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Subject: Re: [PATCH v2 3/5] LoongArch: Add kretprobe support
To:     Huacai Chen <chenhuacai@kernel.org>
Cc:     Tiezhu Yang <yangtiezhu@loongson.cn>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        loongarch@lists.linux.dev, linux-kernel@vger.kernel.org
References: <1664326209-13995-1-git-send-email-yangtiezhu@loongson.cn>
 <1664326209-13995-4-git-send-email-yangtiezhu@loongson.cn>
 <CAAhV-H7N-N2400ivdczJrfJ9Ht12JUbOADxExF87wVPFEj_c_g@mail.gmail.com>
 <dd3fc4ab-6bbb-2fa5-8d5e-b8206b42518c@loongson.cn>
 <CAAhV-H4ok2r2FarFszwYJ6Hah4m7ieNf6egnXyZhUf1ESDUhXg@mail.gmail.com>
From:   Jinyang He <hejinyang@loongson.cn>
Message-ID: <a528e970-29ac-2fcb-b749-cfae3fbe5a18@loongson.cn>
Date:   Mon, 14 Nov 2022 16:32:40 +0800
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On 2022/11/14 下午2:50, Huacai Chen wrote:

> On Mon, Nov 14, 2022 at 2:11 PM Jinyang He <hejinyang@loongson.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@loongson.cn> wrote:
>>>> Use the generic kretprobe trampoline handler to add kretprobe
>>>> support for LoongArch.
>>>>
>>>> Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.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.


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
>>>>
>>>>