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Subject[PATCH 1/2] uprobes/core: Allocate xol slots for uprobes use
From: Srikar Dronamraju <srikar@linux.vnet.ibm.com>

Uprobes executes the original instruction at a probed location out of
line. For this, we allocate a page (per mm) upon the first uprobe hit,
in the process user address space, divide it into slots that are used
to store the actual instructions to be singlestepped. These slots are
known as xol (execution out of line) slots.

Care is taken to ensure that the allocation is in an unmapped area as
close to the top of the user address space as possible, with appropriate
permission settings to keep selinux like frameworks happy.

Upon a uprobe hit, a free slot is acquired, and is released after the
singlestep completes.

[ Folded a fix for build issue on powerpc fixed and reported by Stephen
Rothwell]

Lots of improvements courtesy suggestions/inputs from Peter and Oleg.

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
---
Changelog:
(v5)
- uprobes_xol_area moved from mm to uprobes_state. Also uprobes_xol_area is
now known as xol_area.
- no more spin lock needed for slot allocation.
- use install_special_mapping to add a vma. (previous approach used
init_creds)
- set xol_area while holding map_sem exclusively.

include/linux/mm_types.h | 2
include/linux/uprobes.h | 34 +++++++
kernel/events/uprobes.c | 215 ++++++++++++++++++++++++++++++++++++++++++++++
kernel/fork.c | 2
4 files changed, 253 insertions(+), 0 deletions(-)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 3cc3062..26574c7 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>

@@ -388,6 +389,7 @@ struct mm_struct {
#ifdef CONFIG_CPUMASK_OFFSTACK
struct cpumask cpumask_allocation;
#endif
+ struct uprobes_state uprobes_state;
};

static inline void mm_init_cpumask(struct mm_struct *mm)
diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
index 5ec778f..a111460 100644
--- a/include/linux/uprobes.h
+++ b/include/linux/uprobes.h
@@ -28,6 +28,8 @@
#include <linux/rbtree.h>

struct vm_area_struct;
+struct mm_struct;
+struct inode;

#ifdef CONFIG_ARCH_SUPPORTS_UPROBES
# include <asm/uprobes.h>
@@ -76,6 +78,28 @@ struct uprobe_task {
unsigned long vaddr;
};

+/*
+ * On a breakpoint hit, thread contests for a slot. It frees the
+ * slot after singlestep. Currently a fixed number of slots are
+ * allocated.
+ */
+struct xol_area {
+ wait_queue_head_t wq; /* if all slots are busy */
+ atomic_t slot_count; /* number of 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 */
+};
+
+struct uprobes_state {
+ struct xol_area *xol_area;
+};
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);
@@ -90,7 +114,11 @@ extern int uprobe_pre_sstep_notifier(struct pt_regs *regs);
extern void uprobe_notify_resume(struct pt_regs *regs);
extern bool uprobe_deny_signal(void);
extern bool __weak arch_uprobe_skip_sstep(struct arch_uprobe *aup, struct pt_regs *regs);
+extern void uprobe_clear_state(struct mm_struct *mm);
+extern void uprobe_reset_state(struct mm_struct *mm);
#else /* !CONFIG_UPROBES */
+struct uprobes_state {
+};
static inline int
uprobe_register(struct inode *inode, loff_t offset, struct uprobe_consumer *uc)
{
@@ -121,5 +149,11 @@ static inline void uprobe_free_utask(struct task_struct *t)
static inline void uprobe_copy_process(struct task_struct *t)
{
}
+static inline void uprobe_clear_state(struct mm_struct *mm)
+{
+}
+static inline void uprobe_reset_state(struct mm_struct *mm)
+{
+}
#endif /* !CONFIG_UPROBES */
#endif /* _LINUX_UPROBES_H */
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index b807d15..b395edb 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -35,6 +35,9 @@

#include <linux/uprobes.h>

+#define UINSNS_PER_PAGE (PAGE_SIZE/UPROBE_XOL_SLOT_BYTES)
+#define MAX_UPROBE_XOL_SLOTS UINSNS_PER_PAGE
+
static struct srcu_struct uprobes_srcu;
static struct rb_root uprobes_tree = RB_ROOT;

@@ -1042,6 +1045,213 @@ int uprobe_mmap(struct vm_area_struct *vma)
return ret;
}

+/* Slot allocation for XOL */
+static int xol_add_vma(struct xol_area *area)
+{
+ struct mm_struct *mm;
+ int ret;
+
+ area->page = alloc_page(GFP_HIGHUSER);
+ if (!area->page)
+ return -ENOMEM;
+
+ ret = -EALREADY;
+ mm = current->mm;
+
+ down_write(&mm->mmap_sem);
+ if (mm->uprobes_state.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_xol_area() */
+ mm->uprobes_state.xol_area = area;
+ ret = 0;
+
+fail:
+ up_write(&mm->mmap_sem);
+ if (ret)
+ __free_page(area->page);
+
+ return ret;
+}
+
+static struct xol_area *get_xol_area(struct mm_struct *mm)
+{
+ struct xol_area *area;
+
+ area = mm->uprobes_state.xol_area;
+ smp_read_barrier_depends(); /* pairs with wmb in xol_add_vma() */
+
+ return area;
+}
+
+/*
+ * xol_alloc_area - Allocate process's xol_area.
+ * This area will be used for storing instructions for execution out of
+ * line.
+ *
+ * Returns the allocated area or NULL.
+ */
+static struct xol_area *xol_alloc_area(void)
+{
+ struct 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_xol_area(current->mm);
+}
+
+/*
+ * uprobe_clear_state - Free the area allocated for slots.
+ */
+void uprobe_clear_state(struct mm_struct *mm)
+{
+ struct xol_area *area = mm->uprobes_state.xol_area;
+
+ if (!area)
+ return;
+
+ put_page(area->page);
+ kfree(area->bitmap);
+ kfree(area);
+}
+
+/*
+ * uprobe_reset_state - Free the area allocated for slots.
+ */
+void uprobe_reset_state(struct mm_struct *mm)
+{
+ mm->uprobes_state.xol_area = NULL;
+}
+
+/*
+ * - search for a free slot.
+ */
+static unsigned long xol_take_insn_slot(struct 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 * UPROBE_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 xol_area *area;
+ unsigned long offset;
+ void *vaddr;
+
+ area = get_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->arch.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 xol_area *area;
+ unsigned long vma_end;
+ unsigned long slot_addr;
+
+ if (!tsk->mm || !tsk->mm->uprobes_state.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_state.xol_area;
+ vma_end = area->vaddr + PAGE_SIZE;
+ if (area->vaddr <= slot_addr && slot_addr < vma_end) {
+ unsigned long offset;
+ int slot_nr;
+
+ offset = slot_addr - area->vaddr;
+ slot_nr = offset / UPROBE_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;
+ }
+}
+
/**
* 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
@@ -1070,6 +1280,7 @@ void uprobe_free_utask(struct task_struct *t)
if (utask->active_uprobe)
put_uprobe(utask->active_uprobe);

+ xol_free_insn_slot(t);
kfree(utask);
t->utask = NULL;
}
@@ -1108,6 +1319,9 @@ static struct uprobe_task *add_utask(void)
static int
pre_ssout(struct uprobe *uprobe, struct pt_regs *regs, unsigned long vaddr)
{
+ if (xol_get_insn_slot(uprobe, vaddr) && !arch_uprobe_pre_xol(&uprobe->arch, regs))
+ return 0;
+
return -EFAULT;
}

@@ -1252,6 +1466,7 @@ static void handle_singlestep(struct uprobe_task *utask, struct pt_regs *regs)
utask->active_uprobe = NULL;
utask->state = UTASK_RUNNING;
user_disable_single_step(current);
+ xol_free_insn_slot(current);

spin_lock_irq(&current->sighand->siglock);
recalc_sigpending(); /* see uprobe_deny_signal() */
diff --git a/kernel/fork.c b/kernel/fork.c
index 30affa1..b00f620 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -570,6 +570,7 @@ void mmput(struct mm_struct *mm)
might_sleep();

if (atomic_dec_and_test(&mm->mm_users)) {
+ uprobe_clear_state(mm);
exit_aio(mm);
ksm_exit(mm);
khugepaged_exit(mm); /* must run before exit_mmap */
@@ -804,6 +805,7 @@ struct mm_struct *dup_mm(struct task_struct *tsk)
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
mm->pmd_huge_pte = NULL;
#endif
+ uprobe_reset_state(mm);

if (!mm_init(mm, tsk))
goto fail_nomem;



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