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Subject[PATCH v6 3.2-rc1 18/28] uprobes: slot allocation.

One page of slots are allocated per mm.
On a probehit one free slot is acquired and released after
singlestep operation completes.

Signed-off-by: Jim Keniston <jkenisto@us.ibm.com>
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
---
Changelog (since v5)
- no more spin lock needed for slot allocation.
- use install_special_mapping to add a vma. (previous approach used
init_creds)
- set uprobes_xol_area while holding map_sem exclusively.

include/linux/mm_types.h | 2
include/linux/uprobes.h | 24 +++++
kernel/fork.c | 2
kernel/uprobes.c | 215 +++++++++++++++++++++++++++++++++++++++++++++-
4 files changed, 240 insertions(+), 3 deletions(-)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 544a0b6..2595c9c 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -12,6 +12,7 @@
#include <linux/completion.h>
#include <linux/cpumask.h>
#include <linux/page-debug-flags.h>
+#include <linux/uprobes.h>
#include <asm/page.h>
#include <asm/mmu.h>

@@ -391,6 +392,7 @@ struct mm_struct {
#endif
#ifdef CONFIG_UPROBES
atomic_t mm_uprobes_count;
+ struct uprobes_xol_area *uprobes_xol_area;
#endif
};

diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
index c1378a9..add5222 100644
--- a/include/linux/uprobes.h
+++ b/include/linux/uprobes.h
@@ -90,6 +90,26 @@ struct uprobe_task {
struct uprobe *active_uprobe;
};

+/*
+ * On a breakpoint hit, thread contests for a slot. It free the
+ * slot after singlestep. Only definite number of slots are
+ * allocated.
+ */
+
+struct uprobes_xol_area {
+ wait_queue_head_t wq; /* if all slots are busy */
+ atomic_t slot_count; /* currently in use slots */
+ unsigned long *bitmap; /* 0 = free slot */
+ struct page *page;
+
+ /*
+ * We keep the vma's vm_start rather than a pointer to the vma
+ * itself. The probed process or a naughty kernel module could make
+ * the vma go away, and we must handle that reasonably gracefully.
+ */
+ unsigned long vaddr; /* Page(s) of instruction slots */
+};
+
#ifdef CONFIG_UPROBES
extern int __weak set_bkpt(struct mm_struct *mm, struct uprobe *uprobe,
unsigned long vaddr);
@@ -101,6 +121,7 @@ extern int register_uprobe(struct inode *inode, loff_t offset,
extern void unregister_uprobe(struct inode *inode, loff_t offset,
struct uprobe_consumer *consumer);
extern void free_uprobe_utask(struct task_struct *tsk);
+extern void free_uprobes_xol_area(struct mm_struct *mm);
extern int mmap_uprobe(struct vm_area_struct *vma);
extern void munmap_uprobe(struct vm_area_struct *vma);
extern unsigned long __weak get_uprobe_bkpt_addr(struct pt_regs *regs);
@@ -134,5 +155,8 @@ static inline unsigned long get_uprobe_bkpt_addr(struct pt_regs *regs)
static inline void free_uprobe_utask(struct task_struct *tsk)
{
}
+static inline void free_uprobes_xol_area(struct mm_struct *mm)
+{
+}
#endif /* CONFIG_UPROBES */
#endif /* _LINUX_UPROBES_H */
diff --git a/kernel/fork.c b/kernel/fork.c
index a03f436..c605f2a 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -558,6 +558,7 @@ void mmput(struct mm_struct *mm)
might_sleep();

if (atomic_dec_and_test(&mm->mm_users)) {
+ free_uprobes_xol_area(mm);
exit_aio(mm);
ksm_exit(mm);
khugepaged_exit(mm); /* must run before exit_mmap */
@@ -746,6 +747,7 @@ struct mm_struct *dup_mm(struct task_struct *tsk)
#endif
#ifdef CONFIG_UPROBES
atomic_set(&mm->mm_uprobes_count, 0);
+ mm->uprobes_xol_area = NULL;
#endif

if (!mm_init(mm, tsk))
diff --git a/kernel/uprobes.c b/kernel/uprobes.c
index c4ccb89..9e73cef 100644
--- a/kernel/uprobes.c
+++ b/kernel/uprobes.c
@@ -33,6 +33,9 @@
#include <linux/kdebug.h> /* notifier mechanism */
#include <linux/uprobes.h>

+#define UINSNS_PER_PAGE (PAGE_SIZE/UPROBES_XOL_SLOT_BYTES)
+#define MAX_UPROBES_XOL_SLOTS UINSNS_PER_PAGE
+
static bulkref_t uprobes_srcu;
static struct rb_root uprobes_tree = RB_ROOT;
static DEFINE_SPINLOCK(uprobes_treelock); /* serialize rbtree access */
@@ -1054,6 +1057,201 @@ void munmap_uprobe(struct vm_area_struct *vma)
return;
}

+/* Slot allocation for XOL */
+static int xol_add_vma(struct uprobes_xol_area *area)
+{
+ struct mm_struct *mm;
+ int ret;
+
+ area->page = alloc_page(GFP_HIGHUSER);
+ if (!area->page)
+ return -ENOMEM;
+
+ mm = current->mm;
+ down_write(&mm->mmap_sem);
+ ret = -EALREADY;
+ if (mm->uprobes_xol_area)
+ goto fail;
+
+ ret = -ENOMEM;
+
+ /* Try to map as high as possible, this is only a hint. */
+ area->vaddr = get_unmapped_area(NULL, TASK_SIZE - PAGE_SIZE,
+ PAGE_SIZE, 0, 0);
+ if (area->vaddr & ~PAGE_MASK) {
+ ret = area->vaddr;
+ goto fail;
+ }
+
+ ret = install_special_mapping(mm, area->vaddr, PAGE_SIZE,
+ VM_EXEC|VM_MAYEXEC|VM_DONTCOPY|VM_IO,
+ &area->page);
+ if (ret)
+ goto fail;
+
+ smp_wmb(); /* pairs with get_uprobes_xol_area() */
+ mm->uprobes_xol_area = area;
+ ret = 0;
+
+fail:
+ up_write(&mm->mmap_sem);
+ if (ret)
+ __free_page(area->page);
+
+ return ret;
+}
+
+static struct uprobes_xol_area *get_uprobes_xol_area(struct mm_struct *mm)
+{
+ struct uprobes_xol_area *area = mm->uprobes_xol_area;
+ smp_read_barrier_depends();/* pairs with wmb in xol_add_vma() */
+ return area;
+}
+
+/*
+ * xol_alloc_area - Allocate process's uprobes_xol_area.
+ * This area will be used for storing instructions for execution out of
+ * line.
+ *
+ * Returns the allocated area or NULL.
+ */
+static struct uprobes_xol_area *xol_alloc_area(void)
+{
+ struct uprobes_xol_area *area;
+
+ area = kzalloc(sizeof(*area), GFP_KERNEL);
+ if (unlikely(!area))
+ return NULL;
+
+ area->bitmap = kzalloc(BITS_TO_LONGS(UINSNS_PER_PAGE) * sizeof(long),
+ GFP_KERNEL);
+
+ if (!area->bitmap)
+ goto fail;
+
+ init_waitqueue_head(&area->wq);
+ if (!xol_add_vma(area))
+ return area;
+
+fail:
+ kfree(area->bitmap);
+ kfree(area);
+ return get_uprobes_xol_area(current->mm);
+}
+
+/*
+ * free_uprobes_xol_area - Free the area allocated for slots.
+ */
+void free_uprobes_xol_area(struct mm_struct *mm)
+{
+ struct uprobes_xol_area *area = mm->uprobes_xol_area;
+
+ if (!area)
+ return;
+
+ put_page(area->page);
+ kfree(area->bitmap);
+ kfree(area);
+}
+
+/*
+ * - search for a free slot.
+ */
+static unsigned long xol_take_insn_slot(struct uprobes_xol_area *area)
+{
+ unsigned long slot_addr;
+ int slot_nr;
+
+ do {
+ slot_nr = find_first_zero_bit(area->bitmap, UINSNS_PER_PAGE);
+ if (slot_nr < UINSNS_PER_PAGE) {
+ if (!test_and_set_bit(slot_nr, area->bitmap))
+ break;
+
+ slot_nr = UINSNS_PER_PAGE;
+ continue;
+ }
+ wait_event(area->wq,
+ (atomic_read(&area->slot_count) < UINSNS_PER_PAGE));
+ } while (slot_nr >= UINSNS_PER_PAGE);
+
+ slot_addr = area->vaddr + (slot_nr * UPROBES_XOL_SLOT_BYTES);
+ atomic_inc(&area->slot_count);
+ return slot_addr;
+}
+
+/*
+ * xol_get_insn_slot - If was not allocated a slot, then
+ * allocate a slot.
+ * Returns the allocated slot address or 0.
+ */
+static unsigned long xol_get_insn_slot(struct uprobe *uprobe,
+ unsigned long slot_addr)
+{
+ struct uprobes_xol_area *area;
+ unsigned long offset;
+ void *vaddr;
+
+ area = get_uprobes_xol_area(current->mm);
+ if (!area) {
+ area = xol_alloc_area();
+ if (!area)
+ return 0;
+ }
+ current->utask->xol_vaddr = xol_take_insn_slot(area);
+
+ /*
+ * Initialize the slot if xol_vaddr points to valid
+ * instruction slot.
+ */
+ if (unlikely(!current->utask->xol_vaddr))
+ return 0;
+
+ current->utask->vaddr = slot_addr;
+ offset = current->utask->xol_vaddr & ~PAGE_MASK;
+ vaddr = kmap_atomic(area->page);
+ memcpy(vaddr + offset, uprobe->insn, MAX_UINSN_BYTES);
+ kunmap_atomic(vaddr);
+ return current->utask->xol_vaddr;
+}
+
+/*
+ * xol_free_insn_slot - If slot was earlier allocated by
+ * @xol_get_insn_slot(), make the slot available for
+ * subsequent requests.
+ */
+static void xol_free_insn_slot(struct task_struct *tsk)
+{
+ struct uprobes_xol_area *area;
+ unsigned long vma_end;
+ unsigned long slot_addr;
+
+ if (!tsk->mm || !tsk->mm->uprobes_xol_area || !tsk->utask)
+ return;
+
+ slot_addr = tsk->utask->xol_vaddr;
+
+ if (unlikely(!slot_addr || IS_ERR_VALUE(slot_addr)))
+ return;
+
+ area = tsk->mm->uprobes_xol_area;
+ vma_end = area->vaddr + PAGE_SIZE;
+ if (area->vaddr <= slot_addr && slot_addr < vma_end) {
+ int slot_nr;
+ unsigned long offset = slot_addr - area->vaddr;
+
+ slot_nr = offset / UPROBES_XOL_SLOT_BYTES;
+ if (slot_nr >= UINSNS_PER_PAGE)
+ return;
+
+ clear_bit(slot_nr, area->bitmap);
+ atomic_dec(&area->slot_count);
+ if (waitqueue_active(&area->wq))
+ wake_up(&area->wq);
+ tsk->utask->xol_vaddr = 0;
+ }
+}
+
/**
* get_uprobe_bkpt_addr - compute address of bkpt given post-bkpt regs
* @regs: Reflects the saved state of the task after it has hit a breakpoint
@@ -1082,6 +1280,7 @@ void free_uprobe_utask(struct task_struct *tsk)
if (utask->active_uprobe)
put_uprobe(utask->active_uprobe);

+ xol_free_insn_slot(tsk);
kfree(utask);
tsk->utask = NULL;
}
@@ -1111,7 +1310,8 @@ static struct uprobe_task *add_utask(void)
static int pre_ssout(struct uprobe *uprobe, struct pt_regs *regs,
unsigned long vaddr)
{
- /* TODO: Yet to be implemented */
+ if (xol_get_insn_slot(uprobe, vaddr) && !pre_xol(uprobe, regs))
+ return 0;
return -EFAULT;
}

@@ -1121,8 +1321,16 @@ static int pre_ssout(struct uprobe *uprobe, struct pt_regs *regs,
*/
static bool sstep_complete(struct uprobe *uprobe, struct pt_regs *regs)
{
- /* TODO: Yet to be implemented */
- return false;
+ unsigned long vaddr = instruction_pointer(regs);
+
+ /*
+ * If we have executed out of line, Instruction pointer
+ * cannot be same as virtual address of XOL slot.
+ */
+ if (vaddr == current->utask->xol_vaddr)
+ return false;
+ post_xol(uprobe, regs);
+ return true;
}

/*
@@ -1180,6 +1388,7 @@ void uprobe_notify_resume(struct pt_regs *regs)
utask->active_uprobe = NULL;
utask->state = UTASK_RUNNING;
user_disable_single_step(current);
+ xol_free_insn_slot(current);

/* TODO Stop queueing signals. */
}


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