uretprobes: Reserve the first slot in xol_vma for trampoline

Allocate trampoline page, as the very first one in uprobed
task xol area, and fill it with breakpoint opcode.

Also introduce get_trampoline_vaddr() helper, to wrap the
trampoline address extraction from area->vaddr. That removes
confusion and eases the debug experience in case ->vaddr
notion will be changed.

Signed-off-by: Anton Arapov <anton@redhat.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index eb384e90ac92b27f1eb2fde6fabfc2aca9b5f508..d345b7c6cb2dce5d435fece4474def421c1bfe88 100644 (file)
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1132,6 +1132,7 @@ static struct xol_area *get_xol_area(void)
 {
        struct mm_struct *mm = current->mm;
        struct xol_area *area;
+       uprobe_opcode_t insn = UPROBE_SWBP_INSN;
 
        area = mm->uprobes_state.xol_area;
        if (area)
@@ -1149,7 +1150,12 @@ static struct xol_area *get_xol_area(void)
        if (!area->page)
                goto free_bitmap;
 
+       /* allocate first slot of task's xol_area for the return probes */
+       set_bit(0, area->bitmap);
+       copy_to_page(area->page, 0, &insn, UPROBE_SWBP_INSN_SIZE);
+       atomic_set(&area->slot_count, 1);
        init_waitqueue_head(&area->wq);
+
        if (!xol_add_vma(area))
                return area;
 
@@ -1346,6 +1352,25 @@ static struct uprobe_task *get_utask(void)
        return current->utask;
 }
 
+/*
+ * Current area->vaddr notion assume the trampoline address is always
+ * equal area->vaddr.
+ *
+ * Returns -1 in case the xol_area is not allocated.
+ */
+static unsigned long get_trampoline_vaddr(void)
+{
+       struct xol_area *area;
+       unsigned long trampoline_vaddr = -1;
+
+       area = current->mm->uprobes_state.xol_area;
+       smp_read_barrier_depends();
+       if (area)
+               trampoline_vaddr = area->vaddr;
+
+       return trampoline_vaddr;
+}
+
 /* Prepare to single-step probed instruction out of line. */
 static int
 pre_ssout(struct uprobe *uprobe, struct pt_regs *regs, unsigned long bp_vaddr)