Re: [PATCH] LoongArch: Mark kernel stack padding as TOP_OF_KERNEL_STACK_PADDING

From: Huacai Chen
Date: Tue Sep 06 2022 - 03:00:58 EST

Maybe we can simply remove it, but I still want to ask Xuerui and
Ruoyao's advices.

Huacai

On Tue, Sep 6, 2022 at 2:06 PM Jinyang He <hejinyang@xxxxxxxxxxx> wrote:
>
> Hi, Huacai,
>
> On 2022/9/5 14:21, Huacai Chen wrote:
> > Hi, Jinyang,
> >
> > On Mon, Sep 5, 2022 at 1:39 PM Jinyang He <hejinyang@xxxxxxxxxxx> wrote:
> >> On 2022/9/4 20:19, Huacai Chen wrote:
> >>
> >>> Hi, Jinyang,
> >>>
> >>> I don't know what scenarios this padding is used and how to use.
> >> To be frankly, I don't know its scenarios on LoongArch. It looks like
> >> the MIPS legacy. This patch only keep this padding, which can be
> >> easily modified if needed in the future. Now we can completely
> >> set it to 0 or remove it. Actually, it can be thought of as another
> >> buffer for thread_info, while thread_info usually limit to one cache line.
> > We should make it clear how MIPS use it at least, then we can make a
> > decision whether to remove it or not, otherwise it may break
> > something.
> I think it is related to shadow parameters area of MIPS o32 Calling
> Convention.
> This happened a long time ago, starting is 24, then 4*SZREG, and now 32.
> But there are few commit-logs I can find. See links. I guess these changes
> are due to the ABI, the macro SZREG in [2] is referenced to check stack
> alignment. And pt_regs and sigframe also have padding. If we can't find
> what will be broken, then we should cancel it rather than keeping it.
> We wouldn't have these paddings if we weren't inspired by MIPS.
>
> [1] https://elixir.bootlin.com/linux/1.1.82/source/arch/mips/kernel/head.S
> [2] https://elixir.bootlin.com/linux/2.1.44/source/arch/mips/kernel/head.S
> [3] https://elixir.bootlin.com/linux/2.1.73/source/arch/mips/kernel/head.S
> [4]
> https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit?id=484889fc85a223ef56edc31828b86751b2296b7c
> >>> 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.
> >> How about put it into loongarch.h if we need this macro?
> > That is good because we only need one macro, but you can research
> > whether there is a better place.
>
> As asm and C code need it, how about put it in output_thread_info_defines?
>
> Thanks,
>
> Jinyang
>
> >>> On Fri, Sep 2, 2022 at 11:01 PM Jinyang He <hejinyang@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> >>>> ---
> >>>> 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
> >>>>
>