uprobes/core: Handle breakpoint and singlestep exceptions

Uprobes uses exception notifiers to get to know if a thread hit
a breakpoint or a singlestep exception.

When a thread hits a uprobe or is singlestepping post a uprobe
hit, the uprobe exception notifier sets its TIF_UPROBE bit,
which will then be checked on its return to userspace path
(do_notify_resume() ->uprobe_notify_resume()), where the
consumers handlers are run (in task context) based on the
defined filters.

Uprobe hits are thread specific and hence we need to maintain
information about if a task hit a uprobe, what uprobe was hit,
the slot where the original instruction was copied for xol so
that it can be singlestepped with appropriate fixups.

In some cases, special care is needed for instructions that are
executed out of line (xol). These are architecture specific
artefacts, such as handling RIP relative instructions on x86_64.

Since the instruction at which the uprobe was inserted is
executed out of line, architecture specific fixups are added so
that the thread continues normal execution in the presence of a
uprobe.

Postpone the signals until we execute the probed insn.
post_xol() path does a recalc_sigpending() before return to
user-mode, this ensures the signal can't be lost.

Uprobes relies on DIE_DEBUG notification to notify if a
singlestep is complete.

Adds x86 specific uprobe exception notifiers and appropriate
hooks needed to determine a uprobe hit and subsequent post
processing.

Add requisite x86 fixups for xol for uprobes. Specific cases
needing fixups include relative jumps (x86_64), calls, etc.

Where possible, we check and skip singlestepping the
breakpointed instructions. For now we skip single byte as well
as few multibyte nop instructions. However this can be extended
to other instructions too.

Credits to Oleg Nesterov for suggestions/patches related to
signal, breakpoint, singlestep handling code.

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Cc: Jim Keniston <jkenisto@linux.vnet.ibm.com>
Cc: Linux-mm <linux-mm@kvack.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Arnaldo Carvalho de Melo <acme@infradead.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20120313180011.29771.89027.sendpatchset@srdronam.in.ibm.com
[ Performed various cleanliness edits ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 319 insertions, 4 deletions

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index e56e56a..b807d15 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -30,9 +30,12 @@
 #include <linux/rmap.h>		/* anon_vma_prepare */
 #include <linux/mmu_notifier.h>	/* set_pte_at_notify */
 #include <linux/swap.h>		/* try_to_free_swap */
+#include <linux/ptrace.h>	/* user_enable_single_step */
+#include <linux/kdebug.h>	/* notifier mechanism */
 
 #include <linux/uprobes.h>
 
+static struct srcu_struct uprobes_srcu;
 static struct rb_root uprobes_tree = RB_ROOT;
 
 static DEFINE_SPINLOCK(uprobes_treelock);	/* serialize rbtree access */
@@ -486,6 +489,9 @@ static struct uprobe *insert_uprobe(struct uprobe *uprobe)
 	u = __insert_uprobe(uprobe);
 	spin_unlock_irqrestore(&uprobes_treelock, flags);
 
+	/* For now assume that the instruction need not be single-stepped */
+	uprobe->flags |= UPROBE_SKIP_SSTEP;
+
 	return u;
 }
 
@@ -523,6 +529,21 @@ static struct uprobe *alloc_uprobe(struct inode *inode, loff_t offset)
 	return uprobe;
 }
 
+static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
+{
+	struct uprobe_consumer *uc;
+
+	if (!(uprobe->flags & UPROBE_RUN_HANDLER))
+		return;
+
+	down_read(&uprobe->consumer_rwsem);
+	for (uc = uprobe->consumers; uc; uc = uc->next) {
+		if (!uc->filter || uc->filter(uc, current))
+			uc->handler(uc, regs);
+	}
+	up_read(&uprobe->consumer_rwsem);
+}
+
 /* Returns the previous consumer */
 static struct uprobe_consumer *
 consumer_add(struct uprobe *uprobe, struct uprobe_consumer *uc)
@@ -645,7 +666,7 @@ install_breakpoint(struct uprobe *uprobe, struct mm_struct *mm,
 		if (is_swbp_insn((uprobe_opcode_t *)uprobe->arch.insn))
 			return -EEXIST;
 
-		ret = arch_uprobes_analyze_insn(&uprobe->arch, mm);
+		ret = arch_uprobe_analyze_insn(&uprobe->arch, mm);
 		if (ret)
 			return ret;
 
@@ -662,10 +683,21 @@ remove_breakpoint(struct uprobe *uprobe, struct mm_struct *mm, loff_t vaddr)
 	set_orig_insn(&uprobe->arch, mm, (unsigned long)vaddr, true);
 }
 
+/*
+ * There could be threads that have hit the breakpoint and are entering the
+ * notifier code and trying to acquire the uprobes_treelock. The thread
+ * calling delete_uprobe() that is removing the uprobe from the rb_tree can
+ * race with these threads and might acquire the uprobes_treelock compared
+ * to some of the breakpoint hit threads. In such a case, the breakpoint
+ * hit threads will not find the uprobe. The current unregistering thread
+ * waits till all other threads have hit a breakpoint, to acquire the
+ * uprobes_treelock before the uprobe is removed from the rbtree.
+ */
 static void delete_uprobe(struct uprobe *uprobe)
 {
 	unsigned long flags;
 
+	synchronize_srcu(&uprobes_srcu);
 	spin_lock_irqsave(&uprobes_treelock, flags);
 	rb_erase(&uprobe->rb_node, &uprobes_tree);
 	spin_unlock_irqrestore(&uprobes_treelock, flags);
@@ -1010,6 +1042,288 @@ int uprobe_mmap(struct vm_area_struct *vma)
 	return ret;
 }
 
+/**
+ * uprobe_get_swbp_addr - compute address of swbp given post-swbp regs
+ * @regs: Reflects the saved state of the task after it has hit a breakpoint
+ * instruction.
+ * Return the address of the breakpoint instruction.
+ */
+unsigned long __weak uprobe_get_swbp_addr(struct pt_regs *regs)
+{
+	return instruction_pointer(regs) - UPROBE_SWBP_INSN_SIZE;
+}
+
+/*
+ * Called with no locks held.
+ * Called in context of a exiting or a exec-ing thread.
+ */
+void uprobe_free_utask(struct task_struct *t)
+{
+	struct uprobe_task *utask = t->utask;
+
+	if (t->uprobe_srcu_id != -1)
+		srcu_read_unlock_raw(&uprobes_srcu, t->uprobe_srcu_id);
+
+	if (!utask)
+		return;
+
+	if (utask->active_uprobe)
+		put_uprobe(utask->active_uprobe);
+
+	kfree(utask);
+	t->utask = NULL;
+}
+
+/*
+ * Called in context of a new clone/fork from copy_process.
+ */
+void uprobe_copy_process(struct task_struct *t)
+{
+	t->utask = NULL;
+	t->uprobe_srcu_id = -1;
+}
+
+/*
+ * Allocate a uprobe_task object for the task.
+ * Called when the thread hits a breakpoint for the first time.
+ *
+ * Returns:
+ * - pointer to new uprobe_task on success
+ * - NULL otherwise
+ */
+static struct uprobe_task *add_utask(void)
+{
+	struct uprobe_task *utask;
+
+	utask = kzalloc(sizeof *utask, GFP_KERNEL);
+	if (unlikely(!utask))
+		return NULL;
+
+	utask->active_uprobe = NULL;
+	current->utask = utask;
+	return utask;
+}
+
+/* Prepare to single-step probed instruction out of line. */
+static int
+pre_ssout(struct uprobe *uprobe, struct pt_regs *regs, unsigned long vaddr)
+{
+	return -EFAULT;
+}
+
+/*
+ * If we are singlestepping, then ensure this thread is not connected to
+ * non-fatal signals until completion of singlestep.  When xol insn itself
+ * triggers the signal,  restart the original insn even if the task is
+ * already SIGKILL'ed (since coredump should report the correct ip).  This
+ * is even more important if the task has a handler for SIGSEGV/etc, The
+ * _same_ instruction should be repeated again after return from the signal
+ * handler, and SSTEP can never finish in this case.
+ */
+bool uprobe_deny_signal(void)
+{
+	struct task_struct *t = current;
+	struct uprobe_task *utask = t->utask;
+
+	if (likely(!utask || !utask->active_uprobe))
+		return false;
+
+	WARN_ON_ONCE(utask->state != UTASK_SSTEP);
+
+	if (signal_pending(t)) {
+		spin_lock_irq(&t->sighand->siglock);
+		clear_tsk_thread_flag(t, TIF_SIGPENDING);
+		spin_unlock_irq(&t->sighand->siglock);
+
+		if (__fatal_signal_pending(t) || arch_uprobe_xol_was_trapped(t)) {
+			utask->state = UTASK_SSTEP_TRAPPED;
+			set_tsk_thread_flag(t, TIF_UPROBE);
+			set_tsk_thread_flag(t, TIF_NOTIFY_RESUME);
+		}
+	}
+
+	return true;
+}
+
+/*
+ * Avoid singlestepping the original instruction if the original instruction
+ * is a NOP or can be emulated.
+ */
+static bool can_skip_sstep(struct uprobe *uprobe, struct pt_regs *regs)
+{
+	if (arch_uprobe_skip_sstep(&uprobe->arch, regs))
+		return true;
+
+	uprobe->flags &= ~UPROBE_SKIP_SSTEP;
+	return false;
+}
+
+/*
+ * Run handler and ask thread to singlestep.
+ * Ensure all non-fatal signals cannot interrupt thread while it singlesteps.
+ */
+static void handle_swbp(struct pt_regs *regs)
+{
+	struct vm_area_struct *vma;
+	struct uprobe_task *utask;
+	struct uprobe *uprobe;
+	struct mm_struct *mm;
+	unsigned long bp_vaddr;
+
+	uprobe = NULL;
+	bp_vaddr = uprobe_get_swbp_addr(regs);
+	mm = current->mm;
+	down_read(&mm->mmap_sem);
+	vma = find_vma(mm, bp_vaddr);
+
+	if (vma && vma->vm_start <= bp_vaddr && valid_vma(vma, false)) {
+		struct inode *inode;
+		loff_t offset;
+
+		inode = vma->vm_file->f_mapping->host;
+		offset = bp_vaddr - vma->vm_start;
+		offset += (vma->vm_pgoff << PAGE_SHIFT);
+		uprobe = find_uprobe(inode, offset);
+	}
+
+	srcu_read_unlock_raw(&uprobes_srcu, current->uprobe_srcu_id);
+	current->uprobe_srcu_id = -1;
+	up_read(&mm->mmap_sem);
+
+	if (!uprobe) {
+		/* No matching uprobe; signal SIGTRAP. */
+		send_sig(SIGTRAP, current, 0);
+		return;
+	}
+
+	utask = current->utask;
+	if (!utask) {
+		utask = add_utask();
+		/* Cannot allocate; re-execute the instruction. */
+		if (!utask)
+			goto cleanup_ret;
+	}
+	utask->active_uprobe = uprobe;
+	handler_chain(uprobe, regs);
+	if (uprobe->flags & UPROBE_SKIP_SSTEP && can_skip_sstep(uprobe, regs))
+		goto cleanup_ret;
+
+	utask->state = UTASK_SSTEP;
+	if (!pre_ssout(uprobe, regs, bp_vaddr)) {
+		user_enable_single_step(current);
+		return;
+	}
+
+cleanup_ret:
+	if (utask) {
+		utask->active_uprobe = NULL;
+		utask->state = UTASK_RUNNING;
+	}
+	if (uprobe) {
+		if (!(uprobe->flags & UPROBE_SKIP_SSTEP))
+
+			/*
+			 * cannot singlestep; cannot skip instruction;
+			 * re-execute the instruction.
+			 */
+			instruction_pointer_set(regs, bp_vaddr);
+
+		put_uprobe(uprobe);
+	}
+}
+
+/*
+ * Perform required fix-ups and disable singlestep.
+ * Allow pending signals to take effect.
+ */
+static void handle_singlestep(struct uprobe_task *utask, struct pt_regs *regs)
+{
+	struct uprobe *uprobe;
+
+	uprobe = utask->active_uprobe;
+	if (utask->state == UTASK_SSTEP_ACK)
+		arch_uprobe_post_xol(&uprobe->arch, regs);
+	else if (utask->state == UTASK_SSTEP_TRAPPED)
+		arch_uprobe_abort_xol(&uprobe->arch, regs);
+	else
+		WARN_ON_ONCE(1);
+
+	put_uprobe(uprobe);
+	utask->active_uprobe = NULL;
+	utask->state = UTASK_RUNNING;
+	user_disable_single_step(current);
+
+	spin_lock_irq(&current->sighand->siglock);
+	recalc_sigpending(); /* see uprobe_deny_signal() */
+	spin_unlock_irq(&current->sighand->siglock);
+}
+
+/*
+ * On breakpoint hit, breakpoint notifier sets the TIF_UPROBE flag.  (and on
+ * subsequent probe hits on the thread sets the state to UTASK_BP_HIT) and
+ * allows the thread to return from interrupt.
+ *
+ * On singlestep exception, singlestep notifier sets the TIF_UPROBE flag and
+ * also sets the state to UTASK_SSTEP_ACK and allows the thread to return from
+ * interrupt.
+ *
+ * While returning to userspace, thread notices the TIF_UPROBE flag and calls
+ * uprobe_notify_resume().
+ */
+void uprobe_notify_resume(struct pt_regs *regs)
+{
+	struct uprobe_task *utask;
+
+	utask = current->utask;
+	if (!utask || utask->state == UTASK_BP_HIT)
+		handle_swbp(regs);
+	else
+		handle_singlestep(utask, regs);
+}
+
+/*
+ * uprobe_pre_sstep_notifier gets called from interrupt context as part of
+ * notifier mechanism. Set TIF_UPROBE flag and indicate breakpoint hit.
+ */
+int uprobe_pre_sstep_notifier(struct pt_regs *regs)
+{
+	struct uprobe_task *utask;
+
+	if (!current->mm)
+		return 0;
+
+	utask = current->utask;
+	if (utask)
+		utask->state = UTASK_BP_HIT;
+
+	set_thread_flag(TIF_UPROBE);
+	current->uprobe_srcu_id = srcu_read_lock_raw(&uprobes_srcu);
+
+	return 1;
+}
+
+/*
+ * uprobe_post_sstep_notifier gets called in interrupt context as part of notifier
+ * mechanism. Set TIF_UPROBE flag and indicate completion of singlestep.
+ */
+int uprobe_post_sstep_notifier(struct pt_regs *regs)
+{
+	struct uprobe_task *utask = current->utask;
+
+	if (!current->mm || !utask || !utask->active_uprobe)
+		/* task is currently not uprobed */
+		return 0;
+
+	utask->state = UTASK_SSTEP_ACK;
+	set_thread_flag(TIF_UPROBE);
+	return 1;
+}
+
+static struct notifier_block uprobe_exception_nb = {
+	.notifier_call		= arch_uprobe_exception_notify,
+	.priority		= INT_MAX-1,	/* notified after kprobes, kgdb */
+};
+
 static int __init init_uprobes(void)
 {
 	int i;
@@ -1018,12 +1332,13 @@ static int __init init_uprobes(void)
 		mutex_init(&uprobes_mutex[i]);
 		mutex_init(&uprobes_mmap_mutex[i]);
 	}
-	return 0;
+	init_srcu_struct(&uprobes_srcu);
+
+	return register_die_notifier(&uprobe_exception_nb);
 }
+module_init(init_uprobes);
 
 static void __exit exit_uprobes(void)
 {
 }
-
-module_init(init_uprobes);
 module_exit(exit_uprobes);