uprobes: Kill uprobes_state->count

uprobes_state->count is only needed to avoid the slow path in
uprobe_pre_sstep_notifier(). It is also checked in uprobe_munmap()
but ironically its only goal to decrement this counter. However,
it is very broken. Just some examples:

- uprobe_mmap() can race with uprobe_unregister() and wrongly
  increment the counter if it hits the non-uprobe "int3". Note
  that install_breakpoint() checks ->consumers first and returns
  -EEXIST if it is NULL.

  "atomic_sub() if error" in uprobe_mmap() looks obviously wrong
  too.

- uprobe_munmap() can race with uprobe_register() and wrongly
  decrement the counter by the same reason.

- Suppose an appication tries to increase the mmapped area via
  sys_mremap(). vma_adjust() does uprobe_munmap(whole_vma) first,
  this can nullify the counter temporarily and race with another
  thread which can hit the bp, the application will be killed by
  SIGTRAP.

- Suppose an application mmaps 2 consecutive areas in the same file
  and one (or both) of these areas has uprobes. In the likely case
  mmap_region()->vma_merge() suceeds. Like above, this leads to
  uprobe_munmap/uprobe_mmap from vma_merge()->vma_adjust() but then
  mmap_region() does another uprobe_mmap(resulting_vma) and doubles
  the counter.

This patch only removes this counter and fixes the compile errors,
then we will try to cleanup the changed code and add something else
instead.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>

2 files changed, 3 insertions, 37 deletions

diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
index efe4b3308c74..03ae547c1c31 100644
--- a/include/linux/uprobes.h
+++ b/include/linux/uprobes.h
@@ -99,8 +99,8 @@ struct xol_area {
 
 struct uprobes_state {
         struct xol_area         *xol_area;
-        atomic_t                count;
 };
+
 extern int __weak set_swbp(struct arch_uprobe *aup, struct mm_struct *mm, unsigned long vaddr);
 extern int __weak set_orig_insn(struct arch_uprobe *aup, struct mm_struct *mm,  unsigned long vaddr, bool verify);
 extern bool __weak is_swbp_insn(uprobe_opcode_t *insn);
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 0cefde276641..6f1664d217dc 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -678,18 +678,7 @@ 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, vaddr);
-        if (ret)
-                atomic_dec(&mm->uprobes_state.count);
 
         return ret;
 }
@@ -697,8 +686,7 @@ install_breakpoint(struct uprobe *uprobe, struct mm_struct *mm,
 static void
 remove_breakpoint(struct uprobe *uprobe, struct mm_struct *mm, unsigned long vaddr)
 {
-        if (!set_orig_insn(&uprobe->arch, mm, vaddr, true))
-                atomic_dec(&mm->uprobes_state.count);
+        set_orig_insn(&uprobe->arch, mm, vaddr, true);
 }
 
 /*
@@ -1051,13 +1039,6 @@ int uprobe_mmap(struct vm_area_struct *vma)
 
                                 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)
@@ -1068,9 +1049,6 @@ int uprobe_mmap(struct vm_area_struct *vma)
 
         mutex_unlock(uprobes_mmap_hash(inode));
 
-        if (ret)
-                atomic_sub(count, &vma->vm_mm->uprobes_state.count);
-
         return ret;
 }
 
@@ -1089,9 +1067,6 @@ void uprobe_munmap(struct vm_area_struct *vma, unsigned long start, unsigned lon
         if (!atomic_read(&vma->vm_mm->mm_users)) /* called by mmput() ? */
                 return;
 
-        if (!atomic_read(&vma->vm_mm->uprobes_state.count))
-                return;
-
         inode = vma->vm_file->f_mapping->host;
         if (!inode)
                 return;
@@ -1100,13 +1075,6 @@ void uprobe_munmap(struct vm_area_struct *vma, unsigned long start, unsigned lon
         build_probe_list(inode, vma, start, end, &tmp_list);
 
         list_for_each_entry_safe(uprobe, u, &tmp_list, pending_list) {
-                unsigned long vaddr = offset_to_vaddr(vma, uprobe->offset);
-                /*
-                 * 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));
@@ -1217,7 +1185,6 @@ 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);
 }
 
 /*
@@ -1585,8 +1552,7 @@ int uprobe_pre_sstep_notifier(struct pt_regs *regs)
 {
         struct uprobe_task *utask;
 
-        if (!current->mm || !atomic_read(&current->mm->uprobes_state.count))
-                /* task is currently not uprobed */
+        if (!current->mm)
                 return 0;
 
         utask = current->utask;