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Date: Mon, 14 Nov 2022 17:26:54 +0800
From: Jinyang He <hejinyang@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>
Cc: Tiezhu Yang <yangtiezhu@...ngson.cn>,
Masami Hiramatsu <mhiramat@...nel.org>,
loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/5] LoongArch: Add kretprobe support
On 2022/11/14 下午4:51, Huacai Chen wrote:
> On Mon, Nov 14, 2022 at 4:32 PM Jinyang He <hejinyang@...ngson.cn> wrote:
>> On 2022/11/14 下午2:50, Huacai Chen wrote:
>>
>>> On Mon, Nov 14, 2022 at 2:11 PM Jinyang He <hejinyang@...ngson.cn> wrote:
>>>> On 2022/11/14 下午12:43, Huacai Chen wrote:
>>>>
>>>>> Hi, Tiezhu and Jinyang,
>>>>>
>>>>> On Wed, Sep 28, 2022 at 8:50 AM Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:
>>>>>> Use the generic kretprobe trampoline handler to add kretprobe
>>>>>> support for LoongArch.
>>>>>>
>>>>>> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
>>>>>> ---
>>>>>> arch/loongarch/Kconfig | 1 +
>>>>>> arch/loongarch/kernel/Makefile | 2 +-
>>>>>> arch/loongarch/kernel/kprobes.c | 24 ++++++++
>>>>>> arch/loongarch/kernel/kprobes_trampoline.S | 97 ++++++++++++++++++++++++++++++
>>>>>> 4 files changed, 123 insertions(+), 1 deletion(-)
>>>>>> create mode 100644 arch/loongarch/kernel/kprobes_trampoline.S
>>>>>>
>>>>>> diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
>>>>>> index 8debd70..877be6a 100644
>>>>>> --- a/arch/loongarch/Kconfig
>>>>>> +++ b/arch/loongarch/Kconfig
>>>>>> @@ -95,6 +95,7 @@ config LOONGARCH
>>>>>> select HAVE_IRQ_EXIT_ON_IRQ_STACK
>>>>>> select HAVE_IRQ_TIME_ACCOUNTING
>>>>>> select HAVE_KPROBES
>>>>>> + select HAVE_KRETPROBES
>>>>>> select HAVE_MOD_ARCH_SPECIFIC
>>>>>> select HAVE_NMI
>>>>>> select HAVE_PCI
>>>>>> diff --git a/arch/loongarch/kernel/Makefile b/arch/loongarch/kernel/Makefile
>>>>>> index ff98d8a..48f50607 100644
>>>>>> --- a/arch/loongarch/kernel/Makefile
>>>>>> +++ b/arch/loongarch/kernel/Makefile
>>>>>> @@ -33,6 +33,6 @@ obj-$(CONFIG_UNWINDER_PROLOGUE) += unwind_prologue.o
>>>>>>
>>>>>> obj-$(CONFIG_PERF_EVENTS) += perf_event.o perf_regs.o
>>>>>>
>>>>>> -obj-$(CONFIG_KPROBES) += kprobes.o
>>>>>> +obj-$(CONFIG_KPROBES) += kprobes.o kprobes_trampoline.o
>>>>>>
>>>>>> CPPFLAGS_vmlinux.lds := $(KBUILD_CFLAGS)
>>>>>> diff --git a/arch/loongarch/kernel/kprobes.c b/arch/loongarch/kernel/kprobes.c
>>>>>> index c11f6e0..ca3f1dc 100644
>>>>>> --- a/arch/loongarch/kernel/kprobes.c
>>>>>> +++ b/arch/loongarch/kernel/kprobes.c
>>>>>> @@ -306,6 +306,30 @@ int __init arch_populate_kprobe_blacklist(void)
>>>>>> (unsigned long)__irqentry_text_end);
>>>>>> }
>>>>>>
>>>>>> +/* Called from __kretprobe_trampoline */
>>>>>> +void __used *trampoline_probe_handler(struct pt_regs *regs)
>>>>>> +{
>>>>>> + return (void *)kretprobe_trampoline_handler(regs, NULL);
>>>>>> +}
>>>>>> +NOKPROBE_SYMBOL(trampoline_probe_handler);
>>>>>> +
>>>>>> +void arch_prepare_kretprobe(struct kretprobe_instance *ri,
>>>>>> + struct pt_regs *regs)
>>>>>> +{
>>>>>> + ri->ret_addr = (kprobe_opcode_t *)regs->regs[1];
>>>>>> + ri->fp = NULL;
>>>>>> +
>>>>>> + /* Replace the return addr with trampoline addr */
>>>>>> + regs->regs[1] = (unsigned long)&__kretprobe_trampoline;
>>>>>> +}
>>>>>> +NOKPROBE_SYMBOL(arch_prepare_kretprobe);
>>>>>> +
>>>>>> +int arch_trampoline_kprobe(struct kprobe *p)
>>>>>> +{
>>>>>> + return 0;
>>>>>> +}
>>>>>> +NOKPROBE_SYMBOL(arch_trampoline_kprobe);
>>>>>> +
>>>>>> int __init arch_init_kprobes(void)
>>>>>> {
>>>>>> return 0;
>>>>>> diff --git a/arch/loongarch/kernel/kprobes_trampoline.S b/arch/loongarch/kernel/kprobes_trampoline.S
>>>>>> new file mode 100644
>>>>>> index 0000000..9888ab8
>>>>>> --- /dev/null
>>>>>> +++ b/arch/loongarch/kernel/kprobes_trampoline.S
>>>>>> @@ -0,0 +1,97 @@
>>>>>> +/* SPDX-License-Identifier: GPL-2.0+ */
>>>>>> +#include <linux/linkage.h>
>>>>>> +#include <asm/stackframe.h>
>>>>>> +
>>>>>> + .text
>>>>>> +
>>>>>> + .macro save_all_base_regs
>>>>>> + cfi_st zero, PT_R0
>>>>>> + cfi_st ra, PT_R1
>>>>>> + cfi_st tp, PT_R2
>>>>>> + cfi_st a0, PT_R4
>>>>>> + cfi_st a1, PT_R5
>>>>>> + cfi_st a2, PT_R6
>>>>>> + cfi_st a3, PT_R7
>>>>>> + cfi_st a4, PT_R8
>>>>>> + cfi_st a5, PT_R9
>>>>>> + cfi_st a6, PT_R10
>>>>>> + cfi_st a7, PT_R11
>>>>>> + cfi_st t0, PT_R12
>>>>>> + cfi_st t1, PT_R13
>>>>>> + cfi_st t2, PT_R14
>>>>>> + cfi_st t3, PT_R15
>>>>>> + cfi_st t4, PT_R16
>>>>>> + cfi_st t5, PT_R17
>>>>>> + cfi_st t6, PT_R18
>>>>>> + cfi_st t7, PT_R19
>>>>>> + cfi_st t8, PT_R20
>>>>>> + cfi_st u0, PT_R21
>>>>>> + cfi_st fp, PT_R22
>>>>>> + cfi_st s0, PT_R23
>>>>>> + cfi_st s1, PT_R24
>>>>>> + cfi_st s2, PT_R25
>>>>>> + cfi_st s3, PT_R26
>>>>>> + cfi_st s4, PT_R27
>>>>>> + cfi_st s5, PT_R28
>>>>>> + cfi_st s6, PT_R29
>>>>>> + cfi_st s7, PT_R30
>>>>>> + cfi_st s8, PT_R31
>>>>>> + addi.d t0, sp, PT_SIZE
>>>>>> + LONG_S t0, sp, PT_R3
>>>>>> + csrrd t0, LOONGARCH_CSR_CRMD
>>>>>> + andi t0, t0, 0x7 /* extract bit[1:0] PLV, bit[2] IE */
>>>>>> + LONG_S t0, sp, PT_PRMD
>>>>>> + .endm
>>>>>> +
>>>>>> + .macro restore_all_base_regs
>>>>>> + cfi_ld zero, PT_R0
>>>>>> + cfi_ld tp, PT_R2
>>>>>> + cfi_ld a0, PT_R4
>>>>>> + cfi_ld a1, PT_R5
>>>>>> + cfi_ld a2, PT_R6
>>>>>> + cfi_ld a3, PT_R7
>>>>>> + cfi_ld a4, PT_R8
>>>>>> + cfi_ld a5, PT_R9
>>>>>> + cfi_ld a6, PT_R10
>>>>>> + cfi_ld a7, PT_R11
>>>>>> + cfi_ld t0, PT_R12
>>>>>> + cfi_ld t1, PT_R13
>>>>>> + cfi_ld t2, PT_R14
>>>>>> + cfi_ld t3, PT_R15
>>>>>> + cfi_ld t4, PT_R16
>>>>>> + cfi_ld t5, PT_R17
>>>>>> + cfi_ld t6, PT_R18
>>>>>> + cfi_ld t7, PT_R19
>>>>>> + cfi_ld t8, PT_R20
>>>>>> + cfi_ld u0, PT_R21
>>>>>> + cfi_ld fp, PT_R22
>>>>>> + cfi_ld s0, PT_R23
>>>>>> + cfi_ld s1, PT_R24
>>>>>> + cfi_ld s2, PT_R25
>>>>>> + cfi_ld s3, PT_R26
>>>>>> + cfi_ld s4, PT_R27
>>>>>> + cfi_ld s5, PT_R28
>>>>>> + cfi_ld s6, PT_R29
>>>>>> + cfi_ld s7, PT_R30
>>>>>> + cfi_ld s8, PT_R31
>>>>>> + LONG_L t0, sp, PT_PRMD
>>>>>> + li.d t1, 0x7 /* mask bit[1:0] PLV, bit[2] IE */
>>>>>> + csrxchg t0, t1, LOONGARCH_CSR_CRMD
>>>>>> + .endm
>>>>> Do you think we need to save and restore all regs here?
>>>>>
>>>>> Huacai
>>>> Hi, Huacai,
>>>>
>>>>
>>>> Note that it is not function context. In the original kprobe design, it is
>>>> triggered by 'break' and then trap into exception with all pt_regs saved.
>>>> The all pt_regs will be visible to the user. So I think in this version
>>>> we should also support all regs to user. BTW, due to all exceptions is
>>>> trapped by 'break' something in pt_regs is not needed, like estat,
>>>> badvaddr and so on.
>>> OK, but I still have some questions:
>>> 1, Why $r0 need save/restore?
>> Surely $r0 can be not saved, as now we do not have strange purpose
>> to make PT_R0 as a flag.
>>
>>
>>> 2, Why save $r1 but not restore?
>> My wrong idea is $r1 should be saved at CSR_ERA, to plays it like
>> exception happened. But its value always equal the address of
>> __kretprobe_trampoline. The kretprobe is something like fgraph. The real
>> return address is returned by trampoline_probe_handler. And at present,
>> the real return address is replaced in pt_regs->csr_era in
>> __kretprobe_trampoline_handler(). So the $r1 saved in CSR_ERA will
>> be destroied at __kretprobe_trampoline_handler() actually.
>> That's why $r1 saved also is not needed.
>>
>> And both way to get return address from return value or get return address
>>
>> from pt_regs is same on LoongArch because arch_kretprobe_fixup_return()
>>
>> does nothing. But I think get return address from pt_regs is more reliable.
>>
>>
>>> 3, What is the purpose of CRMD magic?
>> PT_CRMD magic is just exception context. It gives us a chance e.g.
>> set ie off at func head, and ie on at func return.
> ARM64 and RISC-V don't have such magics, so maybe they are unneeded?
>
ARM64 have noted "Construct a useful saved PSTATE" in kprobes_trampoline.S.
And it seems we provide more interfaces than RISC-V. ?
Thanks,
Jinyang
>>>>>> +
>>>>>> +SYM_CODE_START(__kretprobe_trampoline)
>>>>>> + addi.d sp, sp, -PT_SIZE
>>>>>> + save_all_base_regs
>>>>>> +
>>>>>> + move a0, sp /* pt_regs */
>>>>>> +
>>>>>> + bl trampoline_probe_handler
>>>>>> +
>>>>>> + /* use the result as the return-address */
>>>>>> + move ra, a0
>>>>>> +
>>>>>> + restore_all_base_regs
>>>>>> + addi.d sp, sp, PT_SIZE
>>>>>> +
>>>>>> + jr ra
>>>>>> +SYM_CODE_END(__kretprobe_trampoline)
>>>>>> --
>>>>>> 2.1.0
>>>>>>
>>>>>>
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