RK3566 EBC Reverse-Engineering

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Revision as of 08:15, 1 August 2021 by CounterPillow (talk | contribs) (Oh god what have I got myself into)
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The RK3566 SoC, used in the Quartz64 SBC by PINE64, contains an eInk interface. This is referred to as ebc by Rockchip apparently.

Unfortunately, the driver published for this eInk interface within the BSP kernel is an assembly dump produced by gcc. Fortunately, it contains quite a bit of debug information, which we can use to reverse engineer it.


The ebc driver source is available from the quartz-bsp repository.

The file of interest is ebc_dev_v8.S, which implements a DRM (Direct Rendering Manager) driver for the eInk panel.


Assembly Syntax and Semantics

The Syntax is GNU Assembler (GAS) syntax. This modexp article provides a good introduction to the syntax, calling convention, semantics and some often used instructions.

The ARM Architecture Reference Manual for ARMv8 should be used as reference for any instructions.

At the very least, you should read up on the registers and calling convention used.

Debug Information

Quite a bit of debug info is left in the assembly dump, including function names, file names and line numbers. We can take this to our advantage.

.file file-number file-path

Specifies a number to reference a file by, and its path. All following code until the next .file or .loc statement are to be understood as originating from this file. This is particularly useful to understand which code has been inlined from other files, for which the source is available.

.loc file-number line-number 0

Specifies that the following code is generated from line-number stemming from file number file-number. See the .file directive for this file number to understand which source file it came from.

.type function-name, %function

This tells us that the following code belongs to function function-name. You'll usually see a .cfi_startproc, which signifies the start of the function code, until the matching .cfi_endproc.

A quick grep for %function shows that we are dealing with 30 functions in this file.

.type struct-name, %object

This seems to signify a definition of a C struct named struct-name.

A quick grep for %object shows that we are dealing with around 27 structs in this file.


TODO: This seems to contain the main bulk of the DWARF debug information, including enough info to reverse full structs and function signatures.

Finding Structs and Function Signatures

First, we'll need to assemble the file:

aarch64-linux-gnu-gcc -c -o ebc_dev_v8.o ebc_dev_v8.S

This gives us a ebc_dev_v8.o which we can feed into readelf:

readelf --debug-dump ebc_dev_v8.o

This will produce a lot of output, but we're mainly concerned with the start of the dump. We'll find things like:

 <2><101f8>: Abbrev Number: 0
 <1><101f9>: Abbrev Number: 79 (DW_TAG_subprogram)
    <101fa>   DW_AT_name        : (indirect string, offset: 0xa2b4): ebc_open
    <101fe>   DW_AT_decl_file   : 1
    <101ff>   DW_AT_decl_line   : 1377
    <10201>   DW_AT_prototyped  : 1
    <10201>   DW_AT_type        : <0xc6>
    <10205>   DW_AT_low_pc      : 0x0
    <1020d>   DW_AT_high_pc     : 0xc
    <10215>   DW_AT_frame_base  : 1 byte block: 9c 	(DW_OP_call_frame_cfa)
    <10217>   DW_AT_GNU_all_call_sites: 1
    <10217>   DW_AT_sibling     : <0x1023a>
 <2><1021b>: Abbrev Number: 88 (DW_TAG_formal_parameter)
    <1021c>   DW_AT_name        : (indirect string, offset: 0x1153): inode
    <10220>   DW_AT_decl_file   : 1
    <10221>   DW_AT_decl_line   : 1377
    <10223>   DW_AT_type        : <0x1c54>
    <10227>   DW_AT_location    : 0xd63 (location list)
 <2><1022b>: Abbrev Number: 106 (DW_TAG_formal_parameter)
    <1022c>   DW_AT_name        : (indirect string, offset: 0x8b06): file
    <10230>   DW_AT_decl_file   : 1
    <10231>   DW_AT_decl_line   : 1377
    <10233>   DW_AT_type        : <0x551f>
    <10237>   DW_AT_location    : 1 byte block: 51 	(DW_OP_reg1 (x1))

This essentially tells us the full function signature of ebc_open:

DW_TAG_subprogram tells us of a function, with DW_AT_name letting us know that this is ebc_open. DW_AT_type of 0xc6 let's us know, once we jump to <c6>, that this function's return type is a signed 32-bit integer.

The DW_TAG_formal_parameter that follow tell us of each of the parameter the function takes. The first one is called inode and is of type 0x1c54. Referencing what this type is, we find:

 <1><1c54>: Abbrev Number: 7 (DW_TAG_pointer_type)
    <1c55>   DW_AT_byte_size   : 8
    <1c56>   DW_AT_type        : <0x1970>

which in of itself goes on to reference 0x1970, and looking this one up, we'll find a struct definition:

 <1><1970>: Abbrev Number: 26 (DW_TAG_structure_type)
    <1971>   DW_AT_name        : (indirect string, offset: 0x1153): inode
    <1975>   DW_AT_byte_size   : 672
    <1977>   DW_AT_decl_file   : 31
    <1978>   DW_AT_decl_line   : 611
    <197a>   DW_AT_sibling     : <0x1c4f>
 <2><197e>: Abbrev Number: 27 (DW_TAG_member)
    <197f>   DW_AT_name        : (indirect string, offset: 0x7d00): i_mode
[etc etc...]