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Subject[PATCH 2/2] uprobes/core: Optimize probe hits with help of counter
From: Srikar Dronamraju <srikar@linux.vnet.ibm.com>

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>
---
Changelog:
(v7)
- Move mm->mm_uprobes_count to mm->uprobes_state.count.
- Separate this patch from the patch that implements uprobe_mmap.
- Increment counter only after verifying that the probe lies underneath.

(v5)
- While forking, handle vma's that have VM_DONTCOPY.
- While forking, handle race of new breakpoints being inserted / removed
in the parent process.

include/linux/uprobes.h | 5 ++
kernel/events/uprobes.c | 119 ++++++++++++++++++++++++++++++++++++++++++++---
kernel/fork.c | 3 +
mm/mmap.c | 10 ++++
4 files changed, 128 insertions(+), 9 deletions(-)
diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
index a111460..d594d3b 100644
--- a/include/linux/uprobes.h
+++ b/include/linux/uprobes.h
@@ -99,6 +99,7 @@ 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);
@@ -106,6 +107,7 @@ extern bool __weak is_swbp_insn(uprobe_opcode_t *insn);
extern int uprobe_register(struct inode *inode, loff_t offset, struct uprobe_consumer *uc);
extern void uprobe_unregister(struct inode *inode, loff_t offset, struct uprobe_consumer *uc);
extern int uprobe_mmap(struct vm_area_struct *vma);
+extern void uprobe_munmap(struct vm_area_struct *vma);
extern void uprobe_free_utask(struct task_struct *t);
extern void uprobe_copy_process(struct task_struct *t);
extern unsigned long __weak uprobe_get_swbp_addr(struct pt_regs *regs);
@@ -132,6 +134,9 @@ static inline int uprobe_mmap(struct vm_area_struct *vma)
{
return 0;
}
+static inline void uprobe_munmap(struct vm_area_struct *vma)
+{
+}
static inline void uprobe_notify_resume(struct pt_regs *regs)
{
}
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index b395edb..29e881b 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;
diff --git a/kernel/fork.c b/kernel/fork.c
index b00f620..ca9a384 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -422,6 +422,9 @@ static int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm)

if (retval)
goto out;
+
+ if (file && uprobe_mmap(tmp))
+ goto out;
}
/* a new mm has just been created */
arch_dup_mmap(oldmm, mm);
diff --git a/mm/mmap.c b/mm/mmap.c
index 9ae8650..b17a39f 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -218,6 +218,7 @@ void unlink_file_vma(struct vm_area_struct *vma)
mutex_lock(&mapping->i_mmap_mutex);
__remove_shared_vm_struct(vma, file, mapping);
mutex_unlock(&mapping->i_mmap_mutex);
+ uprobe_munmap(vma);
}
}

@@ -545,8 +546,14 @@ again: remove_next = 1 + (end > next->vm_end);

if (file) {
mapping = file->f_mapping;
- if (!(vma->vm_flags & VM_NONLINEAR))
+ if (!(vma->vm_flags & VM_NONLINEAR)) {
root = &mapping->i_mmap;
+ uprobe_munmap(vma);
+
+ if (adjust_next)
+ uprobe_munmap(next);
+ }
+
mutex_lock(&mapping->i_mmap_mutex);
if (insert) {
/*
@@ -625,6 +632,7 @@ again: remove_next = 1 + (end > next->vm_end);

if (remove_next) {
if (file) {
+ uprobe_munmap(next);
fput(file);
if (next->vm_flags & VM_EXECUTABLE)
removed_exe_file_vma(mm);



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