[RFC PATCH v2 2/4] 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.

todo:
  unify with xol_take_insn_slot()

Signed-off-by: Anton Arapov <anton@redhat.com>
---
 include/linux/uprobes.h |  1 +
 kernel/events/uprobes.c | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
index 95d0002..bddfad6 100644
--- a/include/linux/uprobes.h
+++ b/include/linux/uprobes.h
@@ -91,6 +91,7 @@ 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 473db3f..0ad2ac3 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1208,6 +1208,39 @@ static unsigned long xol_get_insn_slot(struct uprobe *uprobe)
 	return xol_vaddr;
 }
 
+/*
+ * 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;
+	uprobe_opcode_t bp_insn = UPROBE_SWBP_INSN;
+	void *vaddr;
+
+	area = get_xol_area();
+	if (!area)
+		return 0;
+
+	if (!area->rp_trampoline_vaddr)
+		area->rp_trampoline_vaddr = xol_take_insn_slot(area);
+
+	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);
+
+	flush_dcache_page(area->page);
+
+	return area->rp_trampoline_vaddr;
+}
+
 /*
  * xol_free_insn_slot - If slot was earlier allocated by
  * @xol_get_insn_slot(), make the slot available for
-- 
1.8.0.2