ARM kprobes: core code

This is a full implementation of Kprobes including Jprobes and
Kretprobes support.

This ARM implementation does not follow the usual kprobes double-
exception model. The traditional model is where the initial kprobes
breakpoint calls kprobe_handler(), which returns from exception to
execute the instruction in its original context, then immediately
re-enters after a second breakpoint (or single-stepping exception)
into post_kprobe_handler(), each time the probe is hit..  The ARM
implementation only executes one kprobes exception per hit, so no
post_kprobe_handler() phase. All side-effects from the kprobe'd
instruction are resolved before returning from the initial exception.
As a result, all instructions are _always_ effectively boosted
regardless of the type of instruction, and even regardless of whether
or not there is a post-handler for the probe.

Signed-off-by: Abhishek Sagar <sagar.abhishek@gmail.com>
Signed-off-by: Quentin Barnes <qbarnes@gmail.com>
Signed-off-by: Nicolas Pitre <nico@marvell.com>

arch/arm/kernel/Makefile

@@ -19,7 +19,7 @@ obj-$(CONFIG_ISA_DMA)		+= dma-isa.o
 obj-$(CONFIG_PCI)		+= bios32.o isa.o
 obj-$(CONFIG_SMP)		+= smp.o
 obj-$(CONFIG_KEXEC)		+= machine_kexec.o relocate_kernel.o
-obj-$(CONFIG_KPROBES)		+= kprobes-decode.o
+obj-$(CONFIG_KPROBES)		+= kprobes.o kprobes-decode.o
 obj-$(CONFIG_OABI_COMPAT)	+= sys_oabi-compat.o
 
 obj-$(CONFIG_CRUNCH)		+= crunch.o crunch-bits.o

arch/arm/kernel/kprobes.c

@@ -0,0 +1,453 @@
+/*
+ * arch/arm/kernel/kprobes.c
+ *
+ * Kprobes on ARM
+ *
+ * Abhishek Sagar <sagar.abhishek@gmail.com>
+ * Copyright (C) 2006, 2007 Motorola Inc.
+ *
+ * Nicolas Pitre <nico@marvell.com>
+ * Copyright (C) 2007 Marvell Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * 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.
+ */
+
+#include <linux/kernel.h>
+#include <linux/kprobes.h>
+#include <linux/module.h>
+#include <linux/stringify.h>
+#include <asm/traps.h>
+#include <asm/cacheflush.h>
+
+/*
+ * This undefined instruction must be unique and
+ * reserved solely for kprobes' use.
+ */
+#define KPROBE_BREAKPOINT_INSTRUCTION	0xe7f001f8
+
+#define MIN_STACK_SIZE(addr) 				\
+	min((unsigned long)MAX_STACK_SIZE,		\
+	    (unsigned long)current_thread_info() + THREAD_START_SP - (addr))
+
+#define flush_insns(addr, cnt) 				\
+	flush_icache_range((unsigned long)(addr),	\
+			   (unsigned long)(addr) +	\
+			   sizeof(kprobe_opcode_t) * (cnt))
+
+/* Used as a marker in ARM_pc to note when we're in a jprobe. */
+#define JPROBE_MAGIC_ADDR		0xffffffff
+
+DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
+DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
+
+
+int __kprobes arch_prepare_kprobe(struct kprobe *p)
+{
+	kprobe_opcode_t insn;
+	kprobe_opcode_t tmp_insn[MAX_INSN_SIZE];
+	unsigned long addr = (unsigned long)p->addr;
+	int is;
+
+	if (addr & 0x3)
+		return -EINVAL;
+
+	insn = *p->addr;
+	p->opcode = insn;
+	p->ainsn.insn = tmp_insn;
+
+	switch (arm_kprobe_decode_insn(insn, &p->ainsn)) {
+	case INSN_REJECTED:	/* not supported */
+		return -EINVAL;
+
+	case INSN_GOOD:		/* instruction uses slot */
+		p->ainsn.insn = get_insn_slot();
+		if (!p->ainsn.insn)
+			return -ENOMEM;
+		for (is = 0; is < MAX_INSN_SIZE; ++is)
+			p->ainsn.insn[is] = tmp_insn[is];
+		flush_insns(&p->ainsn.insn, MAX_INSN_SIZE);
+		break;
+
+	case INSN_GOOD_NO_SLOT:	/* instruction doesn't need insn slot */
+		p->ainsn.insn = NULL;
+		break;
+	}
+
+	return 0;
+}
+
+void __kprobes arch_arm_kprobe(struct kprobe *p)
+{
+	*p->addr = KPROBE_BREAKPOINT_INSTRUCTION;
+	flush_insns(p->addr, 1);
+}
+
+void __kprobes arch_disarm_kprobe(struct kprobe *p)
+{
+	*p->addr = p->opcode;
+	flush_insns(p->addr, 1);
+}
+
+void __kprobes arch_remove_kprobe(struct kprobe *p)
+{
+	if (p->ainsn.insn) {
+		mutex_lock(&kprobe_mutex);
+		free_insn_slot(p->ainsn.insn, 0);
+		mutex_unlock(&kprobe_mutex);
+		p->ainsn.insn = NULL;
+	}
+}
+
+static void __kprobes save_previous_kprobe(struct kprobe_ctlblk *kcb)
+{
+	kcb->prev_kprobe.kp = kprobe_running();
+	kcb->prev_kprobe.status = kcb->kprobe_status;
+}
+
+static void __kprobes restore_previous_kprobe(struct kprobe_ctlblk *kcb)
+{
+	__get_cpu_var(current_kprobe) = kcb->prev_kprobe.kp;
+	kcb->kprobe_status = kcb->prev_kprobe.status;
+}
+
+static void __kprobes set_current_kprobe(struct kprobe *p)
+{
+	__get_cpu_var(current_kprobe) = p;
+}
+
+static void __kprobes singlestep(struct kprobe *p, struct pt_regs *regs,
+				 struct kprobe_ctlblk *kcb)
+{
+	regs->ARM_pc += 4;
+	p->ainsn.insn_handler(p, regs);
+}
+
+/*
+ * Called with IRQs disabled. IRQs must remain disabled from that point
+ * all the way until processing this kprobe is complete.  The current
+ * kprobes implementation cannot process more than one nested level of
+ * kprobe, and that level is reserved for user kprobe handlers, so we can't
+ * risk encountering a new kprobe in an interrupt handler.
+ */
+void __kprobes kprobe_handler(struct pt_regs *regs)
+{
+	struct kprobe *p, *cur;
+	struct kprobe_ctlblk *kcb;
+	kprobe_opcode_t	*addr = (kprobe_opcode_t *)regs->ARM_pc;
+
+	kcb = get_kprobe_ctlblk();
+	cur = kprobe_running();
+	p = get_kprobe(addr);
+
+	if (p) {
+		if (cur) {
+			/* Kprobe is pending, so we're recursing. */
+			switch (kcb->kprobe_status) {
+			case KPROBE_HIT_ACTIVE:
+			case KPROBE_HIT_SSDONE:
+				/* A pre- or post-handler probe got us here. */
+				kprobes_inc_nmissed_count(p);
+				save_previous_kprobe(kcb);
+				set_current_kprobe(p);
+				kcb->kprobe_status = KPROBE_REENTER;
+				singlestep(p, regs, kcb);
+				restore_previous_kprobe(kcb);
+				break;
+			default:
+				/* impossible cases */
+				BUG();
+			}
+		} else {
+			set_current_kprobe(p);
+			kcb->kprobe_status = KPROBE_HIT_ACTIVE;
+
+			/*
+			 * If we have no pre-handler or it returned 0, we
+			 * continue with normal processing.  If we have a
+			 * pre-handler and it returned non-zero, it prepped
+			 * for calling the break_handler below on re-entry,
+			 * so get out doing nothing more here.
+			 */
+			if (!p->pre_handler || !p->pre_handler(p, regs)) {
+				kcb->kprobe_status = KPROBE_HIT_SS;
+				singlestep(p, regs, kcb);
+				if (p->post_handler) {
+					kcb->kprobe_status = KPROBE_HIT_SSDONE;
+					p->post_handler(p, regs, 0);
+				}
+				reset_current_kprobe();
+			}
+		}
+	} else if (cur) {
+		/* We probably hit a jprobe.  Call its break handler. */
+		if (cur->break_handler && cur->break_handler(cur, regs)) {
+			kcb->kprobe_status = KPROBE_HIT_SS;
+			singlestep(cur, regs, kcb);
+			if (cur->post_handler) {
+				kcb->kprobe_status = KPROBE_HIT_SSDONE;
+				cur->post_handler(cur, regs, 0);
+			}
+		}
+		reset_current_kprobe();
+	} else {
+		/*
+		 * The probe was removed and a race is in progress.
+		 * There is nothing we can do about it.  Let's restart
+		 * the instruction.  By the time we can restart, the
+		 * real instruction will be there.
+		 */
+	}
+}
+
+static int kprobe_trap_handler(struct pt_regs *regs, unsigned int instr)
+{
+	kprobe_handler(regs);
+	return 0;
+}
+
+int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr)
+{
+	struct kprobe *cur = kprobe_running();
+	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
+
+	switch (kcb->kprobe_status) {
+	case KPROBE_HIT_SS:
+	case KPROBE_REENTER:
+		/*
+		 * We are here because the instruction being single
+		 * stepped caused a page fault. We reset the current
+		 * kprobe and the PC to point back to the probe address
+		 * and allow the page fault handler to continue as a
+		 * normal page fault.
+		 */
+		regs->ARM_pc = (long)cur->addr;
+		if (kcb->kprobe_status == KPROBE_REENTER) {
+			restore_previous_kprobe(kcb);
+		} else {
+			reset_current_kprobe();
+		}
+		break;
+
+	case KPROBE_HIT_ACTIVE:
+	case KPROBE_HIT_SSDONE:
+		/*
+		 * We increment the nmissed count for accounting,
+		 * we can also use npre/npostfault count for accounting
+		 * these specific fault cases.
+		 */
+		kprobes_inc_nmissed_count(cur);
+
+		/*
+		 * We come here because instructions in the pre/post
+		 * handler caused the page_fault, this could happen
+		 * if handler tries to access user space by
+		 * copy_from_user(), get_user() etc. Let the
+		 * user-specified handler try to fix it.
+		 */
+		if (cur->fault_handler && cur->fault_handler(cur, regs, fsr))
+			return 1;
+		break;
+
+	default:
+		break;
+	}
+
+	return 0;
+}
+
+int __kprobes kprobe_exceptions_notify(struct notifier_block *self,
+				       unsigned long val, void *data)
+{
+	/*
+	 * notify_die() is currently never called on ARM,
+	 * so this callback is currently empty.
+	 */
+	return NOTIFY_DONE;
+}
+
+/*
+ * When a retprobed function returns, trampoline_handler() is called,
+ * calling the kretprobe's handler. We construct a struct pt_regs to
+ * give a view of registers r0-r11 to the user return-handler.  This is
+ * not a complete pt_regs structure, but that should be plenty sufficient
+ * for kretprobe handlers which should normally be interested in r0 only
+ * anyway.
+ */
+static void __attribute__((naked)) __kprobes kretprobe_trampoline(void)
+{
+	__asm__ __volatile__ (
+		"stmdb	sp!, {r0 - r11}		\n\t"
+		"mov	r0, sp			\n\t"
+		"bl	trampoline_handler	\n\t"
+		"mov	lr, r0			\n\t"
+		"ldmia	sp!, {r0 - r11}		\n\t"
+		"mov	pc, lr			\n\t"
+		: : : "memory");
+}
+
+/* Called from kretprobe_trampoline */
+static __used __kprobes void *trampoline_handler(struct pt_regs *regs)
+{
+	struct kretprobe_instance *ri = NULL;
+	struct hlist_head *head, empty_rp;
+	struct hlist_node *node, *tmp;
+	unsigned long flags, orig_ret_address = 0;
+	unsigned long trampoline_address = (unsigned long)&kretprobe_trampoline;
+
+	INIT_HLIST_HEAD(&empty_rp);
+	spin_lock_irqsave(&kretprobe_lock, flags);
+	head = kretprobe_inst_table_head(current);
+
+	/*
+	 * It is possible to have multiple instances associated with a given
+	 * task either because multiple functions in the call path have
+	 * a return probe installed on them, and/or more than one return
+	 * probe was registered for a target function.
+	 *
+	 * We can handle this because:
+	 *     - instances are always inserted at the head of the list
+	 *     - when multiple return probes are registered for the same
+	 *       function, the first instance's ret_addr will point to the
+	 *       real return address, and all the rest will point to
+	 *       kretprobe_trampoline
+	 */
+	hlist_for_each_entry_safe(ri, node, tmp, head, hlist) {
+		if (ri->task != current)
+			/* another task is sharing our hash bucket */
+			continue;
+
+		if (ri->rp && ri->rp->handler) {
+			__get_cpu_var(current_kprobe) = &ri->rp->kp;
+			get_kprobe_ctlblk()->kprobe_status = KPROBE_HIT_ACTIVE;
+			ri->rp->handler(ri, regs);
+			__get_cpu_var(current_kprobe) = NULL;
+		}
+
+		orig_ret_address = (unsigned long)ri->ret_addr;
+		recycle_rp_inst(ri, &empty_rp);
+
+		if (orig_ret_address != trampoline_address)
+			/*
+			 * This is the real return address. Any other
+			 * instances associated with this task are for
+			 * other calls deeper on the call stack
+			 */
+			break;
+	}
+
+	kretprobe_assert(ri, orig_ret_address, trampoline_address);
+	spin_unlock_irqrestore(&kretprobe_lock, flags);
+
+	hlist_for_each_entry_safe(ri, node, tmp, &empty_rp, hlist) {
+		hlist_del(&ri->hlist);
+		kfree(ri);
+	}
+
+	return (void *)orig_ret_address;
+}
+
+/* Called with kretprobe_lock held. */
+void __kprobes arch_prepare_kretprobe(struct kretprobe_instance *ri,
+				      struct pt_regs *regs)
+{
+	ri->ret_addr = (kprobe_opcode_t *)regs->ARM_lr;
+
+	/* Replace the return addr with trampoline addr. */
+	regs->ARM_lr = (unsigned long)&kretprobe_trampoline;
+}
+
+int __kprobes setjmp_pre_handler(struct kprobe *p, struct pt_regs *regs)
+{
+	struct jprobe *jp = container_of(p, struct jprobe, kp);
+	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
+	long sp_addr = regs->ARM_sp;
+
+	kcb->jprobe_saved_regs = *regs;
+	memcpy(kcb->jprobes_stack, (void *)sp_addr, MIN_STACK_SIZE(sp_addr));
+	regs->ARM_pc = (long)jp->entry;
+	regs->ARM_cpsr |= PSR_I_BIT;
+	preempt_disable();
+	return 1;
+}
+
+void __kprobes jprobe_return(void)
+{
+	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
+
+	__asm__ __volatile__ (
+		/*
+		 * Setup an empty pt_regs. Fill SP and PC fields as
+		 * they're needed by longjmp_break_handler.
+		 */
+		"sub    sp, %0, %1		\n\t"
+		"ldr    r0, ="__stringify(JPROBE_MAGIC_ADDR)"\n\t"
+		"str    %0, [sp, %2]		\n\t"
+		"str    r0, [sp, %3]		\n\t"
+		"mov    r0, sp			\n\t"
+		"bl     kprobe_handler		\n\t"
+
+		/*
+		 * Return to the context saved by setjmp_pre_handler
+		 * and restored by longjmp_break_handler.
+		 */
+		"ldr	r0, [sp, %4]		\n\t"
+		"msr	cpsr_cxsf, r0		\n\t"
+		"ldmia	sp, {r0 - pc}		\n\t"
+		:
+		: "r" (kcb->jprobe_saved_regs.ARM_sp),
+		  "I" (sizeof(struct pt_regs)),
+		  "J" (offsetof(struct pt_regs, ARM_sp)),
+		  "J" (offsetof(struct pt_regs, ARM_pc)),
+		  "J" (offsetof(struct pt_regs, ARM_cpsr))
+		: "memory", "cc");
+}
+
+int __kprobes longjmp_break_handler(struct kprobe *p, struct pt_regs *regs)
+{
+	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
+	long stack_addr = kcb->jprobe_saved_regs.ARM_sp;
+	long orig_sp = regs->ARM_sp;
+	struct jprobe *jp = container_of(p, struct jprobe, kp);
+
+	if (regs->ARM_pc == JPROBE_MAGIC_ADDR) {
+		if (orig_sp != stack_addr) {
+			struct pt_regs *saved_regs =
+				(struct pt_regs *)kcb->jprobe_saved_regs.ARM_sp;
+			printk("current sp %lx does not match saved sp %lx\n",
+			       orig_sp, stack_addr);
+			printk("Saved registers for jprobe %p\n", jp);
+			show_regs(saved_regs);
+			printk("Current registers\n");
+			show_regs(regs);
+			BUG();
+		}
+		*regs = kcb->jprobe_saved_regs;
+		memcpy((void *)stack_addr, kcb->jprobes_stack,
+		       MIN_STACK_SIZE(stack_addr));
+		preempt_enable_no_resched();
+		return 1;
+	}
+	return 0;
+}
+
+static struct undef_hook kprobes_break_hook = {
+	.instr_mask	= 0xffffffff,
+	.instr_val	= KPROBE_BREAKPOINT_INSTRUCTION,
+	.cpsr_mask	= MODE_MASK,
+	.cpsr_val	= SVC_MODE,
+	.fn		= kprobe_trap_handler,
+};
+
+int __init arch_init_kprobes()
+{
+	arm_kprobe_decode_init();
+	register_undef_hook(&kprobes_break_hook);
+	return 0;
+}

include/asm-arm/kprobes.h

@@ -18,6 +18,16 @@
 
 #include <linux/types.h>
 #include <linux/ptrace.h>
+#include <linux/percpu.h>
+
+#define ARCH_SUPPORTS_KRETPROBES
+#define __ARCH_WANT_KPROBES_INSN_SLOT
+#define MAX_INSN_SIZE			2
+#define MAX_STACK_SIZE			64	/* 32 would probably be OK */
+
+#define regs_return_value(regs)		((regs)->ARM_r0)
+#define flush_insn_slot(p)		do { } while (0)
+#define kretprobe_blacklist_size	0
 
 typedef u32 kprobe_opcode_t;
 
@@ -30,6 +40,25 @@ struct arch_specific_insn {
 	kprobe_insn_handler_t	*insn_handler;
 };
 
+struct prev_kprobe {
+	struct kprobe *kp;
+	unsigned int status;
+};
+
+/* per-cpu kprobe control block */
+struct kprobe_ctlblk {
+	unsigned int kprobe_status;
+	struct prev_kprobe prev_kprobe;
+	struct pt_regs jprobe_saved_regs;
+	char jprobes_stack[MAX_STACK_SIZE];
+};
+
+void arch_remove_kprobe(struct kprobe *);
+
+int kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr);
+int kprobe_exceptions_notify(struct notifier_block *self,
+			     unsigned long val, void *data);
+
 enum kprobe_insn {
 	INSN_REJECTED,
 	INSN_GOOD,