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Subject: Re: [PATCH v2 4/6] LoongArch: Strip guess_unwinder out from
 prologue_unwinder
To:     Huacai Chen <chenhuacai@kernel.org>
Cc:     WANG Xuerui <kernel@xen0n.name>,
        Qing Zhang <zhangqing@loongson.cn>, loongarch@lists.linux.dev,
        linux-kernel@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>
References: <20221229061516.31671-1-hejinyang@loongson.cn>
 <20221229061516.31671-5-hejinyang@loongson.cn>
 <CAAhV-H7J7yWpM-xLpK38gwEw3cMNU-BaxF3atVmDLyRxBcVrLg@mail.gmail.com>
From:   Jinyang He <hejinyang@loongson.cn>
Message-ID: <ab0b221e-db13-3cac-dca1-bbd67f739ab3@loongson.cn>
Date:   Tue, 3 Jan 2023 16:40:56 +0800
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On 2023-01-01 17:56, Huacai Chen wrote:

> Hi, Jinyang,
>
> On Thu, Dec 29, 2022 at 2:15 PM Jinyang He <hejinyang@loongson.cn> wrote:
>> The prolugue unwinder rely on symbol info. When PC is not in kernel
>> text address, it cannot find relative symbol info and it will be broken.
>> The guess unwinder will be used in this case. And the guess unwinder
>> codes in prolugue unwinder is redundant. Strip it out and set the
>> unwinder info in unwind_state.
>>
>> Signed-off-by: Jinyang He <hejinyang@loongson.cn>
>> ---
>>   arch/loongarch/include/asm/unwind.h     |  26 ++++-
>>   arch/loongarch/kernel/Makefile          |   3 +-
>>   arch/loongarch/kernel/traps.c           |   3 -
>>   arch/loongarch/kernel/unwind.c          |  52 +++++++++
>>   arch/loongarch/kernel/unwind_guess.c    |  41 ++------
>>   arch/loongarch/kernel/unwind_prologue.c | 133 +++++++++---------------
>>   6 files changed, 132 insertions(+), 126 deletions(-)
>>   create mode 100644 arch/loongarch/kernel/unwind.c
>>
>> diff --git a/arch/loongarch/include/asm/unwind.h b/arch/loongarch/include/asm/unwind.h
>> index 6ece48f0ff77..4a55fd7b77ad 100644
>> --- a/arch/loongarch/include/asm/unwind.h
>> +++ b/arch/loongarch/include/asm/unwind.h
>> @@ -13,20 +13,30 @@
>>   #include <asm/ptrace.h>
>>   #include <asm/stacktrace.h>
>>
>> -enum unwinder_type {
>> -       UNWINDER_GUESS,
>> -       UNWINDER_PROLOGUE,
>> -};
>> +struct unwinder_ops;
>>
>>   struct unwind_state {
>> -       char type; /* UNWINDER_XXX */
>>          struct stack_info stack_info;
>>          struct task_struct *task;
>>          bool first, error, is_ftrace;
>>          int graph_idx;
>>          unsigned long sp, pc, ra;
>> +       const struct unwinder_ops *ops;
>> +};
>> +
>> +struct unwinder_ops {
>> +       void (*unwind_start)(struct unwind_state *state,
>> +                            struct task_struct *task, struct pt_regs *regs);
>> +       bool (*unwind_next_frame)(struct unwind_state *state);
>> +       unsigned long (*unwind_get_return_address)(struct unwind_state *state);
>>   };
>>
>> +extern const struct unwinder_ops *default_unwinder;
>> +extern const struct unwinder_ops unwinder_guess;
>> +#ifdef CONFIG_UNWINDER_PROLOGUE
>> +extern const struct unwinder_ops unwinder_prologue;
>> +#endif
>> +
>>   void unwind_start(struct unwind_state *state,
>>                    struct task_struct *task, struct pt_regs *regs);
>>   bool unwind_next_frame(struct unwind_state *state);
>> @@ -49,4 +59,10 @@ static inline unsigned long unwind_graph_addr(struct unwind_state *state,
>>          return ftrace_graph_ret_addr(state->task, &state->graph_idx,
>>                                       pc, (unsigned long *)(cfa - GRAPH_FAKE_OFFSET));
>>   }
>> +
>> +static inline void unwind_register_unwinder(struct unwind_state *state,
>> +                                         const struct unwinder_ops *unwinder)
>> +{
>> +       state->ops = unwinder;
>> +}
>>   #endif /* _ASM_UNWIND_H */
>> diff --git a/arch/loongarch/kernel/Makefile b/arch/loongarch/kernel/Makefile
>> index 7ca65195f7f8..cb6029ea3ea9 100644
>> --- a/arch/loongarch/kernel/Makefile
>> +++ b/arch/loongarch/kernel/Makefile
>> @@ -8,7 +8,7 @@ extra-y         := vmlinux.lds
>>   obj-y          += head.o cpu-probe.o cacheinfo.o env.o setup.o entry.o genex.o \
>>                     traps.o irq.o idle.o process.o dma.o mem.o io.o reset.o switch.o \
>>                     elf.o syscall.o signal.o time.o topology.o inst.o ptrace.o vdso.o \
>> -                  alternative.o unaligned.o
>> +                  alternative.o unaligned.o unwind.o unwind_guess.o
>>
>>   obj-$(CONFIG_ACPI)             += acpi.o
>>   obj-$(CONFIG_EFI)              += efi.o
>> @@ -42,7 +42,6 @@ obj-$(CONFIG_MAGIC_SYSRQ)     += sysrq.o
>>   obj-$(CONFIG_KEXEC)            += machine_kexec.o relocate_kernel.o
>>   obj-$(CONFIG_CRASH_DUMP)       += crash_dump.o
>>
>> -obj-$(CONFIG_UNWINDER_GUESS)   += unwind_guess.o
>>   obj-$(CONFIG_UNWINDER_PROLOGUE) += unwind_prologue.o
> We do have CONFIG_UNWINDER_GUESS and I don't like to build guess
> unwinder unconditionally, maybe we can refactor it to make the
> unwinder still user selectable.


In order to provide backtrace info to the user, prologue unwinder
will use guess unwinder when its first pc is not a kernel address,
such as the first PC is in ebpf generated codes. This patch is intended
to extract guess unwinder codes to avoid redundant codes. And if we provide
a new unwinder in the future like orc unwinder which take effect after
orc_init is set, we can use another backup unwinder like guess unwinder in
case we need backtrace before orc_init being set.

If you mind this, could we just do as follows,

obj-$(CONFIG_UNWINDER_GUESS)   += unwind_guess.o
obj-$(CONFIG_UNWINDER_PROLOGUE) += unwind_prologue.o unwind_guess.o

Or copy the guess unwinder codes to unwind_prologue.c and set another
name for guess unwinder.


Thanks,

Jinyang

> Huacai
>>   obj-$(CONFIG_PERF_EVENTS)      += perf_event.o perf_regs.o
>> diff --git a/arch/loongarch/kernel/traps.c b/arch/loongarch/kernel/traps.c
>> index 3adc44832205..1ea14f6c18d3 100644
>> --- a/arch/loongarch/kernel/traps.c
>> +++ b/arch/loongarch/kernel/traps.c
>> @@ -72,9 +72,6 @@ static void show_backtrace(struct task_struct *task, const struct pt_regs *regs,
>>          if (!task)
>>                  task = current;
>>
>> -       if (user_mode(regs))
>> -               state.type = UNWINDER_GUESS;
>> -
>>          printk("%sCall Trace:", loglvl);
>>          for (unwind_start(&state, task, pregs);
>>                !unwind_done(&state); unwind_next_frame(&state)) {
>> diff --git a/arch/loongarch/kernel/unwind.c b/arch/loongarch/kernel/unwind.c
>> new file mode 100644
>> index 000000000000..24d2cf99bfb6
>> --- /dev/null
>> +++ b/arch/loongarch/kernel/unwind.c
>> @@ -0,0 +1,52 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/*
>> + * Copyright (C) 2022 Loongson Technology Corporation Limited
>> + */
>> +#include <asm/unwind.h>
>> +
>> +#if defined(CONFIG_UNWINDER_GUESS)
>> +const struct unwinder_ops *default_unwinder = &unwinder_guess;
>> +#elif defined(CONFIG_UNWINDER_PROLOGUE)
>> +const struct unwinder_ops *default_unwinder = &unwinder_prologue;
>> +#endif
>> +
>> +unsigned long unwind_get_return_address(struct unwind_state *state)
>> +{
>> +       if (!state->ops || unwind_done(state))
>> +               return 0;
>> +       return state->ops->unwind_get_return_address(state);
>> +}
>> +EXPORT_SYMBOL_GPL(unwind_get_return_address);
>> +
>> +void unwind_start(struct unwind_state *state, struct task_struct *task,
>> +                   struct pt_regs *regs)
>> +{
>> +       memset(state, 0, sizeof(*state));
>> +       unwind_register_unwinder(state, default_unwinder);
>> +       if (regs) {
>> +               state->sp = regs->regs[3];
>> +               state->pc = regs->csr_era;
>> +               state->ra = regs->regs[1];
>> +       } else if (task == current || task == NULL) {
>> +               state->sp = (unsigned long)__builtin_frame_address(0);
>> +               state->pc = (unsigned long)__builtin_return_address(0);
>> +               state->ra = 0;
>> +       } else {
>> +               state->sp = thread_saved_fp(task);
>> +               state->pc = thread_saved_ra(task);
>> +               state->ra = 0;
>> +       }
>> +       state->task = task;
>> +       get_stack_info(state->sp, state->task, &state->stack_info);
>> +       state->pc = unwind_graph_addr(state, state->pc, state->sp);
>> +       state->ops->unwind_start(state, task, regs);
>> +}
>> +EXPORT_SYMBOL_GPL(unwind_start);
>> +
>> +bool unwind_next_frame(struct unwind_state *state)
>> +{
>> +       if (!state->ops || unwind_done(state))
>> +               return false;
>> +       return state->ops->unwind_next_frame(state);
>> +}
>> +EXPORT_SYMBOL_GPL(unwind_next_frame);
>> diff --git a/arch/loongarch/kernel/unwind_guess.c b/arch/loongarch/kernel/unwind_guess.c
>> index 935d24f8c95c..b7ca2b88ac63 100644
>> --- a/arch/loongarch/kernel/unwind_guess.c
>> +++ b/arch/loongarch/kernel/unwind_guess.c
>> @@ -7,51 +7,23 @@
>>
>>   #include <asm/unwind.h>
>>
>> -unsigned long unwind_get_return_address(struct unwind_state *state)
>> +static unsigned long get_return_address(struct unwind_state *state)
>>   {
>> -       if (unwind_done(state))
>> -               return 0;
>>          return state->pc;
>>   }
>> -EXPORT_SYMBOL_GPL(unwind_get_return_address);
>>
>> -void unwind_start(struct unwind_state *state, struct task_struct *task,
>> +static void start(struct unwind_state *state, struct task_struct *task,
>>                      struct pt_regs *regs)
>>   {
>> -       memset(state, 0, sizeof(*state));
>> -
>> -       if (regs) {
>> -               state->sp = regs->regs[3];
>> -               state->pc = regs->csr_era;
>> -       } else if (task == current || task == NULL) {
>> -               state->sp = (unsigned long)__builtin_frame_address(0);
>> -               state->pc = (unsigned long)__builtin_return_address(0);
>> -       } else {
>> -               state->sp = thread_saved_fp(task);
>> -               state->pc = thread_saved_ra(task);
>> -       }
>> -
>> -       state->task = task;
>> -       state->first = true;
>> -       state->pc = unwind_graph_addr(state, state->pc, state->sp);
>> -       get_stack_info(state->sp, state->task, &state->stack_info);
>> -
>>          if (!unwind_done(state) && !__kernel_text_address(state->pc))
>>                  unwind_next_frame(state);
>>   }
>> -EXPORT_SYMBOL_GPL(unwind_start);
>>
>> -bool unwind_next_frame(struct unwind_state *state)
>> +static bool next_frame(struct unwind_state *state)
>>   {
>>          struct stack_info *info = &state->stack_info;
>>          unsigned long addr;
>>
>> -       if (unwind_done(state))
>> -               return false;
>> -
>> -       if (state->first)
>> -               state->first = false;
>> -
>>          do {
>>                  for (state->sp += sizeof(unsigned long);
>>                       state->sp < info->end;
>> @@ -68,4 +40,9 @@ bool unwind_next_frame(struct unwind_state *state)
>>
>>          return false;
>>   }
>> -EXPORT_SYMBOL_GPL(unwind_next_frame);
>> +
>> +const struct unwinder_ops unwinder_guess = {
>> +       .unwind_start = start,
>> +       .unwind_next_frame = next_frame,
>> +       .unwind_get_return_address = get_return_address,
>> +};
>> diff --git a/arch/loongarch/kernel/unwind_prologue.c b/arch/loongarch/kernel/unwind_prologue.c
>> index f7994ed05f04..beb57ea24da2 100644
>> --- a/arch/loongarch/kernel/unwind_prologue.c
>> +++ b/arch/loongarch/kernel/unwind_prologue.c
>> @@ -19,31 +19,19 @@ static inline void unwind_state_fixup(struct unwind_state *state)
>>   #endif
>>   }
>>
>> -unsigned long unwind_get_return_address(struct unwind_state *state)
>> +static unsigned long get_return_address(struct unwind_state *state)
>>   {
>> -       if (unwind_done(state))
>> -               return 0;
>>          return state->pc;
>>   }
>> -EXPORT_SYMBOL_GPL(unwind_get_return_address);
>> -
>> -static bool unwind_by_guess(struct unwind_state *state)
>> -{
>> -       struct stack_info *info = &state->stack_info;
>> -       unsigned long addr;
>> -
>> -       for (state->sp += sizeof(unsigned long);
>> -            state->sp < info->end;
>> -            state->sp += sizeof(unsigned long)) {
>> -               addr = *(unsigned long *)(state->sp);
>> -               state->pc = unwind_graph_addr(state, addr, state->sp + 8);
>> -               if (__kernel_text_address(state->pc))
>> -                       return true;
>> -       }
>> -
>> -       return false;
>> -}
>>
>> +/*
>> + * LoongArch function prologue like follows,
>> + *     [others instructions not use stack var]
>> + *     addi.d sp, sp, -imm
>> + *     st.d   xx, sp, offset <- save callee saved regs and
>> + *     st.d   yy, sp, offset    save ra if function is nest.
>> + *     [others instructions]
>> + */
>>   static bool unwind_by_prologue(struct unwind_state *state)
>>   {
>>          long frame_ra = -1;
>> @@ -89,6 +77,10 @@ static bool unwind_by_prologue(struct unwind_state *state)
>>                  ip++;
>>          }
>>
>> +       /*
>> +        * Not find stack alloc action, PC may be in a leaf function. Only the
>> +        * first being true is reasonable, otherwise indicate analysis is broken.
>> +        */
>>          if (!frame_size) {
>>                  if (state->first)
>>                          goto first;
>> @@ -106,6 +98,7 @@ static bool unwind_by_prologue(struct unwind_state *state)
>>                  ip++;
>>          }
>>
>> +       /* Not find save $ra action, PC may be in a leaf function, too. */
>>          if (frame_ra < 0) {
>>                  if (state->first) {
>>                          state->sp = state->sp + frame_size;
>> @@ -114,96 +107,63 @@ static bool unwind_by_prologue(struct unwind_state *state)
>>                  return false;
>>          }
>>
>> -       if (state->first)
>> -               state->first = false;
>> -
>>          state->pc = *(unsigned long *)(state->sp + frame_ra);
>>          state->sp = state->sp + frame_size;
>>          goto out;
>>
>>   first:
>> -       state->first = false;
>> -       if (state->pc == state->ra)
>> -               return false;
>> -
>>          state->pc = state->ra;
>>
>>   out:
>> +       state->first = false;
>>          unwind_state_fixup(state);
>>          return !!__kernel_text_address(state->pc);
>>   }
>>
>> -void unwind_start(struct unwind_state *state, struct task_struct *task,
>> +static void start(struct unwind_state *state, struct task_struct *task,
>>                      struct pt_regs *regs)
>>   {
>> -       memset(state, 0, sizeof(*state));
>> -       state->type = UNWINDER_PROLOGUE;
>> -
>> -       if (regs) {
>> -               state->sp = regs->regs[3];
>> -               state->pc = regs->csr_era;
>> -               state->ra = regs->regs[1];
>> -               if (!__kernel_text_address(state->pc))
>> -                       state->type = UNWINDER_GUESS;
>> -       } else if (task == current || task == NULL) {
>> -               state->sp = (unsigned long)__builtin_frame_address(0);
>> -               state->pc = (unsigned long)__builtin_return_address(0);
>> -               state->ra = 0;
>> -       } else {
>> -               state->sp = thread_saved_fp(task);
>> -               state->pc = thread_saved_ra(task);
>> -               state->ra = 0;
>> -       }
>> -
>> -       state->task = task;
>>          state->first = true;
>> -       state->pc = unwind_graph_addr(state, state->pc, state->sp);
>> -       get_stack_info(state->sp, state->task, &state->stack_info);
>>
>> -       if (!unwind_done(state) && !__kernel_text_address(state->pc))
>> -               unwind_next_frame(state);
>> +       /*
>> +        * The current PC is not kernel text address, we cannot find its
>> +        * relative symbol. Thus, prologue analysis will be broken. Luckly,
>> +        * we can use the guess unwinder.
>> +        */
>> +       if (!__kernel_text_address(state->pc)) {
>> +               unwind_register_unwinder(state, &unwinder_guess);
>> +               if (!unwind_done(state))
>> +                       unwind_next_frame(state);
>> +       }
>>   }
>> -EXPORT_SYMBOL_GPL(unwind_start);
>>
>> -bool unwind_next_frame(struct unwind_state *state)
>> +static bool next_frame(struct unwind_state *state)
>>   {
>>          struct stack_info *info = &state->stack_info;
>>          struct pt_regs *regs;
>>          unsigned long pc;
>>
>> -       if (unwind_done(state))
>> -               return false;
>> -
>>          do {
>> -               switch (state->type) {
>> -               case UNWINDER_GUESS:
>> -                       state->first = false;
>> -                       if (unwind_by_guess(state))
>> -                               return true;
>> -                       break;
>> -
>> -               case UNWINDER_PROLOGUE:
>> -                       if (unwind_by_prologue(state)) {
>> -                               state->pc = unwind_graph_addr(state, state->pc, state->sp);
>> -                               return true;
>> -                       }
>> +               if (unwind_by_prologue(state)) {
>> +                       state->pc = unwind_graph_addr(state, state->pc, state->sp);
>> +                       return true;
>> +               }
>>
>> -                       if (info->type == STACK_TYPE_IRQ &&
>> -                               info->end == state->sp) {
>> -                               regs = (struct pt_regs *)info->next_sp;
>> -                               pc = regs->csr_era;
>> +               if (info->type == STACK_TYPE_IRQ &&
>> +                   info->end == state->sp) {
>> +                       regs = (struct pt_regs *)info->next_sp;
>> +                       pc = regs->csr_era;
>>
>> -                               if (user_mode(regs) || !__kernel_text_address(pc))
>> -                                       return false;
>> +                       if (user_mode(regs) || !__kernel_text_address(pc))
>> +                               return false;
>>
>> -                               state->first = true;
>> -                               state->ra = regs->regs[1];
>> -                               state->sp = regs->regs[3];
>> -                               state->pc = pc;
>> -                               get_stack_info(state->sp, state->task, info);
>> +                       state->first = true;
>> +                       state->ra = regs->regs[1];
>> +                       state->sp = regs->regs[3];
>> +                       state->pc = pc;
>> +                       get_stack_info(state->sp, state->task, info);
>>
>> -                               return true;
>> -                       }
>> +                       return true;
>>                  }
>>
>>                  state->sp = info->next_sp;
>> @@ -212,4 +172,9 @@ bool unwind_next_frame(struct unwind_state *state)
>>
>>          return false;
>>   }
>> -EXPORT_SYMBOL_GPL(unwind_next_frame);
>> +
>> +const struct unwinder_ops unwinder_prologue = {
>> +       .unwind_start = start,
>> +       .unwind_next_frame = next_frame,
>> +       .unwind_get_return_address = get_return_address,
>> +};
>> --
>> 2.34.3
>>
>>