* external.cc (cygwin_internal): Implement CW_CYGNAME_FROM_WINNAME.

Add lengthy comment to explain what we do and why.
	* include/sys/cygwin.h (cygwin_getinfo_types): Add
	CW_CYGNAME_FROM_WINNAME.

winsup/cygwin/external.cc

@@ -619,6 +619,61 @@ cygwin_internal (cygwin_getinfo_types t, ...)
 	}
 	break;
 
+      case CW_CYGNAME_FROM_WINNAME:
+	{
+	  /* This functionality has been added mainly for sshd.  Sshd
+	     calls getpwnam() with the username of the non-privileged
+	     user used for privilege separation.  This is usually a
+	     fixed string "sshd".  However, when using usernames from
+	     the Windows DBs, it's no safe bet anymore if the username
+	     is "sshd", it could also be "DOMAIN+sshd".  So what we do
+	     here is this:
+
+	     Sshd calls cygwin_internal (CW_CYGNAME_FROM_WINNAME,
+					 "sshd",
+					 username_buffer,
+					 sizeof username_buffer);
+	     
+	     If this call succeeds, sshd expects the correct Cygwin
+	     username of the unprivileged sshd account in username_buffer.
+
+	     The below code checks for a Windows username matching the
+	     incoming username, and then fetches the Cygwin username with
+	     the matching SID.  This is our username used for privsep then.
+
+	     Of course, other applications with similar needs can use the
+	     same method. */
+	  const char *winname = va_arg (arg, const char *);
+	  char *buffer = va_arg (arg, char *);
+	  size_t buflen = va_arg (arg, size_t);
+
+	  if (!winname || !buffer || !buflen)
+	    break;
+
+	  PWCHAR name;
+	  if (!sys_mbstowcs_alloc (&name, HEAP_BUF, winname))
+	    break;
+
+	  cygsid sid;
+	  DWORD slen = SECURITY_MAX_SID_SIZE;
+	  WCHAR dom[DNLEN + 1];
+	  DWORD dlen = DNLEN + 1;
+	  SID_NAME_USE acc_type;
+
+	  if (!LookupAccountNameW (NULL, name, sid, &slen, dom, &dlen,
+				   &acc_type))
+	    break;
+
+	  struct passwd *pw = internal_getpwsid (sid);
+	  if (!pw)
+	    break;
+
+	  buffer[0] = '\0';
+	  strncat (buffer, pw->pw_name, buflen - 1);
+	  res = 0;
+	}
+	break;
+
       default:
 	set_errno (ENOSYS);
     }