Re: [RFC PATCH -next v2 0/4] arm64/ftrace: support dynamic trampoline

Wangshaobo (bobo)
2022-04-21T01:15:29Z

在 2022/4/21 2:11, Steven Rostedt 写道:
> Is this going anywhere?
>
> -- Steve

Not yet, Steve, ftrace_location() looks has no help to find a right
rec->ip in our case,

ftrace_location() can find a right rec->ip when input ip is in the range
between

sym+0 and sym+$end, but our question is how to  identify rec->ip from
__mcount_loc,

this changed the patchable entry before bti to after in gcc:

[1] https://reviews.llvm.org/D73680

gcc tells the place of first nop of the 5 NOPs when using
-fpatchable-function-entry=5,3,

but not tells the first nop after bti, so we don't know how to adjust
our rec->ip for ftrace.

>
>
> 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.
>>
>> 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(-)
>>
> .