“Access Denied” on writing on Mailslot in Windows Service program

Question

I use Mailslots (in Delphi 7) for inter-programs dialog and all is OK.

But when I use one of my program (in Windows XP) as Windows service I have a message "Mailslot Access Denied", when another (classical admin user's) program try to write to the mailslot. I Understand that it is surely a rights problem since service have SYSTEM rights but...what is the solution ?

Answer 1

When calling CreateMailslot(), specify a SECURITY_DESCRIPTOR that allows all access to the mailslot, eg:

var
  ...
  sd: SECURITY_DESCRIPTOR;
  sa: SECURITY_ATTRIBUTES; 
begin
  ...
  InitializeSecurityDescriptor(@sd, SECURITY_DESCRIPTOR_REVISION);
  SetSecurityDescriptorDacl(@sd, True, nil, False);

  sa.lpSecurityDescriptor := @sd; 
  sa.bInheritHandle := Frue; 

  ... := CreateMailslot(..., @sa);
  ...
end;

Answer 2

I use C++ Embarcardero 2010, and I have to do some modifications to the solution of Remy Lebeau because the CreateMailSlot function receive a pointer of type SECURITY_ATTRIBUTES *, and not a pointer of type SECURITY_DESCRIPTOR *.

My solution in C++ is:

SECURITY_DESCRIPTOR sd;
InitializeSecurityDescriptor(&sd, SECURITY_DESCRIPTOR_REVISION);
SetSecurityDescriptorDacl(&sd, true, NULL, false);

SECURITY_ATTRIBUTES sa;
sa.lpSecurityDescriptor=&sd;
sa.bInheritHandle=true;
this->pHandleMailSlot = CreateMailslot("your mail slot path", 0, -1, &sa);

---

Note: In my case I've three applications:

1. A service with the MailSlot (Embarcadero C++ 2010)
2. A service with a client mailslot (.NET v4)
3. A WPF with a client mailslot (.NET v4)