112 lines
4.5 KiB
Markdown
112 lines
4.5 KiB
Markdown
# CodeWarrior `.comment` section
|
|
|
|
Files built with `mwcc` contain a `.comment` section:
|
|
|
|
```
|
|
$ powerpc-eabi-readelf -We object.o
|
|
|
|
Section Headers:
|
|
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
|
|
[ 0] NULL 00000000 000000 000000 00 0 0 0
|
|
[ 1] .text PROGBITS 00000000 000034 000708 00 AX 0 0 4
|
|
...
|
|
[16] .comment PROGBITS 00000000 00153b 0001b4 01 0 0 1
|
|
```
|
|
|
|
The `.comment` section contains information that `mwld` uses during linking, primarily symbol alignment and a "force active" / export flag.
|
|
|
|
If missing, `mwld` will **not** adjust the alignment of symbols or remove any unused symbols.
|
|
|
|
This behavior is quite useful in some cases. When we split our program into objects, we're working from the final post-aligned, post-stripped result, and don't want the linker to make any changes. Most decompilation projects rely on this behavior unintentionally, since their generated objects don't contain a `.comment` section. (For example, objects built with `powerpc-eabi-as`.)
|
|
|
|
However, we need the `.comment` section for some purposes:
|
|
|
|
- Reproducing the [common BSS inflation bug](common_bss.md#inflation-bug) requires the `.comment` section present, due to the above. The linker inflates the size of the first common BSS symbol in a TU, but won't actually move any data around unless the `.comment` section is present.
|
|
- In newer versions of the linker, using common BSS at all _without_ a valid `.comment` section will cause an internal linker error.
|
|
|
|
When the `.comment` section is generated, decomp-toolkit will mark all global symbols as "exported" to prevent any deadstripping, since the presence of the `.comment` section itself enables deadstripping.
|
|
|
|
Generating the `.comment` section and setting the "export" flag is also useful to prevent the linker from removing entire objects. A missing `.comment` section will prevent the removal of unused symbols _inside_ of an object, but the linker will still remove the entire object itself if it thinks it's unused.
|
|
|
|
## Contents
|
|
|
|
The contents of this section follow a very simple format:
|
|
|
|
### Header
|
|
|
|
`[0x0 size: 0xB]` Magic: `43 6F 64 65 57 61 72 72 69 6F 72` ("CodeWarrior")
|
|
|
|
`[0xB size: 0x1]` Version(?): `XX`
|
|
|
|
It's not known whether this field actually affects `mwld` in any way, but it's configurable for completeness sake. (See `mw_comment_version` in [`config.example.yml`](/config/GAMEID/config.example.yml).)
|
|
|
|
Known values:
|
|
|
|
- `08` (8) - CodeWarrior for GameCube 1.0+
|
|
- `0A` (10) - CodeWarrior for GameCube 1.3.2+
|
|
- `0B` (11), `0C` (12) - CodeWarrior for GameCube 2.7+ (difference unknown)
|
|
- `0E` (14), `0F` (15) - CodeWarrior for GameCube 3.0a3+ (difference unknown)
|
|
|
|
`[0xC size: 0x4]` Compiler version: `XX XX XX XX`
|
|
|
|
First 3 bytes are major, minor, and patch version numbers.
|
|
4th byte is unknown, but is always `01`.
|
|
|
|
Example: `Version 2.3.3 build 144` -> `02 03 00 01`
|
|
Often the `.exe`'s properties (which `--help` reads from) and the internal version number (here) will differ.
|
|
|
|
`[0x10 size: 1]` Pool data: `XX`
|
|
|
|
- `00` - Data pooling disabled
|
|
- `01` - Data pooling enabled
|
|
|
|
`[0x11 size: 1]` Float type: `XX`
|
|
|
|
- `00` - Floating point disabled
|
|
- `01` - Software floating point
|
|
- `02` - Hardware floating point
|
|
|
|
`[0x12 size: 2]` Processor type: `00 16` (Gekko)
|
|
|
|
`[0x14 size: 1]` Unknown, always `2C`. Possibly the start of symbol entries.
|
|
|
|
`[0x15 size: 1]` "Quirk" flags: `XX`
|
|
|
|
Bitfield of miscellaneous flags. Known flags:
|
|
|
|
- `01` - "Incompatible return small structs"
|
|
- `02` - "Incompatible SFPE double params"
|
|
- `04` - "Unsafe global reg vars"
|
|
|
|
`[0x16 size: 22]` Padding until `0x2C`
|
|
|
|
### Symbol entry
|
|
|
|
At `0x2C` is the first symbol entry. There is one 8 byte entry per ELF symbol.
|
|
|
|
This includes the "null" ELF symbol, so the first entry will be all 0's.
|
|
|
|
`[0x0 size: 4]` Alignment: `XX XX XX XX`
|
|
|
|
`[0x4 size: 1]` Visibility flags(?): `XX`
|
|
|
|
Known values:
|
|
|
|
- `00` - Default
|
|
- `0D` - Weak
|
|
- `0E` - Unknown, also weak?
|
|
|
|
`[0x5 size: 1]` Active flags(?): `XX`
|
|
|
|
Known values:
|
|
|
|
- `00` - Default
|
|
- `08` - Force active / export. Prevents the symbol from being deadstripped.
|
|
When applied on a section symbol, the entire section is kept as-is. This is used
|
|
by `mwcc` when data pooling is triggered (indicated by a symbol like `...data.0`), likely to prevent the hard-coded section-relative offsets from breaking.
|
|
Can also be set using `#pragma force_active on` or `__declspec(export)`.
|
|
- `10` - Unknown
|
|
- `20` - Unknown
|
|
|
|
`[0x6 size: 2]` Padding(?): `00 00`
|