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Date: Mon, 14 Nov 2022 16:51:36 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Jinyang He <hejinyang@...ngson.cn>
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 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?
Huacai
>
>
> 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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