Re: [ros-dev] [ros-diffs] [tfaber] 74305: [NTOS:KE] - Gracefully handle page faults during V86 code execution. This is a bit of a hack because with our limited use of V86 code it is unclear how a page fault can even occur. ...

[Thomas Faber]

I said it's unclear how, not that it doesn't happen ;)

This change fixes boot on the machine mentioned in
https://jira.reactos.org/browse/CORE-12993

What happens is there's a bug in the BIOS that causes a ret to an
broken address, then we execute garbage and fault on a stack address.
The stack didn't overflow so I'm not sure what caused the page to
become invalid; but the machine doesn't have a serial port and I'm at
the end of what I'm able (well, willing) to do with printf debugging.
Hence the hack. I'm always open to better suggestions :)


On 2017-04-14 18:51, Alex Ionescu wrote:
>   So why implement this code path if we have nothing
> that hits it?
>
>  Best regards,
>  Alex Ionescu
>
>  On Fri, Apr 14, 2017 at 4:18 AM, <tfaber(a)svn.reactos.org> wrote:
>
> > Author: tfaber
> > Date: Fri Apr 14 11:18:22 2017
> > New Revision: 74305
> >
> > URL: http://svn.reactos.org/svn/reactos?rev=74305&view=rev
> > Log:
> > [NTOS:KE]
> > - Gracefully handle page faults during V86 code execution. This is a bit
> > of a hack because with our limited use of V86 code it is unclear how a page
> > fault can even occur.
> > CORE-12993 #resolve
> >
> > Modified:
> >     trunk/reactos/ntoskrnl/include/internal/i386/ke.h
> >     trunk/reactos/ntoskrnl/ke/i386/traphdlr.c
> >     trunk/reactos/ntoskrnl/vdm/vdmexec.c
> >
> > Modified: trunk/reactos/ntoskrnl/include/internal/i386/ke.h
> > URL: http://svn.reactos.org/svn/reactos/trunk/reactos/
> >
> ntoskrnl/include/internal/i386/ke.h?rev=74305&r1=74304&r2=74305&view=diff
> > ============================================================
> > ==================
> > --- trunk/reactos/ntoskrnl/include/internal/i386/ke.h   [iso-8859-1]
> > (original)
> > +++ trunk/reactos/ntoskrnl/include/internal/i386/ke.h   [iso-8859-1] Fri
> > Apr 14 11:18:22 2017
> > @@ -478,6 +478,12 @@
> >  NTAPI
> >  VdmDispatchBop(
> >      IN PKTRAP_FRAME TrapFrame
> > +);
> > +
> > +BOOLEAN
> > +NTAPI
> > +VdmDispatchPageFault(
> > +    _In_ PKTRAP_FRAME TrapFrame
> >  );
> >
> >  BOOLEAN
> >
> > Modified: trunk/reactos/ntoskrnl/ke/i386/traphdlr.c
> > URL: http://svn.reactos.org/svn/reactos/trunk/reactos/
> > ntoskrnl/ke/i386/traphdlr.c?rev=74305&r1=74304&r2=74305&view=diff
> > ============================================================
> > ==================
> > --- trunk/reactos/ntoskrnl/ke/i386/traphdlr.c   [iso-8859-1] (original)
> > +++ trunk/reactos/ntoskrnl/ke/i386/traphdlr.c   [iso-8859-1] Fri Apr 14
> > 11:18:22 2017
> > @@ -1304,8 +1304,20 @@
> >          UNIMPLEMENTED_FATAL();
> >      }
> >  #endif
> > +
> >      /* Check for VDM trap */
> > -    ASSERT((KiVdmTrap(TrapFrame)) == FALSE);
> > +    if (KiVdmTrap(TrapFrame))
> > +    {
> > +        DPRINT1("VDM PAGE FAULT at %lx:%lx for address %lx\n",
> > +                TrapFrame->SegCs, TrapFrame->Eip, Cr2);
> > +        if (VdmDispatchPageFault(TrapFrame))
> > +        {
> > +            /* Return and end VDM execution */
> > +            DPRINT1("VDM page fault with status 0x%lx resolved\n",
> > Status);
> > +            KiEoiHelper(TrapFrame);
> > +        }
> > +        DPRINT1("VDM page fault with status 0x%lx NOT resolved\n",
> > Status);
> > +    }
> >
> >      /* Either kernel or user trap (non VDM) so dispatch exception */
> >      if (Status == STATUS_ACCESS_VIOLATION)
> >
> > Modified: trunk/reactos/ntoskrnl/vdm/vdmexec.c
> > URL: http://svn.reactos.org/svn/reactos/trunk/reactos/
> > ntoskrnl/vdm/vdmexec.c?rev=74305&r1=74304&r2=74305&view=diff
> > ============================================================
> > ==================
> > --- trunk/reactos/ntoskrnl/vdm/vdmexec.c        [iso-8859-1] (original)
> > +++ trunk/reactos/ntoskrnl/vdm/vdmexec.c        [iso-8859-1] Fri Apr 14
> > 11:18:22 2017
> > @@ -239,7 +239,7 @@
> >          KeLowerIrql(OldIrql);
> >          return STATUS_INVALID_SYSTEM_SERVICE;
> >      }
> > -
> > +
> >      /* Now do the VDM Swap */
> >      VdmSwapContext(VdmFrame, &VdmTib->MonitorContext, &VdmContext);
> >
> > @@ -269,7 +269,7 @@
> >
> >      /* Make a copy of the monitor context */
> >      Context = VdmTib->MonitorContext;
> > -
> > +
> >      /* Check if V86 mode was enabled or the trap was edited */
> >      if ((Context.EFlags & EFLAGS_V86_MASK) || (Context.SegCs &
> > FRAME_EDITED))
> >      {
> > @@ -291,7 +291,7 @@
> >                  /* Remove real IF flag */
> >                  VdmTib->VdmContext.EFlags &= ~EFLAGS_INTERRUPT_MASK;
> >              }
> > -
> > +
> >              /* Turn off VIP and VIF */
> >              TrapFrame->EFlags &= ~(EFLAGS_VIP | EFLAGS_VIF);
> >              VdmTib->VdmContext.EFlags &= ~(EFLAGS_VIP | EFLAGS_VIF);
> > @@ -362,4 +362,45 @@
> >      return TRUE;
> >  }
> >
> > -
> > +BOOLEAN
> > +NTAPI
> > +VdmDispatchPageFault(
> > +    _In_ PKTRAP_FRAME TrapFrame)
> > +{
> > +    NTSTATUS Status;
> > +    PVDM_TIB VdmTib;
> > +
> > +    PAGED_CODE();
> > +
> > +    /* Get the VDM TIB so we can terminate V86 execution */
> > +    Status = VdmpGetVdmTib(&VdmTib);
> > +    if (!NT_SUCCESS(Status))
> > +    {
> > +        /* Not a proper VDM fault, keep looking */
> > +        DPRINT1("VdmDispatchPageFault: no VDM TIB, Vdm=%p\n",
> > NtCurrentTeb()->Vdm);
> > +        return FALSE;
> > +    }
> > +
> > +    /* Must be coming from V86 code */
> > +    ASSERT(TrapFrame->EFlags & EFLAGS_V86_MASK);
> > +
> > +    _SEH2_TRY
> > +    {
> > +        /* Fill out a VDM Event */
> > +        VdmTib->EventInfo.Event = VdmMemAccess;
> > +        VdmTib->EventInfo.InstructionSize = 0;
> > +
> > +        /* End VDM Execution */
> > +        VdmEndExecution(TrapFrame, VdmTib);
> > +    }
> > +    _SEH2_EXCEPT(EXCEPTION_EXECUTE_HANDLER)
> > +    {
> > +        Status = _SEH2_GetExceptionCode();
> > +    }
> > +    _SEH2_END;
> > +
> > +    /* Consider the exception handled if we succeeded */
> > +    DPRINT1("VdmDispatchPageFault EFlags %lx exit with 0x%lx\n",
> > TrapFrame->EFlags, Status);
> > +    return NT_SUCCESS(Status);
> > +}
> > +
> >
> >