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    Subject[RFC PATCH 2/6] uretprobes: trampoline implementation
    Date
    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


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