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From:   Huacai Chen <chenhuacai@kernel.org>
Date:   Sun, 4 Sep 2022 20:19:20 +0800
Message-ID: <CAAhV-H4vNpbHbT55z8_ub3q=Kv7eVb--7Q3pXz2te-1DVWhTWA@mail.gmail.com>
Subject: Re: [PATCH] LoongArch: Mark kernel stack padding as TOP_OF_KERNEL_STACK_PADDING
To:     Jinyang He <hejinyang@loongson.cn>
Cc:     WANG Xuerui <kernel@xen0n.name>, Oleg Nesterov <oleg@redhat.com>,
        loongarch@lists.linux.dev, LKML <linux-kernel@vger.kernel.org>
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Hi, Jinyang,

I don't know what scenarios this padding is used and how to use. And
in x86 there is only TOP_OF_KERNEL_STACK_PADDING but we need
TOP_OF_KERNEL_STACK_PADDING and _TOP_OF_KERNEL_STACK_PADDING. Two
macros are hard to maintain.

Huacai

On Fri, Sep 2, 2022 at 11:01 PM Jinyang He <hejinyang@loongson.cn> wrote:
>
> Mark the top reserved bytes of the kernel stack by a macro. It collects
> that info so that we can modify or discarded (let it be 0) this padding
> if needed. Its name is referenced from x86_64. It seems that x86_64
> think the padding as extension on top of the kernel stack in other mode
> (normally the top is only pt_regs). Although LoongArch not produce
> such an extension at present, collect them because of the uncertainty
> of the future. Note that this patch does not modify macro size. It
> requires another strict evaluation.
>
> Signed-off-by: Jinyang He <hejinyang@loongson.cn>
> ---
>  arch/loongarch/include/asm/processor.h  | 3 ++-
>  arch/loongarch/include/asm/ptrace.h     | 3 ++-
>  arch/loongarch/include/asm/stackframe.h | 1 +
>  arch/loongarch/kernel/head.S            | 2 +-
>  arch/loongarch/kernel/process.c         | 4 ++--
>  arch/loongarch/kernel/switch.S          | 2 +-
>  6 files changed, 9 insertions(+), 6 deletions(-)
>
> diff --git a/arch/loongarch/include/asm/processor.h b/arch/loongarch/include/asm/processor.h
> index 1c4b43083..6077fd8e3 100644
> --- a/arch/loongarch/include/asm/processor.h
> +++ b/arch/loongarch/include/asm/processor.h
> @@ -193,8 +193,9 @@ static inline void flush_thread(void)
>
>  unsigned long __get_wchan(struct task_struct *p);
>
> +#define TOP_OF_KERNEL_STACK_PADDING 32
>  #define __KSTK_TOS(tsk) ((unsigned long)task_stack_page(tsk) + \
> -                        THREAD_SIZE - 32 - sizeof(struct pt_regs))
> +                        THREAD_SIZE - TOP_OF_KERNEL_STACK_PADDING - sizeof(struct pt_regs))
>  #define task_pt_regs(tsk) ((struct pt_regs *)__KSTK_TOS(tsk))
>  #define KSTK_EIP(tsk) (task_pt_regs(tsk)->csr_era)
>  #define KSTK_ESP(tsk) (task_pt_regs(tsk)->regs[3])
> diff --git a/arch/loongarch/include/asm/ptrace.h b/arch/loongarch/include/asm/ptrace.h
> index 17838c6b7..cdceeaa60 100644
> --- a/arch/loongarch/include/asm/ptrace.h
> +++ b/arch/loongarch/include/asm/ptrace.h
> @@ -133,7 +133,8 @@ static inline void die_if_kernel(const char *str, struct pt_regs *regs)
>  #define current_pt_regs()                                              \
>  ({                                                                     \
>         unsigned long sp = (unsigned long)__builtin_frame_address(0);   \
> -       (struct pt_regs *)((sp | (THREAD_SIZE - 1)) + 1 - 32) - 1;      \
> +       (struct pt_regs *)((sp | (THREAD_SIZE - 1)) + 1 -               \
> +                          TOP_OF_KERNEL_STACK_PADDING) - 1;            \
>  })
>
>  /* Helpers for working with the user stack pointer */
> diff --git a/arch/loongarch/include/asm/stackframe.h b/arch/loongarch/include/asm/stackframe.h
> index 4ca953062..e9f849b29 100644
> --- a/arch/loongarch/include/asm/stackframe.h
> +++ b/arch/loongarch/include/asm/stackframe.h
> @@ -216,4 +216,5 @@
>         RESTORE_SP_AND_RET \docfi
>         .endm
>
> +#define _TOP_OF_KERNEL_STACK_PADDING 32
>  #endif /* _ASM_STACKFRAME_H */
> diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
> index eb3f641d5..290c53847 100644
> --- a/arch/loongarch/kernel/head.S
> +++ b/arch/loongarch/kernel/head.S
> @@ -75,7 +75,7 @@ SYM_CODE_START(kernel_entry)                  # kernel entry point
>
>         la.pcrel        tp, init_thread_union
>         /* Set the SP after an empty pt_regs.  */
> -       PTR_LI          sp, (_THREAD_SIZE - 32 - PT_SIZE)
> +       PTR_LI          sp, (_THREAD_SIZE - _TOP_OF_KERNEL_STACK_PADDING - PT_SIZE)
>         PTR_ADD         sp, sp, tp
>         set_saved_sp    sp, t0, t1
>         PTR_ADDI        sp, sp, -4 * SZREG      # init stack pointer
> diff --git a/arch/loongarch/kernel/process.c b/arch/loongarch/kernel/process.c
> index 660492f06..b51cf7975 100644
> --- a/arch/loongarch/kernel/process.c
> +++ b/arch/loongarch/kernel/process.c
> @@ -129,7 +129,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
>         unsigned long clone_flags = args->flags;
>         struct pt_regs *childregs, *regs = current_pt_regs();
>
> -       childksp = (unsigned long)task_stack_page(p) + THREAD_SIZE - 32;
> +       childksp = (unsigned long)task_stack_page(p) + THREAD_SIZE - TOP_OF_KERNEL_STACK_PADDING;
>
>         /* set up new TSS. */
>         childregs = (struct pt_regs *) childksp - 1;
> @@ -236,7 +236,7 @@ bool in_task_stack(unsigned long stack, struct task_struct *task,
>                         struct stack_info *info)
>  {
>         unsigned long begin = (unsigned long)task_stack_page(task);
> -       unsigned long end = begin + THREAD_SIZE - 32;
> +       unsigned long end = begin + THREAD_SIZE - TOP_OF_KERNEL_STACK_PADDING;
>
>         if (stack < begin || stack >= end)
>                 return false;
> diff --git a/arch/loongarch/kernel/switch.S b/arch/loongarch/kernel/switch.S
> index 43ebbc399..1797aceae 100644
> --- a/arch/loongarch/kernel/switch.S
> +++ b/arch/loongarch/kernel/switch.S
> @@ -26,7 +26,7 @@ SYM_FUNC_START(__switch_to)
>         move    tp, a2
>         cpu_restore_nonscratch a1
>
> -       li.w            t0, _THREAD_SIZE - 32
> +       li.w            t0, _THREAD_SIZE - _TOP_OF_KERNEL_STACK_PADDING
>         PTR_ADD         t0, t0, tp
>         set_saved_sp    t0, t1, t2
>
> --
> 2.31.1
>