IPC Marshalling

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IPC Command Structure

This is an array of u32's, usually located in Thread Local Storage.

Word Bits Description
0 15-0 Type. 4=Request, 5=Control
0 19-16 Number of buf X descriptors (each: 2 words).
0 23-20 Number of buf A descriptors (each: 3 words).
0 27-24 Number of buf B descriptors (each: 3 words).
0 31-28 Number of buf W desciptors (each: 3 words), never observed.
1 9-0 Size of raw data section in u32s.
1 13-10 Flags for buf C descriptor.
1 31 Enable handle descriptor.
... Handle descriptor, if enabled.
... Buf X descriptors, each one 2 words.
... Buf A descriptors, each one 3 words.
... Buf B descriptors, each one 3 words.
... Type W descriptors, each one 3 words.
... Raw data section (including padding before and after aligned data section).
... Buf C descriptors, each one 2 words.

Handle descriptor

There can only be one of this descriptor type. It is enabled by bit31 of the second word.

Word Bits Description
0 0 Send current PID.
0 4-1 Number of handles to copy
0 8-5 Number of handles to move
... 8-byte PID if enabled
... Handles to copy
... Handles to move

Sysmodules load the last u64 of rawdata when handling the PID. This is not written by kernel. For sysmodule handling:

  • In some cases: these commands require a placeholder u64 value passed in the input parameters, as mentioned above. In these cases the OverwriteClientProcessId method is called to replace the value before it is used.
  • In other cases: The rawdata_u64 is compared with the PID from the descriptor. On mismatch and when rawdata_u64!=0, error 0x60A is returned. The PID value passed to the cmdhandler vtable funcptr is the rawdata_u64.

Buffer descriptor X "Pointer"

This one is packed even worse than A, they inserted the bit38-36 of the address on top of the counter field.

Officially, the counter is known as "receive index". This one writes to the buffer described in the ReceiveList.

Word Bits Description
0 5-0 Bits 5-0 of counter.
0 8-6 Bit 38-36 of address.
0 11-9 Bits 11-9 of counter.
0 15-12 Bit 35-32 of address.
0 31-16 Size
1 Lower 32-bits of address.

Buffer descriptor A/B/W "Send"/"Receive"/"Exchange"

This packing is so unnecessarily complex.

Word Bits Description
0 Lower 32-bits of size.
1 Lower 32-bits of address.
2 1-0 Flags. Always set to 0, 1 or 3. Determines what MemoryState to use with the mapped memory in the sysmodule.
2 4-2 Bit 38-36 of address.
2 27-24 Bit 35-32 of size.
2 31-28 Bit 35-32 of address.

Buffer descriptor C "ReceiveList"

There's a 4-bit flag in the main header controlling the behavior of C descriptors.

If it has value 0, the C descriptor functionality is disabled. If it has value 1, there is no C descriptor. If it has value 2, there is a single C descriptor.

Otherwise it has (flag-2) C descriptors.

Word Bits Description
0 Lower 32-bits of address.
1 15-0 Rest of address.
1 31-16 Size

IPC buffers

Buffer descriptor A/B/... map memory into the sysmodule process. For the mapped memory in the sysmodule the permissions are: desc-A = R--, desc-B = RW-. The buffer is automatically unmapped while the kernel handles the cmdreply, the sysmodule doesn't need to specify anything in the cmdreply to trigger this.

This memory is mapped in the sysmodule to the same vaddr from the original user-process cmd-request, except with with bits >=(~28(?)) changed to a different ASLR'd region.

No user-process->sysmodule memcpy is done for outbufs, only sysmodule->user-process.

Raw data section

An example of an IPC message with a type 0xA buffer in it. Red is headers/descriptors, yellow is padding, and blue is data/buffer lengths. Note that the size of the u16 array for type A lengths is padded to fill up a whole word.
Word Description
... Padding to align to 16 bytes.
... If sent to an object domain, a domain message, otherwise a data payload
... Padding
... Buffer type 0xA lengths (u16 array)

The total amount of padding within the raw data section is always 0x10 bytes. This means that if no padding is required before the message, there will be 0x10 bytes of padding after the message (before the buffer type 0xA lengths).

Domain message

This header is used to wrap up requests sent to domains instead of sessions.

Word Bits Description
0 7-0 Command. 1=send message, 2=close virtual handle
0 31-16 Length of data payload in bytes.
1 Object ID (from cmd 0 in Control).
2 Padding to align to u64
4... Data payload

Data payload

This is an array of u32's, but individual parameters are generally stored as u64's.

Word Description
0 Magic ("SFCI" for requests, "SFCO" for responses) as u64.
2 Command id as u64 for requests, error code as u64 for responses.
4... Input parameters or return values

Official marshalling code

The official marshalling function takes an array of (buf_ptr, size) pairs and a type-field for each such pair.

Bitmask 0x10 seems to indicate null-terminated strings, but that flag is ignored by the marshalling code.

Type Mask Description Direction
4 + 1 Creates a A descriptor with flags=0. In
0x40 + 4 + 1 Creates a A descriptor with flags=1. In
0x80 + 4 + 1 Creates a A descriptor with flags=3. In
4 + 2 Creates a B descriptor with flags=0. Out
0x40 + 4 + 2 Creates a B descriptor with flags=1. Out
0x80 + 4 + 2 Creates a B descriptor with flags=3. Out
8 + 1 Creates an X descriptor In
8 + 2 Creates a C descriptor, and writes the u16 size to an offset into raw data. Out
0x10 + 8 + 2 Creates a C descriptor Out
0x20 + 1 Creates both an A and X descriptor In
0x20 + 2 Creates both an B and C descriptor Out

C and X (Pointer and ReceiveList) descriptors are backed by the "pointer buffer", a buffer in the service process. Its size is a u16, which is retrieved using the "QueryPointerBufferSize" control message. If the client code determines all buffers with flag 8 do not fit in the pointer buffer, it returns error 0x11A0B.

For buffers with flag 0x20 it creates two descriptors (A+X or B+C), but one descriptor is NULL (zero size and pointer), while the other holds the expected values. X/C descriptors are used as the non-NULL descriptor where possible, but if they don't fit in the pointer buffer, A/B descriptors are used instead. The code defers processing of type 0x20 buffers with sizes that fit in a u16 (and may therefore fit in the pointer buffer). This ensures all type 8 buffers get pointer-buffer space before any type 0x20.

(The order in which the deferred type 0x20 buffers are processed is determined by a convoluted loop.)

Official IPC Cmd Structure

Official struct that is stored for each IPC command. It contains precalculated offsets for different portions of the command structure.

All offsets are given is in number of u32 words.

struct IpcCmdStruct {
  u8  unk0;
  u8  has_handle_descriptor;
  u8  pad0[2];
  u32 cmd0;
  u32 cmd1;
  u32 offset_handle_descriptor;
  u32 pad1;
  u32 offset_handles;          
  u32 pad2;
  u32 offset_x_descriptors;
  u32 offset_a_descriptors;
  u32 offset_b_descriptors;
  u32 offset_w_descriptors; /* this is a guess */
  u32 offset_raw_data;
  u32 offset_c_descriptors;
  u32 unk2;
  u32 unk3;


When type == 5 you are talking to the IPC manager. These are processed by the sysmodule.

Cmd Name Arguments Output
0 ConvertSessionToDomain None
1 ConvertDomainToSession u32 domain
2 DuplicateSession None
3 QueryPointerBufferSize None u32 as u16-size
4 DuplicateSessionEx u32 unknown