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Subject: Re: [PATCH v9 2/4] LoongArch: Add kprobe support
To:     Jinyang He <hejinyang@loongson.cn>,
        Huacai Chen <chenhuacai@kernel.org>,
        Tiezhu Yang <yangtiezhu@loongson.cn>
References: <1670506868-15771-1-git-send-email-yangtiezhu@loongson.cn>
 <1670506868-15771-3-git-send-email-yangtiezhu@loongson.cn>
 <CAAhV-H54yruHAe7ey8L9hHPonryEWEd8nt-iS6s7-odrDdrutw@mail.gmail.com>
 <b1b4614c-7d7f-0773-dced-9182a510a80a@loongson.cn>
Cc:     WANG Xuerui <kernel@xen0n.name>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        loongarch@lists.linux.dev, linux-kernel@vger.kernel.org
From:   Youling Tang <tangyouling@loongson.cn>
Message-ID: <039cf92d-ec36-1d4c-e191-1d31a7d11bd3@loongson.cn>
Date:   Fri, 9 Dec 2022 11:44:58 +0800
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Hi, Jinyang

On 12/09/2022 11:31 AM, Jinyang He wrote:
> On 2022-12-09 10:22, Huacai Chen wrote:
>
>>    cas
>>
>> On Thu, Dec 8, 2022 at 9:41 PM Tiezhu Yang <yangtiezhu@loongson.cn>
>> wrote:
>>> Kprobes allows you to trap at almost any kernel address and
>>> execute a callback function, this commit adds kprobe support
>>> for LoongArch.
>>>
>>> Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
>>> ---
>>>   arch/loongarch/Kconfig               |   1 +
>>>   arch/loongarch/include/asm/inst.h    |  15 ++
>>>   arch/loongarch/include/asm/kprobes.h |  59 +++++++
>>>   arch/loongarch/kernel/Makefile       |   2 +
>>>   arch/loongarch/kernel/kprobes.c      | 332
>>> +++++++++++++++++++++++++++++++++++
>>>   arch/loongarch/kernel/traps.c        |  13 +-
>>>   arch/loongarch/mm/fault.c            |   3 +
>>>   7 files changed, 421 insertions(+), 4 deletions(-)
>>>   create mode 100644 arch/loongarch/include/asm/kprobes.h
>>>   create mode 100644 arch/loongarch/kernel/kprobes.c
>>>
>>> diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
>>> index 16bf1b6..f6fc156 100644
>>> --- a/arch/loongarch/Kconfig
>>> +++ b/arch/loongarch/Kconfig
>>> @@ -102,6 +102,7 @@ config LOONGARCH
>>>          select HAVE_IOREMAP_PROT
>>>          select HAVE_IRQ_EXIT_ON_IRQ_STACK
>>>          select HAVE_IRQ_TIME_ACCOUNTING
>>> +       select HAVE_KPROBES
>>>          select HAVE_MOD_ARCH_SPECIFIC
>>>          select HAVE_NMI
>>>          select HAVE_PCI
>>> diff --git a/arch/loongarch/include/asm/inst.h
>>> b/arch/loongarch/include/asm/inst.h
>>> index 19cf692..e3f2378 100644
>>> --- a/arch/loongarch/include/asm/inst.h
>>> +++ b/arch/loongarch/include/asm/inst.h
>>> @@ -24,6 +24,10 @@
>>>
>>>   #define ADDR_IMM(addr, INSN)   ((addr & ADDR_IMMMASK_##INSN) >>
>>> ADDR_IMMSHIFT_##INSN)
>>>
>>> +enum reg0i15_op {
>>> +       break_op        = 0x54,
>>> +};
>>> +
>>>   enum reg0i26_op {
>>>          b_op            = 0x14,
>>>          bl_op           = 0x15,
>>> @@ -180,6 +184,11 @@ enum reg3sa2_op {
>>>          alsld_op        = 0x16,
>>>   };
>>>
>>> +struct reg0i15_format {
>>> +       unsigned int immediate : 15;
>>> +       unsigned int opcode : 17;
>>> +};
>>> +
>>>   struct reg0i26_format {
>>>          unsigned int immediate_h : 10;
>>>          unsigned int immediate_l : 16;
>>> @@ -265,6 +274,7 @@ struct reg3sa2_format {
>>>
>>>   union loongarch_instruction {
>>>          unsigned int word;
>>> +       struct reg0i15_format   reg0i15_format;
>>>          struct reg0i26_format   reg0i26_format;
>>>          struct reg1i20_format   reg1i20_format;
>>>          struct reg1i21_format   reg1i21_format;
>>> @@ -335,6 +345,11 @@ static inline bool is_branch_ins(union
>>> loongarch_instruction *ip)
>>>                  ip->reg1i21_format.opcode <= bgeu_op;
>>>   }
>>>
>>> +static inline bool is_break_ins(union loongarch_instruction *ip)
>>> +{
>>> +       return ip->reg0i15_format.opcode == break_op;
>>> +}
>>> +
>>>   static inline bool is_ra_save_ins(union loongarch_instruction *ip)
>>>   {
>>>          /* st.d $ra, $sp, offset */
>>> diff --git a/arch/loongarch/include/asm/kprobes.h
>>> b/arch/loongarch/include/asm/kprobes.h
>>> new file mode 100644
>>> index 0000000..d3903f3
>>> --- /dev/null
>>> +++ b/arch/loongarch/include/asm/kprobes.h
>>> @@ -0,0 +1,59 @@
>>> +/* SPDX-License-Identifier: GPL-2.0-only */
>>> +#ifndef __ASM_LOONGARCH_KPROBES_H
>>> +#define __ASM_LOONGARCH_KPROBES_H
>>> +
>>> +#include <asm-generic/kprobes.h>
>>> +#include <asm/cacheflush.h>
>>> +
>>> +#ifdef CONFIG_KPROBES
>>> +
>>> +#include <asm/inst.h>
>>> +
>>> +#define __ARCH_WANT_KPROBES_INSN_SLOT
>>> +#define MAX_INSN_SIZE                  2
>>> +
>>> +#define
>>> flush_insn_slot(p)                                             \
>>> +do
>>> {                                                                   \
>>> +       if
>>> (p->addr)                                                    \
>>> +               flush_icache_range((unsigned
>>> long)p->addr,              \
>>> +                          (unsigned long)p->addr
>>> +                     \
>>> +                          (MAX_INSN_SIZE *
>>> sizeof(kprobe_opcode_t)));  \
>>> +} while (0)
>>> +
>>> +#define kretprobe_blacklist_size       0
>>> +
>>> +typedef union loongarch_instruction kprobe_opcode_t;
>>> +
>>> +/* Architecture specific copy of original instruction */
>>> +struct arch_specific_insn {
>>> +       /* copy of the original instruction */
>>> +       kprobe_opcode_t *insn;
>>> +};
>>> +
>>> +struct prev_kprobe {
>>> +       struct kprobe *kp;
>>> +       unsigned long status;
>>> +       unsigned long saved_irq;
>>> +       unsigned long saved_era;
>>> +};
>>> +
>>> +/* per-cpu kprobe control block */
>>> +struct kprobe_ctlblk {
>>> +       unsigned long kprobe_status;
>>> +       unsigned long kprobe_saved_irq;
>>> +       unsigned long kprobe_saved_era;
>>> +       struct prev_kprobe prev_kprobe;
>>> +};
>>> +
>>> +void arch_remove_kprobe(struct kprobe *p);
>>> +bool kprobe_fault_handler(struct pt_regs *regs, int trapnr);
>>> +bool kprobe_breakpoint_handler(struct pt_regs *regs);
>>> +bool kprobe_singlestep_handler(struct pt_regs *regs);
>>> +
>>> +#else /* !CONFIG_KPROBES */
>>> +
>>> +static inline bool kprobe_breakpoint_handler(struct pt_regs *regs) {
>>> return 0; }
>>> +static inline bool kprobe_singlestep_handler(struct pt_regs *regs) {
>>> return 0; }
>>> +
>>> +#endif /* CONFIG_KPROBES */
>>> +#endif /* __ASM_LOONGARCH_KPROBES_H */
>>> diff --git a/arch/loongarch/kernel/Makefile
>>> b/arch/loongarch/kernel/Makefile
>>> index fcaa024..6fe4a4e 100644
>>> --- a/arch/loongarch/kernel/Makefile
>>> +++ b/arch/loongarch/kernel/Makefile
>>> @@ -47,4 +47,6 @@ obj-$(CONFIG_UNWINDER_PROLOGUE) += unwind_prologue.o
>>>
>>>   obj-$(CONFIG_PERF_EVENTS)      += perf_event.o perf_regs.o
>>>
>>> +obj-$(CONFIG_KPROBES)          += kprobes.o
>>> +
>>>   CPPFLAGS_vmlinux.lds           := $(KBUILD_CFLAGS)
>>> diff --git a/arch/loongarch/kernel/kprobes.c
>>> b/arch/loongarch/kernel/kprobes.c
>>> new file mode 100644
>>> index 0000000..aadfc9ae
>>> --- /dev/null
>>> +++ b/arch/loongarch/kernel/kprobes.c
>>> @@ -0,0 +1,332 @@
>>> +// SPDX-License-Identifier: GPL-2.0-only
>>> +#include <linux/kprobes.h>
>>> +#include <linux/kdebug.h>
>>> +#include <linux/preempt.h>
>>> +#include <asm/break.h>
>>> +
>>> +static const union loongarch_instruction breakpoint_insn = {
>>> +       .reg0i15_format = {
>>> +               .opcode = break_op,
>>> +               .immediate = BRK_KPROBE_BP,
>>> +       }
>>> +};
>>> +
>>> +static const union loongarch_instruction singlestep_insn = {
>>> +       .reg0i15_format = {
>>> +               .opcode = break_op,
>>> +               .immediate = BRK_KPROBE_SSTEPBP,
>>> +       }
>>> +};
>>> +
>>> +DEFINE_PER_CPU(struct kprobe *, current_kprobe);
>>> +DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
>>> +
>>> +static bool insns_are_not_supported(union loongarch_instruction insn)
>> Use  insns_not_supported for short.
>>
>>> +{
>>> +       switch (insn.reg2i14_format.opcode) {
>>> +       case llw_op:
>>> +       case lld_op:
>>> +       case scw_op:
>>> +       case scd_op:
>>> +               pr_notice("kprobe: ll and sc instructions are not
>>> supported\n");
>>> +               return true;
>>> +       }
>>> +
>>> +       switch (insn.reg1i21_format.opcode) {
>>> +       case bceqz_op:
>>> +               pr_notice("kprobe: bceqz and bcnez instructions are
>>> not supported\n");
>>> +               return true;
>>> +       }
>>> +
>>> +       return false;
>>> +}
>>> +NOKPROBE_SYMBOL(insns_are_not_supported);
>>> +
>>> +int arch_prepare_kprobe(struct kprobe *p)
>>> +{
>>> +       union loongarch_instruction insn;
>>> +
>>> +       insn = p->addr[0];
>>> +       if (insns_are_not_supported(insn))
>>> +               return -EINVAL;
>>> +
>>> +       p->ainsn.insn = get_insn_slot();
>>> +       if (!p->ainsn.insn)
>>> +               return -ENOMEM;
>>> +
>>> +       p->ainsn.insn[0] = *p->addr;
>>> +       p->ainsn.insn[1] = singlestep_insn;
>>> +
>>> +       p->opcode = *p->addr;
>>> +
>>> +       return 0;
>>> +}
>>> +NOKPROBE_SYMBOL(arch_prepare_kprobe);
>>> +
>>> +/* Install breakpoint in text */
>>> +void arch_arm_kprobe(struct kprobe *p)
>>> +{
>>> +       *p->addr = breakpoint_insn;
>>> +       flush_insn_slot(p);
>>> +}
>>> +NOKPROBE_SYMBOL(arch_arm_kprobe);
>>> +
>>> +/* Remove breakpoint from text */
>>> +void arch_disarm_kprobe(struct kprobe *p)
>>> +{
>>> +       *p->addr = p->opcode;
>>> +       flush_insn_slot(p);
>>> +}
>>> +NOKPROBE_SYMBOL(arch_disarm_kprobe);
>>> +
>>> +void arch_remove_kprobe(struct kprobe *p)
>>> +{
>>> +       if (p->ainsn.insn) {
>>> +               free_insn_slot(p->ainsn.insn, 0);
>>> +               p->ainsn.insn = NULL;
>>> +       }
>>> +}
>>> +NOKPROBE_SYMBOL(arch_remove_kprobe);
>>> +
>>> +static void save_previous_kprobe(struct kprobe_ctlblk *kcb)
>>> +{
>>> +       kcb->prev_kprobe.kp = kprobe_running();
>>> +       kcb->prev_kprobe.status = kcb->kprobe_status;
>>> +       kcb->prev_kprobe.saved_irq = kcb->kprobe_saved_irq;
>>> +       kcb->prev_kprobe.saved_era = kcb->kprobe_saved_era;
>>> +}
>>> +NOKPROBE_SYMBOL(save_previous_kprobe);
>>> +
>>> +static void restore_previous_kprobe(struct kprobe_ctlblk *kcb)
>>> +{
>>> +       __this_cpu_write(current_kprobe, kcb->prev_kprobe.kp);
>>> +       kcb->kprobe_status = kcb->prev_kprobe.status;
>>> +       kcb->kprobe_saved_irq = kcb->prev_kprobe.saved_irq;
>>> +       kcb->kprobe_saved_era = kcb->prev_kprobe.saved_era;
>>> +}
>>> +NOKPROBE_SYMBOL(restore_previous_kprobe);
>>> +
>>> +static void set_current_kprobe(struct kprobe *p, struct pt_regs *regs,
>>> +                              struct kprobe_ctlblk *kcb)
>>> +{
>>> +       __this_cpu_write(current_kprobe, p);
>>> +       kcb->kprobe_saved_irq = regs->csr_prmd & CSR_PRMD_PIE;
>>> +       kcb->kprobe_saved_era = regs->csr_era;
>>> +}
>>> +NOKPROBE_SYMBOL(set_current_kprobe);
>>> +
>>> +static bool insns_are_not_simulated(struct kprobe *p, struct pt_regs
>>> *regs)
>> Use  insns_not_simulated for short.
>>> +{
>>> +       if (is_branch_ins(&p->opcode)) {
>>> +               simu_branch(regs, p->opcode);
>>> +               return false;
>>> +       } else if (is_pc_ins(&p->opcode)) {
>>> +               simu_pc(regs, p->opcode);
>>> +               return false;
>>> +       } else {
>>> +               return true;
>>> +       }
>>> +}
>>> +NOKPROBE_SYMBOL(insns_are_not_simulated);
>>> +
>>> +static void setup_singlestep(struct kprobe *p, struct pt_regs *regs,
>>> +                            struct kprobe_ctlblk *kcb, int reenter)
>>> +{
>>> +       if (reenter) {
>>> +               save_previous_kprobe(kcb);
>>> +               set_current_kprobe(p, regs, kcb);
>>> +               kcb->kprobe_status = KPROBE_REENTER;
>>> +       } else {
>>> +               kcb->kprobe_status = KPROBE_HIT_SS;
>>> +       }
>>> +
>>> +       regs->csr_prmd &= ~CSR_PRMD_PIE;
>>> +
>>> +       if (p->ainsn.insn->word == breakpoint_insn.word) {
>>> +               regs->csr_prmd |= kcb->kprobe_saved_irq;
>>> +               preempt_enable_no_resched();
>>> +               return;
>>> +       }
>>> +
>>> +       if (insns_are_not_simulated(p, regs)) {
>>> +               kcb->kprobe_status = KPROBE_HIT_SS;
>>> +               regs->csr_era = (unsigned long)&p->ainsn.insn[0];
>>> +       } else {
>>> +               kcb->kprobe_status = KPROBE_HIT_SSDONE;
>>> +               if (p->post_handler)
>>> +                       p->post_handler(p, regs, 0);
>>> +               reset_current_kprobe();
>>> +               preempt_enable_no_resched();
>>> +       }
>>> +}
>>> +NOKPROBE_SYMBOL(setup_singlestep);
>>> +
>>> +static bool reenter_kprobe(struct kprobe *p, struct pt_regs *regs,
>>> +                         struct kprobe_ctlblk *kcb)
>>> +{
>>> +       switch (kcb->kprobe_status) {
>>> +       case KPROBE_HIT_SSDONE:
>>> +       case KPROBE_HIT_ACTIVE:
>>> +               kprobes_inc_nmissed_count(p);
>>> +               setup_singlestep(p, regs, kcb, 1);
>>> +               break;
>>> +       case KPROBE_HIT_SS:
>>> +       case KPROBE_REENTER:
>>> +               pr_warn("Failed to recover from reentered kprobes.\n");
>>> +               dump_kprobe(p);
>>> +               BUG();
>>> +               break;
>>> +       default:
>>> +               WARN_ON(1);
>>> +               return false;
>>> +       }
>>> +
>>> +       return true;
>>> +}
>>> +NOKPROBE_SYMBOL(reenter_kprobe);
>>> +
>>> +bool kprobe_breakpoint_handler(struct pt_regs *regs)
>>> +{
>>> +       struct kprobe_ctlblk *kcb;
>>> +       struct kprobe *p, *cur_kprobe;
>>> +       kprobe_opcode_t *addr = (kprobe_opcode_t *)regs->csr_era;
>>> +
>>> +       /*
>>> +        * We don't want to be preempted for the entire
>>> +        * duration of kprobe processing.
>>> +        */
>>> +       preempt_disable();
>>> +       kcb = get_kprobe_ctlblk();
>>> +       cur_kprobe = kprobe_running();
>>> +
>>> +       p = get_kprobe(addr);
>>> +       if (p) {
>>> +               if (cur_kprobe) {
>>> +                       if (reenter_kprobe(p, regs, kcb))
>>> +                               return true;
>>> +               } else {
>>> +                       /* Probe hit */
>>> +                       set_current_kprobe(p, regs, kcb);
>>> +                       kcb->kprobe_status = KPROBE_HIT_ACTIVE;
>>> +
>>> +                       /*
>>> +                        * If we have no pre-handler or it returned
>>> 0, we
>>> +                        * continue with normal processing.  If we
>>> have a
>>> +                        * pre-handler and it returned non-zero, it will
>>> +                        * modify the execution path and no need to
>>> single
>>> +                        * stepping. Let's just reset current kprobe
>>> and exit.
>>> +                        *
>>> +                        * pre_handler can hit a breakpoint and can
>>> step thru
>>> +                        * before return.
>>> +                        */
>>> +                       if (!p->pre_handler || !p->pre_handler(p,
>>> regs)) {
>>> +                               setup_singlestep(p, regs, kcb, 0);
>>> +                       } else {
>>> +                               reset_current_kprobe();
>>> +                               preempt_enable_no_resched();
>>> +                       }
>>> +               }
>>> +               return true;
>>> +       }
>>> +
>>> +       if (!is_break_ins(addr)) {
>>> +               /*
>>> +                * The breakpoint instruction was removed right
>>> +                * after we hit it.  Another cpu has removed
>>> +                * either a probepoint or a debugger breakpoint
>>> +                * at this address.  In either case, no further
>>> +                * handling of this interrupt is appropriate.
>>> +                * Return back to original instruction, and continue.
>>> +                */
>>> +               preempt_enable_no_resched();
>>> +               return true;
>>> +       }
>>> +
>>> +       preempt_enable_no_resched();
>>> +       return false;
>>> +}
>>> +NOKPROBE_SYMBOL(kprobe_breakpoint_handler);
>>> +
>>> +bool kprobe_singlestep_handler(struct pt_regs *regs)
>>> +{
>>> +       struct kprobe *cur = kprobe_running();
>>> +       struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
>>> +
>>> +       if (!cur)
>>> +               return false;
>>> +
>>> +       /* Restore back the original saved kprobes variables and
>>> continue */
>>> +       if (kcb->kprobe_status == KPROBE_REENTER) {
>>> +               restore_previous_kprobe(kcb);
>>> +               goto out;
>>> +       }
>>> +
>>> +       /* Call post handler */
>>> +       kcb->kprobe_status = KPROBE_HIT_SSDONE;
>>> +       if (cur->post_handler)
>>> +               cur->post_handler(cur, regs, 0);
>> I'm not very familiar, but I think you should set KPROBE_HIT_SSDONE
>> only when post_handler is not NULL.
>>
>>> +
>>> +       regs->csr_era = kcb->kprobe_saved_era + LOONGARCH_INSN_SIZE;
>>> +       regs->csr_prmd |= kcb->kprobe_saved_irq;
>>> +
>>> +       reset_current_kprobe();
>>> +out:
>>> +       preempt_enable_no_resched();
>>> +       return true;
>>> +}
>>> +NOKPROBE_SYMBOL(kprobe_singlestep_handler);
>>> +
>>> +bool kprobe_fault_handler(struct pt_regs *regs, int trapnr)
>>> +{
>>> +       struct kprobe *cur = kprobe_running();
>>> +       struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
>>> +
>>> +       switch (kcb->kprobe_status) {
>>> +       case KPROBE_HIT_SS:
>>> +       case KPROBE_REENTER:
>>> +               /*
>>> +                * We are here because the instruction being single
>>> +                * stepped caused a page fault. We reset the current
>>> +                * kprobe and the ip points back to the probe address
>>> +                * and allow the page fault handler to continue as a
>>> +                * normal page fault.
>>> +                */
>>> +               regs->csr_era = (unsigned long) cur->addr;
>>> +               BUG_ON(!instruction_pointer(regs));
>>> +
>>> +               if (kcb->kprobe_status == KPROBE_REENTER) {
>>> +                       restore_previous_kprobe(kcb);
>>> +               } else {
>>> +                       regs->csr_prmd |= kcb->kprobe_saved_irq;
>>> +                       reset_current_kprobe();
>>> +                       preempt_enable_no_resched();
>>> +               }
>> I think the KPROBE_REENTER case also needs to enable preemption.
>>
>>> +
>>> +               break;
>>> +       case KPROBE_HIT_ACTIVE:
>>> +       case KPROBE_HIT_SSDONE:
>>> +               /*
>>> +                * In case the user-specified fault handler returned
>>> +                * zero, try to fix up.
>>> +                */
>>> +               if (fixup_exception(regs))
>>> +                       return true;
>> Does here need to enable preemption too?
>>
>>> +       }
>>> +       return false;
>>> +}
>>> +NOKPROBE_SYMBOL(kprobe_fault_handler);
>>> +
>>> +/*
>>> + * Provide a blacklist of symbols identifying ranges which cannot be
>>> kprobed.
>>> + * This blacklist is exposed to userspace via debugfs
>>> (kprobes/blacklist).
>>> + */
>>> +int __init arch_populate_kprobe_blacklist(void)
>>> +{
>>> +       return kprobe_add_area_blacklist((unsigned
>>> long)__irqentry_text_start,
>>> +                                        (unsigned
>>> long)__irqentry_text_end);
>>> +}
>>> +
>>> +int __init arch_init_kprobes(void)
>>> +{
>>> +       return 0;
>>> +}
>>> diff --git a/arch/loongarch/kernel/traps.c
>>> b/arch/loongarch/kernel/traps.c
>>> index a19bb32..4d9f775 100644
>>> --- a/arch/loongarch/kernel/traps.c
>>> +++ b/arch/loongarch/kernel/traps.c
>>> @@ -448,14 +448,12 @@ asmlinkage void noinstr do_bp(struct pt_regs
>>> *regs)
>>>           */
>>>          switch (bcode) {
>>>          case BRK_KPROBE_BP:
>>> -               if (notify_die(DIE_BREAK, "Kprobe", regs, bcode,
>>> -                              current->thread.trap_nr, SIGTRAP) ==
>>> NOTIFY_STOP)
>>> +               if (kprobe_breakpoint_handler(regs))
>>>                          goto out;
>>>                  else
>>>                          break;
>>>          case BRK_KPROBE_SSTEPBP:
>>> -               if (notify_die(DIE_SSTEPBP, "Kprobe_SingleStep",
>>> regs, bcode,
>>> -                              current->thread.trap_nr, SIGTRAP) ==
>>> NOTIFY_STOP)
>>> +               if (kprobe_singlestep_handler(regs))
>>>                          goto out;
>>>                  else
>>>                          break;
>>> @@ -479,6 +477,13 @@ asmlinkage void noinstr do_bp(struct pt_regs *regs)
>>>                          break;
>>>          }
>>>
>>> +       if (bcode == BRK_KPROBE_BP) {
>>> +               if (__get_inst(&opcode, (u32 *)era, user))
>>> +                       goto out_sigsegv;
>>> +
>>> +               bcode = (opcode & 0x7fff);
>>> +       }
>>> +
>> Why? we already got bcode at the beginning.
>
> Hi, Huacai,
>
>
> I think it is because kprobe_breakpoint_handler() may deal with that
> case other cpu may remove its breakpoint. In that case, normally we can
> exit this break exception and go on. But if
> the orignal instruction is_break_ins(), we will trigger trap aggain. So
> it is better to avoid double traps and let it go ahead when
> kprobe_breakpoint_handler() returns.
Perhaps it is not described here as avoiding the trap twice, but rather
that if we don't re-fetch the bcode when the original instruction
`break` will go to the default branch regardless of what the bcode of
the original instruction is (since the bcode is BRK_KPROBE_BP at this
point). IMHO, so we re-get the bcode to get the original instruction
`break` is the correct bcode to ensure that it goes to the appropriate
case branch.

Thanks,
Youling.

> Differnt from many other archs, e.g. riscv, we switch bcode to decide
> which handlers should go on. Thus, we need get bcode again.
>
> Thanks,
>
> Jinyang
>
>
>>>          switch (bcode) {
>>>          case BRK_BUG:
>>>                  bug_handler(regs);
>>> diff --git a/arch/loongarch/mm/fault.c b/arch/loongarch/mm/fault.c
>>> index 1ccd536..449087b 100644
>>> --- a/arch/loongarch/mm/fault.c
>>> +++ b/arch/loongarch/mm/fault.c
>>> @@ -135,6 +135,9 @@ static void __kprobes __do_page_fault(struct
>>> pt_regs *regs,
>>>          struct vm_area_struct *vma = NULL;
>>>          vm_fault_t fault;
>>>
>>> +       if (kprobe_page_fault(regs, current->thread.trap_nr))
>>> +               return;
>>> +
>>>          /*
>>>           * We fault-in kernel-space virtual memory on-demand. The
>>>           * 'reference' page table is init_mm.pgd.
>>> --
>>> 2.1.0
>>>
>>>
>