lglaf/protocol.md

LG LAF Protocol

This document is a reverse-engineered protocol description for LG Advanced Flash (LAF), the download mode offered by various LG models. It is based on analysis on the Send_Command.exe utility and LGD855_20140526_LGFLASHv160.dll file and a USB trace using Wireshark and usbmon on Linux. Some commands were found in the /sbin/lafd binary.

This document uses the following conventions for types:

• \xaa\xbb\xcc\xdd denotes a byte pattern aa bb cc dd.
• 0xddccbbaa denotes a 32-bit integer in hexadecimal format. It represents the same byte pattern as \xaa\xbb\xcc\xdd.

Overview

LAF is a simple request/response protocol operating over USB. The USB details are described at the end of the document, the messages are described below.

Each message consists of a header, followed by an optional body. The header contains 32-bit DWORDs, integers are encoded in little-endian form:

| Offset (hex) | Offset (dec) | Type | Description | ----:| --:| --- | 0x00 | 0 | char[4] | Command | 0x04 | 4 | var | Argument 1 | 0x08 | 8 | var | Argument 2 | 0x0c | 12 | var | Argument 3 | 0x10 | 16 | var | Argument 4 | 0x14 | 20 | int | Body length | 0x18 | 24 | int | CRC-16 | 0x1c | 28 | char[4] | Bit-wise invertion of command at offset 0

Arguments can be integers or character sequences depending on the command.

The CRC field is the CRC-16-CCITT calculation (LSB-first) over the header and the body with zeroes in place of CRC.

Each request is followed by a response with a matching command field. If an error occurs, the response contains command is FAIL with argument 1 being the error code and the original request header as body.

Commands

OPEN - Open File

Opens a file path.

Arguments:

• arg1 (response): DWORD file descriptor. Request body: NUL-terminated file path that should be opened for reading or an empty string to open /dev/block/mmcblk0 in read/write mode. (at most 276 (0x114) bytes?)

Non-existing files result in FAIL with error code 0x80000001.

CLSE - Close File

Closes a file descriptor which was returned by the OPEN command.

Arguments:

• arg1: DWORD file descriptor (same in request and response).

Note: this allows you to close any file descriptor that are in use by the lafd process, not just the one returned by OPEN. You can discover the current file descriptors via ls -l /proc/$pid/fd where $pid is found by ps | grep lafd.

HELO - Hello

Arguments:

• arg1: DWORD Protocol Version (\1\0\0\1) (resp must match req.)
• arg2 (response): Minimum Protocol Version (\0\0\x80\0 was observed)

CTRL - Control

Arguments:

• arg1: "RSET" (reboots device), "POFF" (powers device off) or "ONRS"

Note: CTRL(RSET) with no body is sent by the Send_Command.exe utility for the LEAVE command.

LG Flash DLL waits 5000 milliseconds after this command.

WRTE - Write File

Writes to a file descriptor.

Arguments:

• arg1: file descriptor (must be open for writing!)
• arg2 (request): offset in blocks (multiple of 512 bytes).
• arg2 (response): offset in bytes. Request body: the data to be written. Can be of any size (including 1 or 513).

Note: writing to a file descriptor which was opened for reading results in FAIL with code 0x82000002. This command is likely used for writing to partitions.

Integer overflow in the response offset is ignored. That is, the block offset 30736384 (0x1d50000) is 0x3aa000000 bytes, but will appear as 0xaa000000.

READ - Read File

Reads from a file descriptor.

Arguments:

• arg1: file descriptor.
• arg2: offset in blocks (multiple of 512 bytes).
• arg3: requested length in bytes (at most 8MiB).
• arg4: "whence" seek mode (see below). Response body: data in file at given offset and requested length.

Note: be sure not to read past the end of the file (512 * offset + length), this will hang the communication, requiring a reset (pull out battery)!

Arg4 affects the seek mode, values for request:

• 0 (SEEK_SET) - seek to 512 * offset.
• 1 (SEEK_CUR) - read from current position (offset argument is ignored).
• 2 (SEEK_END) - kind of useless when all offsets are unsigned...
• 3 (SEEK_DATA) - FAILs with 0x80000001 when used on /proc/kmsg or /dev/block/mmcblk0p44. Works on a regular file though. The response matches the request (masked with 0x3).

If the length is larger than somewhere between 227 MiB and 228 MiB, an 0x80000001 error will be raised (observed with /dev/block/mmcblk0). Requesting lengths larger than 8 MiB however already seem to hang the communication.

ERSE - Erase Block

TRIMs a block (IOCTL_TRIM_CMD).

Arguments:

• arg1: file descriptor (open /dev/block/mmcblk0 for writing).
• arg2: start address (in sectors).
• arg3: count (in sectors).
• arg4: unknown, set to zero. Request body: none.

Note: after sending TRIM, reading the block still returned old values. After a reboot, everything was zeroed out though.

EXEC - Execute Command

Arguments: none Request body: NUL-terminated command. Response body: standard output of the command.

The command is probably split on space and then passes to execve. In order to see standard error, use variables and globbing, use a command such as:

sh -c "$@" -- eval 2>&1 </dev/null echo $PATH

If you need to read dmesg (or other blocking files), try to put busybox on the device (e.g. by writing to an unused partition) and execute:

/data/busybox timeout -s 2 cat /proc/kmsg

INFO

Arguments:

• arg1: action (GPRO - Get Properties, SPRO - Set Properties) Request body: must begin with two bytes (08 0b). Response body: 2824 (0xb08) bytes of binary info.

See scripts/parse-props.py for the structure of the property body.

UNLK - Unlink

Delete a file.

Arguments: none Request body: NUL-terminated file name

Responds with FAIL code 0x80000001 if the file name is invalid (missing) or file does not exist. Deleting directories is also not possible, giving the same FAIL code 0x80000001.

RSVD - Reserved

Arguments: none

IOCT - ioctl

Unknown.

MISC

Unknown.

KILO

Unknown.

DIFF

Unknown.

USB layer

The LG Windows driver (via LGMobileDriver_WHQL_Ver_4.0.3.exe) exposes two serial ports, LGANDNETMDM0 and LGANDNETDIAG1. The LGANDNETDIAG1 port is used for LAF.

The LG G3 (D855) has Vendor ID 0x1004 and Product ID 0x633e.

There is only one configuration descriptor and LAF uses bulk transfers over endpoints 5 (for input from the device) and endpoint 3 (for output to the device).

For other descriptors, see info/lsusb.txt.