uprobes/core: Optimize probe hits with the help of a counter

Maintain a per-mm counter: number of uprobes that are inserted
on this process address space.

This counter can be used at probe hit time to determine if we
need a lookup in the uprobes rbtree. Everytime a probe gets
inserted successfully, the probe count is incremented and
everytime a probe gets removed, the probe count is decremented.

The new uprobe_munmap hook ensures the count is correct on a
unmap or remap of a region. We expect that once a
uprobe_munmap() is called, the vma goes away.  So
uprobe_unregister() finding a probe to unregister would either
mean unmap event hasnt occurred yet or a mmap event on the same
executable file occured after a unmap event.

Additionally, uprobe_mmap hook now also gets called:

 a. on every executable vma that is COWed at fork.
 b. a vma of interest is newly mapped; breakpoint insertion also
    happens at the required address.

On process creation, make sure the probes count in the child is
set correctly.

Special cases that are taken care include:

 a. mremap
 b. VM_DONTCOPY vmas on fork()
 c. insertion/removal races in the parent during fork().

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: Anton Arapov <anton@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20120330182646.10018.85805.sendpatchset@srdronam.in.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 111 insertions, 8 deletions

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index b395edb97f53..29e881b0137d 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -642,6 +642,29 @@ copy_insn(struct uprobe *uprobe, struct vm_area_struct *vma, unsigned long addr)
         return __copy_insn(mapping, vma, uprobe->arch.insn, bytes, uprobe->offset);
 }
 
+/*
+ * How mm->uprobes_state.count gets updated
+ * uprobe_mmap() increments the count if
+ *      - it successfully adds a breakpoint.
+ *      - it cannot add a breakpoint, but sees that there is a underlying
+ *        breakpoint (via a is_swbp_at_addr()).
+ *
+ * uprobe_munmap() decrements the count if
+ *      - it sees a underlying breakpoint, (via is_swbp_at_addr)
+ *        (Subsequent uprobe_unregister wouldnt find the breakpoint
+ *        unless a uprobe_mmap kicks in, since the old vma would be
+ *        dropped just after uprobe_munmap.)
+ *
+ * uprobe_register increments the count if:
+ *      - it successfully adds a breakpoint.
+ *
+ * uprobe_unregister decrements the count if:
+ *      - it sees a underlying breakpoint and removes successfully.
+ *        (via is_swbp_at_addr)
+ *        (Subsequent uprobe_munmap wouldnt find the breakpoint
+ *        since there is no underlying breakpoint after the
+ *        breakpoint removal.)
+ */
 static int
 install_breakpoint(struct uprobe *uprobe, struct mm_struct *mm,
                         struct vm_area_struct *vma, loff_t vaddr)
@@ -675,7 +698,19 @@ install_breakpoint(struct uprobe *uprobe, struct mm_struct *mm,
 
                 uprobe->flags |= UPROBE_COPY_INSN;
         }
+
+        /*
+         * Ideally, should be updating the probe count after the breakpoint
+         * has been successfully inserted. However a thread could hit the
+         * breakpoint we just inserted even before the probe count is
+         * incremented. If this is the first breakpoint placed, breakpoint
+         * notifier might ignore uprobes and pass the trap to the thread.
+         * Hence increment before and decrement on failure.
+         */
+        atomic_inc(&mm->uprobes_state.count);
         ret = set_swbp(&uprobe->arch, mm, addr);
+        if (ret)
+                atomic_dec(&mm->uprobes_state.count);
 
         return ret;
 }
@@ -683,7 +718,8 @@ install_breakpoint(struct uprobe *uprobe, struct mm_struct *mm,
 static void
 remove_breakpoint(struct uprobe *uprobe, struct mm_struct *mm, loff_t vaddr)
 {
-        set_orig_insn(&uprobe->arch, mm, (unsigned long)vaddr, true);
+        if (!set_orig_insn(&uprobe->arch, mm, (unsigned long)vaddr, true))
+                atomic_dec(&mm->uprobes_state.count);
 }
 
 /*
@@ -1009,7 +1045,7 @@ int uprobe_mmap(struct vm_area_struct *vma)
         struct list_head tmp_list;
         struct uprobe *uprobe, *u;
         struct inode *inode;
-        int ret;
+        int ret, count;
 
         if (!atomic_read(&uprobe_events) || !valid_vma(vma, true))
                 return 0;
@@ -1023,6 +1059,7 @@ int uprobe_mmap(struct vm_area_struct *vma)
         build_probe_list(inode, &tmp_list);
 
         ret = 0;
+        count = 0;
 
         list_for_each_entry_safe(uprobe, u, &tmp_list, pending_list) {
                 loff_t vaddr;
@@ -1030,21 +1067,85 @@ int uprobe_mmap(struct vm_area_struct *vma)
                 list_del(&uprobe->pending_list);
                 if (!ret) {
                         vaddr = vma_address(vma, uprobe->offset);
-                        if (vaddr >= vma->vm_start && vaddr < vma->vm_end) {
-                                ret = install_breakpoint(uprobe, vma->vm_mm, vma, vaddr);
-                                /* Ignore double add: */
-                                if (ret == -EEXIST)
-                                        ret = 0;
+
+                        if (vaddr < vma->vm_start || vaddr >= vma->vm_end) {
+                                put_uprobe(uprobe);
+                                continue;
                         }
+
+                        ret = install_breakpoint(uprobe, vma->vm_mm, vma, vaddr);
+
+                        /* Ignore double add: */
+                        if (ret == -EEXIST) {
+                                ret = 0;
+
+                                if (!is_swbp_at_addr(vma->vm_mm, vaddr))
+                                        continue;
+
+                                /*
+                                 * Unable to insert a breakpoint, but
+                                 * breakpoint lies underneath. Increment the
+                                 * probe count.
+                                 */
+                                atomic_inc(&vma->vm_mm->uprobes_state.count);
+                        }
+
+                        if (!ret)
+                                count++;
                 }
                 put_uprobe(uprobe);
         }
 
         mutex_unlock(uprobes_mmap_hash(inode));
 
+        if (ret)
+                atomic_sub(count, &vma->vm_mm->uprobes_state.count);
+
         return ret;
 }
 
+/*
+ * Called in context of a munmap of a vma.
+ */
+void uprobe_munmap(struct vm_area_struct *vma)
+{
+        struct list_head tmp_list;
+        struct uprobe *uprobe, *u;
+        struct inode *inode;
+
+        if (!atomic_read(&uprobe_events) || !valid_vma(vma, false))
+                return;
+
+        if (!atomic_read(&vma->vm_mm->uprobes_state.count))
+                return;
+
+        inode = vma->vm_file->f_mapping->host;
+        if (!inode)
+                return;
+
+        INIT_LIST_HEAD(&tmp_list);
+        mutex_lock(uprobes_mmap_hash(inode));
+        build_probe_list(inode, &tmp_list);
+
+        list_for_each_entry_safe(uprobe, u, &tmp_list, pending_list) {
+                loff_t vaddr;
+
+                list_del(&uprobe->pending_list);
+                vaddr = vma_address(vma, uprobe->offset);
+
+                if (vaddr >= vma->vm_start && vaddr < vma->vm_end) {
+                        /*
+                         * An unregister could have removed the probe before
+                         * unmap. So check before we decrement the count.
+                         */
+                        if (is_swbp_at_addr(vma->vm_mm, vaddr) == 1)
+                                atomic_dec(&vma->vm_mm->uprobes_state.count);
+                }
+                put_uprobe(uprobe);
+        }
+        mutex_unlock(uprobes_mmap_hash(inode));
+}
+
 /* Slot allocation for XOL */
 static int xol_add_vma(struct xol_area *area)
 {
@@ -1150,6 +1251,7 @@ void uprobe_clear_state(struct mm_struct *mm)
 void uprobe_reset_state(struct mm_struct *mm)
 {
         mm->uprobes_state.xol_area = NULL;
+        atomic_set(&mm->uprobes_state.count, 0);
 }
 
 /*
@@ -1504,7 +1606,8 @@ int uprobe_pre_sstep_notifier(struct pt_regs *regs)
 {
         struct uprobe_task *utask;
 
-        if (!current->mm)
+        if (!current->mm || !atomic_read(&current->mm->uprobes_state.count))
+                /* task is currently not uprobed */
                 return 0;
 
         utask = current->utask;