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Message-ID: <1b5eb372-0e62-bdc8-ebdc-f49eab220d1b@xen0n.name>
Date: Sat, 14 May 2022 21:53:05 +0800
From: WANG Xuerui <kernel@...0n.name>
To: Huacai Chen <chenhuacai@...ngson.cn>,
Arnd Bergmann <arnd@...db.de>,
Andy Lutomirski <luto@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>,
Andrew Morton <akpm@...ux-foundation.org>,
David Airlie <airlied@...ux.ie>,
Jonathan Corbet <corbet@....net>,
Linus Torvalds <torvalds@...ux-foundation.org>
Cc: linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, Xuefeng Li <lixuefeng@...ngson.cn>,
Yanteng Si <siyanteng@...ngson.cn>,
Huacai Chen <chenhuacai@...il.com>,
Guo Ren <guoren@...nel.org>, Xuerui Wang <kernel@...0n.name>,
Jiaxun Yang <jiaxun.yang@...goat.com>,
Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH V10 13/22] LoongArch: Add system call support
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@...ngson.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@...ngson.cn>
> + * Huacai Chen <chenhuacai@...ngson.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], ®s->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@...ngson.cn>
> + * Huacai Chen <chenhuacai@...ngson.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@...ngson.cn>
> + * Huacai Chen <chenhuacai@...ngson.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);
> +}
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