[ros-kernel] got kmode SEH to work

[Royce Mitchell III]

however.... Skywing is pretty sure my fix isn't "correct".

By changing the return value in NtContinue() from STATUS_SUCCESS to 
(NTSTATUS)TrapFrame->Eax, I was able to get SEH to work correctly with 
the following code in drivers/dd/null/null.c. (It turns out EAX gets 
clobbered somewhere between NtContinue() returning and landing back at 
our excepted address - perhaps NtContinue should do it's own special 
dethunking since we *do* want to restore EAX also?)

static DWORD scratch=0;

EXCEPTION_DISPOSITION
__cdecl
_except_handler(
    struct _EXCEPTION_RECORD *ExceptionRecord,
    void * EstablisherFrame,
    PCONTEXT ContextRecord,
    void * DispatcherContext )
{
    // Indicate that we made it to our exception handler
    DbgPrint ( "null.sys:_except_handler() EAX=0x%x, EIP=0x%x, ESP=0x%x, 
EBP=0x%x\n",
        ContextRecord->Eax, ContextRecord->Eip, ContextRecord->Esp, 
ContextRecord->Ebp );

    // Change EAX in the context record so that it points to someplace
    // where we can successfully write
    ContextRecord->Eax = (DWORD)&scratch;

    DbgPrint ( "null.sys:_except_handler() changed EAX to 0x%x", 
ContextRecord->Eax );

    // Tell the OS to restart the faulting instruction
    return ExceptionContinueExecution;
}

[snip]

    case IRP_MJ_CREATE:
    case IRP_MJ_CLOSE:
        {
            DWORD handler = (DWORD)_except_handler;
            DbgPrint ( "null.sys.NullDispatch(): setting up SEH 'frame', 
_except_handler=0x%x\n", handler );
            __asm__ ("\tpushl %0\n" : "=m" (handler) );
            __asm__ ("\tpushl %fs:(0)\n" );
            __asm__ ("\tmovl %esp,%fs:(0)\n" );
            DbgPrint ( "null.sys.NullDispatch(): inside SEH 'frame' - 
about to cause exception\n" );
           
            __asm__ ("\tmovl $0, %eax\n"
                "\tmovl $1, (%eax)\n");

            DbgPrint ( "null.sys.NullDispatch(): after exception, 
closing SEH 'frame'\n" );

            __asm__ ("\tmovl (%esp),%eax\n"
                "\tmovl %eax, %fs:(0)\n"
                "\taddl $8, %esp\n");


Skywing said he'll respond - or whatever - with anything else he finds, 
but I'm going to bed... way too late...