[RFC PATCH 2/6] uretprobes: trampoline implementation

[Anton Arapov <anton@redhat.com>]

The first time a uprobe with return consumer is hit for a process, a
trampoline slot is obtained in the xol_area and initialized with a
breakpoint instruction. This slot is subsequently used by all uretprobes.

Signed-off-by: Anton Arapov <anton@redhat.com>
Signed-off-by: Anithra P Janakiraman <anithra@linux.vnet.ibm.com>
---
 include/linux/uprobes.h |  1 +
 kernel/events/uprobes.c | 38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
index a5db480..68780b6 100644
--- a/include/linux/uprobes.h
+++ b/include/linux/uprobes.h
@@ -88,6 +88,7 @@ struct xol_area {
 	 * the vma go away, and we must handle that reasonably gracefully.
 	 */
 	unsigned long 		vaddr;		/* Page(s) of instruction slots */
+	unsigned long		rp_trampoline_vaddr; /* address of trampolines */
 };
 
 struct uprobes_state {
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 30f2ebc..f0dbbd0 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1246,6 +1246,44 @@ static void xol_free_insn_slot(struct task_struct *tsk)
 	}
 }
 
+/*
+ * xol_get_trampoline_slot - A trampoline slot is obtained the first
+ * time a uprobe with return consumer is hit for a process. Use one
+ * trampoline slot for all probes pertaining to a process, i.e.
+ * one per xol_area.
+ */
+static unsigned long xol_get_trampoline_slot(void)
+{
+	struct xol_area *area;
+	unsigned long offset;
+	void *vaddr;
+	uprobe_opcode_t bp_insn = UPROBE_SWBP_INSN;
+
+	area = get_xol_area(current->mm);
+	if (!area) {
+		area = xol_alloc_area();
+		if (!area)
+			return 0;
+	}
+
+	if (!area->rp_trampoline_vaddr)
+		area->rp_trampoline_vaddr = xol_take_insn_slot(area);
+
+	/*
+	 * Initialize the slot if rp_trampoline_vaddr points to valid
+	 * instruction slot.
+	 */
+	if (unlikely(!area->rp_trampoline_vaddr))
+		return 0;
+
+	offset = area->rp_trampoline_vaddr & ~PAGE_MASK;
+	vaddr = kmap_atomic(area->page);
+	memcpy(vaddr + offset, &bp_insn, UPROBE_SWBP_INSN_SIZE);
+	kunmap_atomic(vaddr);
+
+	return area->rp_trampoline_vaddr;
+}
+
 /**
  * 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
-- 
1.8.0.2