The R0_ReadFile and R0_WriteFile macros in my ifsmgr.h follow an incorrect entry in the DDK documentation
to the effect that the number of bytes actually read or written comes back in ECX. In reality, this
value comes back in EAX. The macros should therefore be as follows:
int VXDINLINE R0_ReadFile(BOOL incontext, DWORD handle, DWORD count, DWORD pos, PBYTE buffer, PDWORD nread)
{
DWORD opcode = incontext ? R0_READFILE_IN_CONTEXT : R0_READFILE;
WORD result;
_asm
{
mov eax, opcode
mov ebx, handle
mov ecx, count
mov edx, pos
mov esi, buffer
}
VxDCall(IFSMgr_Ring0_FileIO)
_asm
{
jc error
mov ebx, nread
mov [ebx], eax <== this is the change
xor eax, eax
error:
mov result, ax
}
return result;
}
int VXDINLINE R0_WriteFile(BOOL incontext, DWORD handle, DWORD count, DWORD pos, const BYTE* buffer, PDWORD nwritten)
{
DWORD opcode = incontext ? R0_WRITEFILE_IN_CONTEXT : R0_WRITEFILE;
WORD result;
_asm
{
mov eax, opcode
mov ebx, handle
mov ecx, count
mov edx, pos
mov esi, buffer
}
VxDCall(IFSMgr_Ring0_FileIO)
_asm
{
jc error
mov ebx, nwritten
mov [ebx], eax <== this is the change
xor eax, eax
error:
mov result, ax
}
return result;
}
(Thanks to Monte Creasor)