handler.c

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/*++
 
Module Name:
 
    syscall.c
 
Purpose:
 
    This module contains the implementation of the syscall C handler.
 
Author:
 
    slep (Matanel) 2025.
 
Revision History:
 
--*/
 
#include "../../includes/mt.h"
#include "../../includes/me.h"
#include "../../includes/ps.h"
#include "../../mtstatus.h"
 
extern SyscallHandler Ssdt[];
 
void
MtSyscallHandler(
    IN PTRAP_FRAME TrapFrame
)
 
/*++
 
    Routine description:
 
        Handles a system call from user mode.
 
    Arguments:
 
        The TRAP_FRAME of syscall entry.
 
    Return Values:
 
        Decided by the system call, could be void, or MTSTATUS (zero extended to RAX)
 
    Notes:
 
        This function must be called ONLY FROM the MtSyscallEntry routine in assembly.
 
--*/
 
{
    // Set previous mode to user mode, this is a system call.
    MeGetCurrentThread()->PreviousMode = UserMode;
 
    // Increment system call count (cool)
    InterlockedIncrementU64((volatile uint64_t*)&MeGetCurrentProcessor()->SystemCallCount);
 
    // Just for future incase. (this must be kept here since after interrupts are enabled UserRsp could very much change)
    // DO NOT Access the RSP in PTRAP_FRAME, it does not exist.
    // And DO NOT grab the UserRsp after this sti call, as it may change immediately even, save it to a local.
    //uint64_t* UserStack = (uint64_t*)MeGetCurrentProcessor()->UserRsp;
 
    // Enable interrupts, its safe now.
    __sti();    
    // Grab arguments
    // The return value is stored in RAX, and I dont want to do more assembly
    // Spare me.
    uint64_t* ReturnValue = &TrapFrame->rax;
 
    // Syscall number is in RAX.
    uint64_t SyscallNumber = TrapFrame->rax;
 
    // >= because 256 is an invalid index in the array (0-255)
    if (SyscallNumber >= MAX_SYSCALLS || Ssdt[SyscallNumber] == NULL) {
        *ReturnValue = MT_INVALID_SYSTEM_SERVICE;
        return;
    }
 
    // Arugments are in RDI RSI RDX R10 (not RCX in Syscalls, since CPU clobbers it for RIP) R8 R9
    // Above 6 arguments we receive from user stack.
    // For now, support 6.
    // To support more args we need a syscall that actually takes more than 6 args
    uint64_t Arg1 = TrapFrame->rdi;
    uint64_t Arg2 = TrapFrame->rsi;
    uint64_t Arg3 = TrapFrame->rdx;
    uint64_t Arg4 = TrapFrame->r10;
    uint64_t Arg5 = TrapFrame->r8;
    uint64_t Arg6 = TrapFrame->r9;
    
    // Todo regular SSDT. (with limits, no direct indexing)
    *ReturnValue = Ssdt[SyscallNumber](Arg1, Arg2, Arg3, Arg4, Arg5, Arg6);
}