:
+		ret = trace_seq_printf(s, " W  ");
+		break;
+	case HW_BREAKPOINT_RW:
+		ret = trace_seq_printf(s, " RW ");
+		break;
+	default:
+		return TRACE_TYPE_PARTIAL_LINE;
+	}
+
+	if (!ret)
+		return TRACE_TYPE_PARTIAL_LINE;
+
+	sprint_symbol(str, field->ip);
+	ret = trace_seq_printf(s, "%-20s\n", str);
+	if (!ret)
+		return TRACE_TYPE_PARTIAL_LINE;
+
+	return TRACE_TYPE_HANDLED;
+}
+
+struct tracer ksym_tracer __read_mostly =
+{
+	.name		= "ksym_tracer",
+	.init		= ksym_trace_init,
+	.reset		= ksym_trace_reset,
+#ifdef CONFIG_FTRACE_SELFTEST
+	.selftest	= trace_selftest_startup_ksym,
+#endif
+	.print_header   = ksym_trace_print_header,
+	.print_line	= ksym_trace_output
+};
+
+__init static int init_ksym_trace(void)
+{
+	struct dentry *d_tracer;
+	struct dentry *entry;
+
+	d_tracer = tracing_init_dentry();
+	ksym_filter_entry_count = 0;
+
+	entry = debugfs_create_file("ksym_trace_filter", 0644, d_tracer,
+				    NULL, &ksym_tracing_fops);
+	if (!entry)
+		pr_warning("Could not create debugfs "
+			   "'ksym_trace_filter' file\n");
+
+	return register_tracer(&ksym_tracer);
+}
+device_initcall(init_ksym_trace);
+
+
+#ifdef CONFIG_PROFILE_KSYM_TRACER
+static int ksym_tracer_stat_headers(struct seq_file *m)
+{
+	seq_printf(m, "   Access type    ");
+	seq_printf(m, "            Symbol                     Counter     \n");
+	return 0;
+}
+
+static int ksym_tracer_stat_show(struct seq_file *m, void *v)
+{
+	struct hlist_node *stat = v;
+	struct trace_ksym *entry;
+	int access_type = 0;
+	char fn_name[KSYM_NAME_LEN];
+
+	entry = hlist_entry(stat, struct trace_ksym, ksym_hlist);
+
+	if (entry->ksym_hbp)
+		access_type = entry->ksym_hbp->info.type;
+
+	switch (access_type) {
+	case HW_BREAKPOINT_WRITE:
+		seq_printf(m, "     W     ");
+		break;
+	case HW_BREAKPOINT_RW:
+		seq_printf(m, "     RW    ");
+		break;
+	default:
+		seq_printf(m, "     NA    ");
+	}
+
+	if (lookup_symbol_name(entry->ksym_addr, fn_name) >= 0)
+		seq_printf(m, "               %s                 ", fn_name);
+	else
+		seq_printf(m, "               <NA>                ");
+
+	seq_printf(m, "%15lu\n", entry->counter);
+	return 0;
+}
+
+static void *ksym_tracer_stat_start(struct tracer_stat *trace)
+{
+	return &(ksym_filter_head.first);
+}
+
+static void *
+ksym_tracer_stat_next(void *v, int idx)
+{
+	struct hlist_node *stat = v;
+
+	return stat->next;
+}
+
+static struct tracer_stat ksym_tracer_stats = {
+	.name = "ksym_tracer",
+	.stat_start = ksym_tracer_stat_start,
+	.stat_next = ksym_tracer_stat_next,
+	.stat_headers = ksym_tracer_stat_headers,
+	.stat_show = ksym_tracer_stat_show
+};
+
+__init static int ksym_tracer_stat_init(void)
+{
+	int ret;
+
+	ret = register_stat_tracer(&ksym_tracer_stats);
+	if (ret) {
+		printk(KERN_WARNING "Warning: could not register "
+				    "ksym tracer stats\n");
+		return 1;
+	}
+
+	return 0;
+}
+fs_initcall(ksym_tracer_stat_init);
+#endif /* CONFIG_PROFILE_KSYM_TRACER */

kernel/trace/trace_selftest.c

@@ -17,6 +17,7 @@ static inline int trace_valid_entry(struct trace_entry *entry)
 	case TRACE_GRAPH_ENT:
 	case TRACE_GRAPH_RET:
 	case TRACE_HW_BRANCHES:
+	case TRACE_KSYM:
 		return 1;
 	}
 	return 0;
@@ -807,3 +808,55 @@ trace_selftest_startup_hw_branches(struct tracer *trace,
 	return ret;
 }
 #endif /* CONFIG_HW_BRANCH_TRACER */
+
+#ifdef CONFIG_KSYM_TRACER
+static int ksym_selftest_dummy;
+
+int
+trace_selftest_startup_ksym(struct tracer *trace, struct trace_array *tr)
+{
+	unsigned long count;
+	int ret;
+
+	/* start the tracing */
+	ret = tracer_init(trace, tr);
+	if (ret) {
+		warn_failed_init_tracer(trace, ret);
+		return ret;
+	}
+
+	ksym_selftest_dummy = 0;
+	/* Register the read-write tracing request */
+	ret = process_new_ksym_entry(KSYM_SELFTEST_ENTRY, HW_BREAKPOINT_RW,
+					(unsigned long)(&ksym_selftest_dummy));
+
+	if (ret < 0) {
+		printk(KERN_CONT "ksym_trace read-write startup test failed\n");
+		goto ret_path;
+	}
+	/* Perform a read and a write operation over the dummy variable to
+	 * trigger the tracer
+	 */
+	if (ksym_selftest_dummy == 0)
+		ksym_selftest_dummy++;
+
+	/* stop the tracing. */
+	tracing_stop();
+	/* check the trace buffer */
+	ret = trace_test_buffer(tr, &count);
+	trace->reset(tr);
+	tracing_start();
+
+	/* read & write operations - one each is performed on the dummy variable
+	 * triggering two entries in the trace buffer
+	 */
+	if (!ret && count != 2) {
+		printk(KERN_CONT "Ksym tracer startup test failed");
+		ret = -1;
+	}
+
+ret_path:
+	return ret;
+}
+#endif /* CONFIG_KSYM_TRACER */
+