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Message-ID: <1b5eb372-0e62-bdc8-ebdc-f49eab220d1b@xen0n.name>
Date:   Sat, 14 May 2022 21:53:05 +0800
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Subject: Re: [PATCH V10 13/22] LoongArch: Add system call support
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To:     Huacai Chen <chenhuacai@loongson.cn>,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Airlie <airlied@linux.ie>,
        Jonathan Corbet <corbet@lwn.net>,
        Linus Torvalds <torvalds@linux-foundation.org>
Cc:     linux-arch@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, Xuefeng Li <lixuefeng@loongson.cn>,
        Yanteng Si <siyanteng@loongson.cn>,
        Huacai Chen <chenhuacai@gmail.com>,
        Guo Ren <guoren@kernel.org>, Xuerui Wang <kernel@xen0n.name>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>
References: <20220514080402.2650181-1-chenhuacai@loongson.cn>
 <20220514080402.2650181-14-chenhuacai@loongson.cn>
From:   WANG Xuerui <kernel@xen0n.name>
In-Reply-To: <20220514080402.2650181-14-chenhuacai@loongson.cn>
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Precedence: bulk

Hi,

On 5/14/22 16:03, Huacai Chen wrote:
> Add system call support and related uaccess.h for LoongArch.
>
> Q: Why keep _ARCH_WANT_SYS_CLONE definition while there is clone3:
> A: The latest glibc release (2.35) has some basic support for clone3 but
>     it isn't complete. E.g., pthread_create() and spawni() have converted
>     to use clone3 but fork() will still use clone. Moreover, some seccomp
>     related applications can still not work perfectly with clone3. E.g.,
>     Chromium sandbox cannot work at all and there is no solution for it,
>     which is more terrible than the fork() story [1].
>
> [1] https://chromium-review.googlesource.com/c/chromium/src/+/2936184
>
> Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
> ---
>   arch/loongarch/include/asm/seccomp.h     |  20 ++
>   arch/loongarch/include/asm/syscall.h     |  74 +++++++
>   arch/loongarch/include/asm/uaccess.h     | 269 +++++++++++++++++++++++
>   arch/loongarch/include/asm/unistd.h      |  11 +
>   arch/loongarch/include/uapi/asm/unistd.h |   5 +
>   arch/loongarch/kernel/entry.S            |  89 ++++++++
>   arch/loongarch/kernel/syscall.c          |  63 ++++++
>   7 files changed, 531 insertions(+)
>   create mode 100644 arch/loongarch/include/asm/seccomp.h
>   create mode 100644 arch/loongarch/include/asm/syscall.h
>   create mode 100644 arch/loongarch/include/asm/uaccess.h
>   create mode 100644 arch/loongarch/include/asm/unistd.h
>   create mode 100644 arch/loongarch/include/uapi/asm/unistd.h
>   create mode 100644 arch/loongarch/kernel/entry.S
>   create mode 100644 arch/loongarch/kernel/syscall.c
>
> diff --git a/arch/loongarch/include/asm/seccomp.h b/arch/loongarch/include/asm/seccomp.h
> new file mode 100644
> index 000000000000..31d6ab42e43e
> --- /dev/null
> +++ b/arch/loongarch/include/asm/seccomp.h
> @@ -0,0 +1,20 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _ASM_SECCOMP_H
> +#define _ASM_SECCOMP_H
> +
> +#include <asm/unistd.h>
> +
> +#include <asm-generic/seccomp.h>
> +
> +#ifdef CONFIG_32BIT
> +# define SECCOMP_ARCH_NATIVE		AUDIT_ARCH_LOONGARCH32
> +# define SECCOMP_ARCH_NATIVE_NR		NR_syscalls
> +# define SECCOMP_ARCH_NATIVE_NAME	"loongarch32"
> +#else
> +# define SECCOMP_ARCH_NATIVE		AUDIT_ARCH_LOONGARCH64
> +# define SECCOMP_ARCH_NATIVE_NR		NR_syscalls
> +# define SECCOMP_ARCH_NATIVE_NAME	"loongarch64"
> +#endif
> +
> +#endif /* _ASM_SECCOMP_H */
> diff --git a/arch/loongarch/include/asm/syscall.h b/arch/loongarch/include/asm/syscall.h
> new file mode 100644
> index 000000000000..e286dc58476e
> --- /dev/null
> +++ b/arch/loongarch/include/asm/syscall.h
> @@ -0,0 +1,74 @@
> +/* SPDX-License-Identifier: GPL-2.0+ */
> +/*
> + * Author: Hanlu Li <lihanlu@loongson.cn>
> + *         Huacai Chen <chenhuacai@loongson.cn>
> + *
> + * Copyright (C) 2020-2022 Loongson Technology Corporation Limited
> + */
> +
> +#ifndef __ASM_LOONGARCH_SYSCALL_H
> +#define __ASM_LOONGARCH_SYSCALL_H
> +
> +#include <linux/compiler.h>
> +#include <uapi/linux/audit.h>
> +#include <linux/elf-em.h>
> +#include <linux/kernel.h>
> +#include <linux/sched.h>
> +#include <linux/uaccess.h>
> +#include <asm/ptrace.h>
> +#include <asm/unistd.h>
> +
> +extern void *sys_call_table[];
> +
> +static inline long syscall_get_nr(struct task_struct *task,
> +				  struct pt_regs *regs)
> +{
> +	return regs->regs[11];
It may be better readability-wise to somehow change this 11 to something 
resembling A7, but it's okay as-is.
> +}
> +
> +static inline void syscall_rollback(struct task_struct *task,
> +				    struct pt_regs *regs)
> +{
> +        regs->regs[4] = regs->orig_a0;
> +}
> +
> +static inline long syscall_get_error(struct task_struct *task,
> +				     struct pt_regs *regs)
> +{
> +	unsigned long error = regs->regs[4];
> +
> +	return IS_ERR_VALUE(error) ? error : 0;
> +}
> +
> +static inline long syscall_get_return_value(struct task_struct *task,
> +					    struct pt_regs *regs)
> +{
> +	return regs->regs[4];
> +}
> +
> +static inline void syscall_set_return_value(struct task_struct *task,
> +					    struct pt_regs *regs,
> +					    int error, long val)
> +{
> +	regs->regs[4] = (long) error ? error : val;
> +}
> +
> +static inline void syscall_get_arguments(struct task_struct *task,
> +					 struct pt_regs *regs,
> +					 unsigned long *args)
> +{
> +	args[0] = regs->orig_a0;
> +	memcpy(&args[1], &regs->regs[5], 5 * sizeof(long));
> +}
> +
> +static inline int syscall_get_arch(struct task_struct *task)
> +{
> +	return AUDIT_ARCH_LOONGARCH64;
> +}
> +
> +static inline bool arch_syscall_is_vdso_sigreturn(struct pt_regs *regs)
> +{
> +	return false;
> +}
> +
> +#endif	/* __ASM_LOONGARCH_SYSCALL_H */
> diff --git a/arch/loongarch/include/asm/uaccess.h b/arch/loongarch/include/asm/uaccess.h
> new file mode 100644
> index 000000000000..217c6a3727b1
> --- /dev/null
> +++ b/arch/loongarch/include/asm/uaccess.h
> @@ -0,0 +1,269 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Copyright (C) 2020-2022 Loongson Technology Corporation Limited
> + *
> + * Derived from MIPS:
> + * Copyright (C) 1996, 1997, 1998, 1999, 2000, 03, 04 by Ralf Baechle
> + * Copyright (C) 1999, 2000 Silicon Graphics, Inc.
> + * Copyright (C) 2007  Maciej W. Rozycki
> + * Copyright (C) 2014, Imagination Technologies Ltd.
> + */
> +#ifndef _ASM_UACCESS_H
> +#define _ASM_UACCESS_H
> +
> +#include <linux/kernel.h>
> +#include <linux/string.h>
> +#include <linux/extable.h>
> +#include <asm/pgtable.h>
> +#include <asm-generic/extable.h>
> +#include <asm-generic/access_ok.h>
> +
> +extern u64 __ua_limit;
> +
> +#define __UA_ADDR	".dword"
> +#define __UA_LA		"la.abs"
> +#define __UA_LIMIT	__ua_limit
> +
> +/*
> + * get_user: - Get a simple variable from user space.
> + * @x:	 Variable to store result.
> + * @ptr: Source address, in user space.
> + *
> + * Context: User context only. This function may sleep if pagefaults are
> + *          enabled.
> + *
> + * This macro copies a single simple variable from user space to kernel
> + * space.  It supports simple types like char and int, but not larger
> + * data types like structures or arrays.
> + *
> + * @ptr must have pointer-to-simple-variable type, and the result of
> + * dereferencing @ptr must be assignable to @x without a cast.
> + *
> + * Returns zero on success, or -EFAULT on error.
> + * On error, the variable @x is set to zero.
> + */
> +#define get_user(x, ptr) \
> +({									\
> +	const __typeof__(*(ptr)) __user *__p = (ptr);			\
> +									\
> +	might_fault();							\
> +	access_ok(__p, sizeof(*__p)) ? __get_user((x), __p) :		\
> +				       ((x) = 0, -EFAULT);		\
> +})
> +
> +/*
> + * put_user: - Write a simple value into user space.
> + * @x:	 Value to copy to user space.
> + * @ptr: Destination address, in user space.
> + *
> + * Context: User context only. This function may sleep if pagefaults are
> + *          enabled.
> + *
> + * This macro copies a single simple value from kernel space to user
> + * space.  It supports simple types like char and int, but not larger
> + * data types like structures or arrays.
> + *
> + * @ptr must have pointer-to-simple-variable type, and @x must be assignable
> + * to the result of dereferencing @ptr.
> + *
> + * Returns zero on success, or -EFAULT on error.
> + */
> +#define put_user(x, ptr) \
> +({									\
> +	__typeof__(*(ptr)) __user *__p = (ptr);				\
> +									\
> +	might_fault();							\
> +	access_ok(__p, sizeof(*__p)) ? __put_user((x), __p) : -EFAULT;	\
> +})
> +
> +/*
> + * __get_user: - Get a simple variable from user space, with less checking.
> + * @x:	 Variable to store result.
> + * @ptr: Source address, in user space.
> + *
> + * Context: User context only. This function may sleep if pagefaults are
> + *          enabled.
> + *
> + * This macro copies a single simple variable from user space to kernel
> + * space.  It supports simple types like char and int, but not larger
> + * data types like structures or arrays.
> + *
> + * @ptr must have pointer-to-simple-variable type, and the result of
> + * dereferencing @ptr must be assignable to @x without a cast.
> + *
> + * Caller must check the pointer with access_ok() before calling this
> + * function.
> + *
> + * Returns zero on success, or -EFAULT on error.
> + * On error, the variable @x is set to zero.
> + */
> +#define __get_user(x, ptr) \
> +({									\
> +	int __gu_err = 0;						\
> +									\
> +	__chk_user_ptr(ptr);						\
> +	__get_user_common((x), sizeof(*(ptr)), ptr);			\
> +	__gu_err;							\
> +})
> +
> +/*
> + * __put_user: - Write a simple value into user space, with less checking.
> + * @x:	 Value to copy to user space.
> + * @ptr: Destination address, in user space.
> + *
> + * Context: User context only. This function may sleep if pagefaults are
> + *          enabled.
> + *
> + * This macro copies a single simple value from kernel space to user
> + * space.  It supports simple types like char and int, but not larger
> + * data types like structures or arrays.
> + *
> + * @ptr must have pointer-to-simple-variable type, and @x must be assignable
> + * to the result of dereferencing @ptr.
> + *
> + * Caller must check the pointer with access_ok() before calling this
> + * function.
> + *
> + * Returns zero on success, or -EFAULT on error.
> + */
> +#define __put_user(x, ptr) \
> +({									\
> +	int __pu_err = 0;						\
> +	__typeof__(*(ptr)) __pu_val;					\
> +									\
> +	__pu_val = (x);							\
> +	__chk_user_ptr(ptr);						\
> +	__put_user_common(ptr, sizeof(*(ptr)));				\
> +	__pu_err;							\
> +})
> +
> +struct __large_struct { unsigned long buf[100]; };
> +#define __m(x) (*(struct __large_struct __user *)(x))
> +
> +#define __get_user_common(val, size, ptr)				\
> +do {									\
> +	switch (size) {							\
> +	case 1: __get_data_asm(val, "ld.b", ptr); break;		\
> +	case 2: __get_data_asm(val, "ld.h", ptr); break;		\
> +	case 4: __get_data_asm(val, "ld.w", ptr); break;		\
> +	case 8: __get_data_asm(val, "ld.d", ptr); break;		\
> +	default: BUILD_BUG(); break;					\
> +	}								\
> +} while (0)
> +
> +#define __get_kernel_common(val, size, ptr) __get_user_common(val, size, ptr)
> +
> +#define __get_data_asm(val, insn, ptr)					\
> +{									\
> +	long __gu_tmp;							\
> +									\
> +	__asm__ __volatile__(						\
> +	"1:	" insn "	%1, %2				\n"	\
> +	"2:							\n"	\
> +	"	.section .fixup,\"ax\"				\n"	\
> +	"3:	li.w	%0, %3					\n"	\
> +	"	or	%1, $r0, $r0				\n"	\
> +	"	b	2b					\n"	\
> +	"	.previous					\n"	\
> +	"	.section __ex_table,\"a\"			\n"	\
> +	"	"__UA_ADDR "\t1b, 3b				\n"	\
> +	"	.previous					\n"	\
> +	: "+r" (__gu_err), "=r" (__gu_tmp)				\
> +	: "m" (__m(ptr)), "i" (-EFAULT));				\
> +									\
> +	(val) = (__typeof__(*(ptr))) __gu_tmp;				\
> +}
> +
> +#define __put_user_common(ptr, size)					\
> +do {									\
> +	switch (size) {							\
> +	case 1: __put_data_asm("st.b", ptr); break;			\
> +	case 2: __put_data_asm("st.h", ptr); break;			\
> +	case 4: __put_data_asm("st.w", ptr); break;			\
> +	case 8: __put_data_asm("st.d", ptr); break;			\
> +	default: BUILD_BUG(); break;					\
> +	}								\
> +} while (0)
> +
> +#define __put_kernel_common(ptr, size) __put_user_common(ptr, size)
> +
> +#define __put_data_asm(insn, ptr)					\
> +{									\
> +	__asm__ __volatile__(						\
> +	"1:	" insn "	%z2, %1		# __put_user_asm\n"	\
> +	"2:							\n"	\
> +	"	.section	.fixup,\"ax\"			\n"	\
> +	"3:	li.w	%0, %3					\n"	\
> +	"	b	2b					\n"	\
> +	"	.previous					\n"	\
> +	"	.section	__ex_table,\"a\"		\n"	\
> +	"	" __UA_ADDR "	1b, 3b				\n"	\
> +	"	.previous					\n"	\
> +	: "+r" (__pu_err), "=m" (__m(ptr))				\
> +	: "Jr" (__pu_val), "i" (-EFAULT));				\
> +}
> +
> +#define __get_kernel_nofault(dst, src, type, err_label)			\
> +do {									\
> +	int __gu_err = 0;						\
> +									\
> +	__get_kernel_common(*((type *)(dst)), sizeof(type),		\
> +			    (__force type *)(src));			\
> +	if (unlikely(__gu_err))						\
> +		goto err_label;						\
> +} while (0)
> +
> +#define __put_kernel_nofault(dst, src, type, err_label)			\
> +do {									\
> +	type __pu_val;							\
> +	int __pu_err = 0;						\
> +									\
> +	__pu_val = *(__force type *)(src);				\
> +	__put_kernel_common(((type *)(dst)), sizeof(type));		\
> +	if (unlikely(__pu_err))						\
> +		goto err_label;						\
> +} while (0)
> +
> +extern unsigned long __copy_user(void *to, const void *from, __kernel_size_t n);
> +
> +static inline unsigned long __must_check
> +raw_copy_from_user(void *to, const void __user *from, unsigned long n)
> +{
> +	return __copy_user(to, from, n);
> +}
> +
> +static inline unsigned long __must_check
> +raw_copy_to_user(void __user *to, const void *from, unsigned long n)
> +{
> +	return __copy_user(to, from, n);
> +}
> +
> +#define INLINE_COPY_FROM_USER
> +#define INLINE_COPY_TO_USER
> +
> +/*
> + * __clear_user: - Zero a block of memory in user space, with less checking.
> + * @addr: Destination address, in user space.
> + * @size: Number of bytes to zero.
> + *
> + * Zero a block of memory in user space.  Caller must check
> + * the specified block with access_ok() before calling this function.
> + *
> + * Returns number of bytes that could not be cleared.
> + * On success, this will be zero.
> + */
> +extern unsigned long __clear_user(void __user *addr, __kernel_size_t size);
> +
> +#define clear_user(addr, n)						\
> +({									\
> +	void __user *__cl_addr = (addr);				\
> +	unsigned long __cl_size = (n);					\
> +	if (__cl_size && access_ok(__cl_addr, __cl_size))		\
> +		__cl_size = __clear_user(__cl_addr, __cl_size);		\
> +	__cl_size;							\
> +})
> +
> +extern long strncpy_from_user(char *to, const char __user *from, long n);
> +extern long strnlen_user(const char __user *str, long n);
> +
> +#endif /* _ASM_UACCESS_H */
> diff --git a/arch/loongarch/include/asm/unistd.h b/arch/loongarch/include/asm/unistd.h
> new file mode 100644
> index 000000000000..cfddb0116a8c
> --- /dev/null
> +++ b/arch/loongarch/include/asm/unistd.h
> @@ -0,0 +1,11 @@
> +/* SPDX-License-Identifier: GPL-2.0+ */
> +/*
> + * Author: Hanlu Li <lihanlu@loongson.cn>
> + *         Huacai Chen <chenhuacai@loongson.cn>
> + *
> + * Copyright (C) 2020-2022 Loongson Technology Corporation Limited
> + */
> +
> +#include <uapi/asm/unistd.h>
> +
> +#define NR_syscalls (__NR_syscalls)
> diff --git a/arch/loongarch/include/uapi/asm/unistd.h b/arch/loongarch/include/uapi/asm/unistd.h
> new file mode 100644
> index 000000000000..fcb668984f03
> --- /dev/null
> +++ b/arch/loongarch/include/uapi/asm/unistd.h
> @@ -0,0 +1,5 @@
> +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> +#define __ARCH_WANT_SYS_CLONE
> +#define __ARCH_WANT_SYS_CLONE3
Having read the discussion in v9 regarding the sandbox scenario, I agree 
leaving clone alone would probably be the better choice. It's relieving 
seeing the stat cruft dropped... World rebuilds of new-world distros 
pending on the horizon. :-D
> +
> +#include <asm-generic/unistd.h>
> diff --git a/arch/loongarch/kernel/entry.S b/arch/loongarch/kernel/entry.S
> new file mode 100644
> index 000000000000..d5b3dbcf5425
> --- /dev/null
> +++ b/arch/loongarch/kernel/entry.S
> @@ -0,0 +1,89 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Copyright (C) 2020-2022 Loongson Technology Corporation Limited
> + *
> + * Derived from MIPS:
> + * Copyright (C) 1994 - 2000, 2001, 2003 Ralf Baechle
> + * Copyright (C) 1999, 2000 Silicon Graphics, Inc.
> + * Copyright (C) 2001 MIPS Technologies, Inc.
> + */
> +
> +#include <asm/asm.h>
> +#include <asm/asmmacro.h>
> +#include <asm/loongarch.h>
> +#include <asm/regdef.h>
> +#include <asm/stackframe.h>
> +#include <asm/thread_info.h>
> +
> +	.text
> +	.cfi_sections	.debug_frame
> +	.align	5
> +SYM_FUNC_START(handle_syscall)
> +	csrrd	t0, PERCPU_BASE_KS
> +	la.abs	t1, kernelsp
> +	add.d	t1, t1, t0
> +	move	t2, sp
> +	ld.d	sp, t1, 0
> +
> +	addi.d	sp, sp, -PT_SIZE
> +	cfi_st	t2, PT_R3
> +	cfi_rel_offset  sp, PT_R3
> +	st.d	zero, sp, PT_R0
> +	csrrd	t2, LOONGARCH_CSR_PRMD
> +	st.d	t2, sp, PT_PRMD
> +	csrrd	t2, LOONGARCH_CSR_CRMD
> +	st.d	t2, sp, PT_CRMD
> +	csrrd	t2, LOONGARCH_CSR_EUEN
> +	st.d	t2, sp, PT_EUEN
> +	csrrd	t2, LOONGARCH_CSR_ECFG
> +	st.d	t2, sp, PT_ECFG
> +	csrrd	t2, LOONGARCH_CSR_ESTAT
> +	st.d	t2, sp, PT_ESTAT
> +	cfi_st	ra, PT_R1
> +	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
> +	csrrd	ra, LOONGARCH_CSR_ERA
> +	st.d	ra, sp, PT_ERA
> +	cfi_rel_offset ra, PT_ERA
> +
> +	cfi_st	tp, PT_R2
> +	cfi_st	u0, PT_R21
> +	cfi_st	fp, PT_R22
> +
> +	SAVE_STATIC
> +
> +	move	u0, t0
> +	li.d	tp, ~_THREAD_MASK
> +	and	tp, tp, sp
> +
> +	move	a0, sp
> +	bl	do_syscall
> +
> +	RESTORE_ALL_AND_RET
> +SYM_FUNC_END(handle_syscall)
> +
> +SYM_CODE_START(ret_from_fork)
> +	bl	schedule_tail		# a0 = struct task_struct *prev
> +	move	a0, sp
> +	bl 	syscall_exit_to_user_mode
> +	RESTORE_STATIC
> +	RESTORE_SOME
> +	RESTORE_SP_AND_RET
> +SYM_CODE_END(ret_from_fork)
> +
> +SYM_CODE_START(ret_from_kernel_thread)
> +	bl	schedule_tail		# a0 = struct task_struct *prev
> +	move	a0, s1
> +	jirl	ra, s0, 0
> +	move	a0, sp
> +	bl	syscall_exit_to_user_mode
> +	RESTORE_STATIC
> +	RESTORE_SOME
> +	RESTORE_SP_AND_RET
> +SYM_CODE_END(ret_from_kernel_thread)
> diff --git a/arch/loongarch/kernel/syscall.c b/arch/loongarch/kernel/syscall.c
> new file mode 100644
> index 000000000000..3fc4211db989
> --- /dev/null
> +++ b/arch/loongarch/kernel/syscall.c
> @@ -0,0 +1,63 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +/*
> + * Author: Hanlu Li <lihanlu@loongson.cn>
> + *         Huacai Chen <chenhuacai@loongson.cn>
> + *
> + * Copyright (C) 2020-2022 Loongson Technology Corporation Limited
> + */
> +#include <linux/capability.h>
> +#include <linux/entry-common.h>
> +#include <linux/errno.h>
> +#include <linux/linkage.h>
> +#include <linux/syscalls.h>
> +#include <linux/unistd.h>
> +
> +#include <asm/asm.h>
> +#include <asm/signal.h>
> +#include <asm/switch_to.h>
> +#include <asm-generic/syscalls.h>
> +
> +#undef __SYSCALL
> +#define __SYSCALL(nr, call)	[nr] = (call),
> +
> +SYSCALL_DEFINE6(mmap, unsigned long, addr, unsigned long, len, unsigned long,
> +		prot, unsigned long, flags, unsigned long, fd, off_t, offset)
> +{
> +	if (offset & ~PAGE_MASK)
> +		return -EINVAL;
> +
> +	return ksys_mmap_pgoff(addr, len, prot, flags, fd, offset >> PAGE_SHIFT);
> +}
> +
> +void *sys_call_table[__NR_syscalls] = {
> +	[0 ... __NR_syscalls - 1] = sys_ni_syscall,
> +#include <asm/unistd.h>
> +};
> +
> +typedef long (*sys_call_fn)(unsigned long, unsigned long,
> +	unsigned long, unsigned long, unsigned long, unsigned long);
> +
> +void noinstr do_syscall(struct pt_regs *regs)
> +{
> +	unsigned long nr;
> +	sys_call_fn syscall_fn;
> +
> +	nr = regs->regs[11];
> +	/* Set for syscall restarting */
> +	if (nr < NR_syscalls)
> +		regs->regs[0] = nr + 1;
> +
> +	regs->csr_era += 4;
> +	regs->orig_a0 = regs->regs[4];
> +	regs->regs[4] = -ENOSYS;
> +
> +	nr = syscall_enter_from_user_mode(regs, nr);
> +
> +	if (nr < NR_syscalls) {

I saw in the manual that there's the BADI (出错指令) CSR that records the 
faulting instruction word for exceptions other than INT, GCHC and MERR; 
this implies we can read the trapping syscall insn word from this CSR, 
to inspect its immediate field, so we can reserve non-zero immediates on 
the syscall insn for future use.

This would take 3 insns to complete: csrrd, bitfield extraction, and 
bail if non-zero; this should be an acceptable tradeoff for a lot of 
extensibility and scalability?

> +		syscall_fn = sys_call_table[nr];
> +		regs->regs[4] = syscall_fn(regs->orig_a0, regs->regs[5], regs->regs[6],
> +					   regs->regs[7], regs->regs[8], regs->regs[9]);
> +	}
> +
> +	syscall_exit_to_user_mode(regs);
> +}