Taken from https://github.com/nelhage/reptyr/commit/e26724cc1ae5fe7af0c9fb6369f6cf09d1d12900

From ae0b4ec014c1a01b1c3409e5404cf0fa0102c349 Mon Sep 17 00:00:00 2001
From: Ast-x64 <Ast-x64@protonmail.com>
Date: Wed, 10 Nov 2021 09:39:45 +0800
Subject: [PATCH] Support riscv64 on Linux.

---
 platform/linux/arch/riscv64.h | 68 +++++++++++++++++++++++++++++++++++
 platform/linux/linux_ptrace.c |  2 ++
 ptrace.h                      |  3 ++
 3 files changed, 73 insertions(+)
 create mode 100644 platform/linux/arch/riscv64.h

diff --git a/platform/linux/arch/riscv64.h b/platform/linux/arch/riscv64.h
new file mode 100644
index 0000000..96221c3
--- /dev/null
+++ b/platform/linux/arch/riscv64.h
@@ -0,0 +1,68 @@
+/*
+ * Copyright (C) 2021 by Ast-x64
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+static struct ptrace_personality arch_personality[1] = {
+    {
+        offsetof(struct user_regs_struct, a0),
+        offsetof(struct user_regs_struct, a0),
+        offsetof(struct user_regs_struct, a1),
+        offsetof(struct user_regs_struct, a2),
+        offsetof(struct user_regs_struct, a3),
+        offsetof(struct user_regs_struct, a4),
+        offsetof(struct user_regs_struct, a5),
+        offsetof(struct user_regs_struct, pc),
+    }
+};
+
+static inline void arch_fixup_regs(struct ptrace_child *child) {
+    child->regs.pc -= 4;
+}
+
+static inline int arch_set_syscall(struct ptrace_child *child,
+                                   unsigned long sysno) {
+    unsigned long x_reg[18];
+    struct iovec reg_iovec = {
+        .iov_base = x_reg,
+        .iov_len = sizeof(x_reg)
+    };
+    if (ptrace_command(child, PTRACE_GETREGSET, NT_PRSTATUS, &reg_iovec) < 0)
+        return -1;
+
+    x_reg[17] = sysno;
+    return ptrace_command(child, PTRACE_SETREGSET, NT_PRSTATUS, &reg_iovec);
+}
+
+static inline int arch_save_syscall(struct ptrace_child *child) {
+    unsigned long x_reg[18];
+    struct iovec reg_iovec = {
+        .iov_base = x_reg,
+        .iov_len = sizeof(x_reg)
+    };
+    if (ptrace_command(child, PTRACE_GETREGSET, NT_PRSTATUS, &reg_iovec) < 0)
+        return -1;
+
+    child->saved_syscall = x_reg[17];
+    return 0;
+}
+
+static inline int arch_restore_syscall(struct ptrace_child *child) {
+    return arch_set_syscall(child, child->saved_syscall);
+}
diff --git a/platform/linux/linux_ptrace.c b/platform/linux/linux_ptrace.c
index d065199..bcbe600 100644
--- a/platform/linux/linux_ptrace.c
+++ b/platform/linux/linux_ptrace.c
@@ -84,6 +84,8 @@ static struct ptrace_personality *personality(struct ptrace_child *child);
 #include "arch/aarch64.h"
 #elif defined(__powerpc__)
 #include "arch/powerpc.h"
+#elif defined(__riscv) && __riscv_xlen == 64
+#include "arch/riscv64.h"
 #else
 #error Unsupported architecture.
 #endif
diff --git a/ptrace.h b/ptrace.h
index ee05bd7..8e3a7f4 100644
--- a/ptrace.h
+++ b/ptrace.h
@@ -25,6 +25,9 @@
 #ifdef __powerpc__
 #include <asm/ptrace.h>
 #endif
+#ifdef __riscv
+#include <asm/ptrace.h>
+#endif
 #include <sys/ptrace.h>
 #include <sys/types.h>
 #include <sys/user.h>