media/doc/LPC.txt

   1 ==============================================================
   2 =                                                            =
   3 =               NOTES FROM THE UNDERGROUND                   =
   4 =                                                            =
   5 ==============================================================
   6 Below are some of Alex's notes on the mysterious LPC Subsystem
   7
   8 =========================
   9 1. Sizes, sizes, sizes...
  10 =========================
  11
  12 There are four imporant LPC Sizes to keep in mind. Try to understand them:
  13
  14 /*
  15  * This determines the absolute maximum message size (0x100 bytes). For
  16  * larger values, use a section-backed message.
  17  */
  18 #define PORT_MAXIMUM_MESSAGE_LENGTH 256
  19
  20 /*
  21  * This determines the maximum length of an LPC request. It is the largest
  22  * amount of bytes that an LPC request can take. To calculate this, assume
  23  * that this is a CONNECTION_REQUEST message, which includes the additionnal
  24  * LPCP_CONNECTION_MESSAGE structure as well. Therefore, we add the kernel LPC,
  25  * header, the maximum port size and the size of the connection request
  26  * structure. This gives a value of 0x15C. However, one must note that NT
  27  * allocates the Lookaside List using a 16-byte aligned value, making this
  28  * number 0x160.
  29  */
  30 #define LPCP_MAX_MESSAGE_SIZE   ROUND_UP(PORT_MAXIMUM_MESSAGE_LENGTH + \
  31                                          sizeof(LPCP_MESSAGE) + \
  32                                          sizeof(LPCP_CONNECTION_MESSAGE), 16)
  33
  34 /*
  35  * Now, for an actual LPC Request size, we remove the kernel LPC header, which
  36  * yields the size of the actual LPC Data that follows the Header, making this
  37  * number 0x148.
  38  */
  39 #define LPC_MAX_MESSAGE_LENGTH (LPCP_MAX_MESSAGE_SIZE - \
  40                                 FIELD_OFFSET(LPCP_MESSAGE, Request))
  41
  42 /*
  43  * Finally, we'll calculate the maximum size of the Connection Info, giving us
  44  * 0x104
  45  */
  46 #define LPC_MAX_DATA_LENGTH     (LPC_MAX_MESSAGE_LENGTH - \
  47                                  sizeof(PORT_MESSAGE) - \
  48                                  sizeof(LPCP_CONNECTION_MESSAGE))
  49
  50 ==========================
  51 2. Structures
  52 ==========================
  53 SOON. TODO.