Handout

kernel/syscall/handler.asm

@@ -0,0 +1,41 @@
+; Generic high-level syscall handler
+[EXTERN syscall_handler]
+
+; Low-level interrupt based system call entry function
+[GLOBAL syscall_entry]
+
+[SECTION .text]
+
+; The SystemV ABI for x64 uses the register `rdi`, `rsi`, `rdx`, `rcx`, `r8` and
+; `r9` to pass the first six parameters of a function. Since  every syscall can
+; have a maximum of 5 parameters, these register will just be forwarded (without
+; requiring any additional operation), having the syscall number stored in `rdi`.
+; The return value will be stored in `rax`.
+ALIGN 8
+syscall_entry:
+	; Interrupt Context Pointer (7th Parameter)
+	push rsp
+
+	; Clear direction flag for string operations
+	cld
+
+	; Call the high-level (C++) system call handler
+	call syscall_handler
+
+	; Optional: Prevent kernel information leakage
+	; by zeroing scratch registers
+	mov rdi, 0
+	mov rsi, 0
+	mov rdx, 0
+	mov rcx, 0
+	mov r8,  0
+	mov r9,  0
+	mov r10, 0
+	mov r11, 0
+
+	; Drop 7th parameter
+	add rsp, 8
+
+	; Return from interrupt entry function
+	iretq
+