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    SubjectRe: [RFC PATCH -next v2 0/4] arm64/ftrace: support dynamic trampoline
    On Wed, 16 Mar 2022 18:01:28 +0800
    Wang ShaoBo <bobo.shaobowang@huawei.com> wrote:

    > This implements dynamic trampoline in ARM64, as reference said, we
    > complete whole design of supporting long jump in dynamic trampoline:
    >
    > .text section:
    > funcA: | funcA(): funcB():|
    > `-> +-----+ | | ... mov x9 |
    > | ... | | | adrp <- bl <> |
    > | nop | | | mov
    > | nop | | | br x16 ---+
    > funcB | nop | | | ftrace_(regs_)caller_tramp:
    > `-> +-----+ | `--> +---------------------+
    > | nop | | | ... |
    > | nop | | ftrace callsite +---------------------+
    > | ... | | `-----> | PLT entry: |
    > | nop | | | adrp |
    > | nop | | | add |
    > funcC: | nop | | ftrace graph callsite | br x16 |
    > `-> +-----+ | `-----> +---------------------+
    > | nop | | | ... |
    > | nop | | +---------------------+
    >
    > But there is still a tricky problem that is how to adjust tracing ip,
    > waiting to be solved:
    >
    > For ARM64, somecases there may be extra instructions inserted into the
    > head of tracable functions(but not all) by compiler, for instance BTI[1].
    >
    > This dump vmlinux with CONFIG_BTI=y:
    >
    > (1) function gic_handle_irq has bti in its head, so we adjust rec->ip+=5 to last nop
    > ffffffc0080100e0: d53cd042 mrs x2, tpidr_el2
    > ...
    > ffffffc0080100f0: d503201f nop //__mcount_loc tells the rec->ip
    > ffffffc0080100f4: d503201f nop
    > ffffffc0080100f8: d503201f nop
    >
    > ffffffc0080100fc <gic_handle_irq>:
    > ffffffc0080100fc: d503245f bti c
    > ffffffc008010100: d503201f nop
    > ffffffc008010104: d503201f nop //we adjust origin rec->ip+5 to here
    > ffffffc008010108: d503233f paciasp
    > (2) name_to_dev_t.part.0 do not has bti in its head, so we should adjust rec->ip+=4 to last nop
    > ffff8000080137d4: d503201f nop
    > ffff8000080137d8: d503201f nop
    > ffff8000080137dc: d503201f nop
    >
    > ffff8000080137e0 <name_to_dev_t.part.0>:
    > ffff8000080137e0: d503201f nop
    > ffff8000080137e4: d503201f nop
    > ffff8000080137e8: d503233f paciasp
    >
    > So at this time we have no idea to identify rec->ip for each tracable function.

    This looks like the same issue that Peter Zijlstra is handling for IBT on
    x86, which I think can be useful for you too.

    https://lore.kernel.org/all/20220308153011.021123062@infradead.org/

    Specifically this patch:

    https://lore.kernel.org/all/20220308154318.227581603@infradead.org/

    Which modifies the ftrace_location() to return the rec->ip if you pass in
    the ip of the function and kallsyms returns that the ip passed in has an
    offset of zero.

    -- Steve

    >
    > we are looking forward to follow-up discussions.
    >
    > References:
    > [1] https://developer.arm.com/documentation/100076/0100/a64-instruction-set-reference/a64-general-instructions/bti
    > [2] https://lore.kernel.org/linux-arm-kernel/20200109142736.1122-1-cj.chengjian@huawei.com/
    >
    > Cheng Jian (4):
    > arm64: introduce aarch64_insn_gen_load_literal
    > arm64/ftrace: introduce ftrace dynamic trampoline entrances
    > arm64/ftrace: support dynamically allocated trampolines
    > arm64/ftrace: implement long jump for dynamic trampolines
    >
    > arch/arm64/Makefile | 2 +-
    > arch/arm64/include/asm/ftrace.h | 10 +-
    > arch/arm64/include/asm/insn.h | 6 +
    > arch/arm64/include/asm/module.h | 9 +
    > arch/arm64/kernel/entry-ftrace.S | 88 ++++++--
    > arch/arm64/kernel/ftrace.c | 366 ++++++++++++++++++++++++++++---
    > arch/arm64/kernel/module-plts.c | 50 +++++
    > arch/arm64/lib/insn.c | 49 +++++
    > 8 files changed, 532 insertions(+), 48 deletions(-)
    >

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