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| Version | Date | Author | Comment |
|---|---|---|---|
| 1.0 | 2023-10-20 | migthymax | Initial approach to document the PPC 32bit AmigaOS ABI |
| 1.1 | 2023-10-29 | FlynnTheAvatar | Small typo and grammar fixes |
| 1.2 | 2023-10-29 | rjd324 | Small typo and grammar fixes |
| 1.3 | 2023-10-30 | migthymax | Added local copies of external linked PDF |
| 1.4 | 2023-10-31 | kas1e | Updated part about start address |
| 1.5 | 2023-11-03 | migthmax | Updated BREL relocation values upon frank (vbcc) comments |
| 1.6 | 2023-11-05 | migthmax | Updated Table of content, Small data model From frank (vbcc) comments: - corrected targetOS meber to e_ident[EI_OSABI] - added information about __amigaos4__ symbol- added information about _SDA_BASE_ symbol- added information about Baserel & Relocations |
| 1.7 | 2023-12-25 | migthmax | Updated Special Sections chapter regarding .rodata section with information from Joerg and Oliver |
| 1.8 | 2024-01-07 | migthmax | Clarified some information with information from Frank Wille and fixed internal links |
This article is an attempt to further understand and document the AmigaOS 32bit PPC ABI (shorten with pcc-amigaos); inspired, in no small part, due to a lack of existing documentation that is publicly available. There is information scattered around the internet in forums and porting progress, but, they describe just a tiny bit and it is hard to get the whole picture from the source. This document tries to give a complete picture of the specific needs of pcc-amigaos as single source of documentation.
Special thanks go to the following members of various forums for giving motivation and helpful insight information (in no particular order):
- afxgroup
- elfpipe
- futura
- joerg
- kas1e
- rjd324
- walkero
- and many more, which I forget to mention (sorry)
To understand the Application Binary Interface used by ppc-amigaos other documents also need to be considered:
The System V Application Binary Interface, or System V ABI, defines a system interface for compiled application programs. Its purpose is to establish a standard binary interface for application programs on systems that implement the interfaces defined in the System V Interface Definition, Issue 3. This includes systems that have implemented UNIX System V Release 4.
The System V Application Binary Interface e500 Processor Supplement (e500 Processor ABI Supplement) described in PowerPC™ e500 Application Binary Interface User’s Guide (local copy) is a supplement to the generic System V ABI and contains information specific to System V implementations built on the e500 Architecture. The generic System V ABI and this supplement together constitute a complete System V Application Binary Interface specification for systems that implement the e500 architecture of the e500 processor family.
The Power Architecture™ 32.bit Application Binary Interface Supplement 1.0 – Linux® & Embbedded (local copy) is the processor-specific supplement for use with ELF on 32-bit Power Architecture processor systems.
This document is the AmigaOS specific supplement for use with ELF on 32-bit big-endian Power Architecture process systems.
The e_ident[EI_OSABI] member of the ELF Header cannot identify the target operation system ABI, because ppc-amigaos has not been assigned a dedicated value. Thus, this value is ignored to identify an object file for ppc-amigaos. Instead, a marker symbol is used. See chapter Marker Symbols for further information.
As know, currently the dynamic linker does ignore this field and the marker symbol. It just assumes the loaded object file is for the ppc-amigaos target. Hence the marker and targetOS field is just used by a linker and its tool.
The entry member of the ELF Header has a fixed start address of 0×01000000. Even that it is not needed for ppc-amigaos, because all programs run in a shared address model. Thus, during loading the program always gets relocated. So, the start address will never fit. Instead, the dynamic linker uses the _start symbol to find the entry point from where the process starts executing.
The start address has not always been 0×01000000. Till the SDK 53.3 (out in 2008), the start address had the value 0×00000000. Then when shared objects (.so) support were added in said SDK 53.3, it were necessary to change start address to anything else, since these did add a .plt section, potentially in front of the .text (which means 0 as text base address was no longer working). So while start address is a placeholder, it shouldn’t be 0×00000000 at least.
That change of the start addresses to be non-zero had unexpected side effects of the profiling executable: Profiling is a complex task, and diverse binaries, API, and so on must support it. And while current C libraries for OS4 such as CLIB2 and NewLIB had profiling code supporting “gprof” (by generating gmon.out and using performance.monitor feature of kernel) , code were hard-coded and expected that the start address was always zero. Thus profiling were broken for a long time, until it were rewritten for new CLIB4, to not depending on hard-coded values of the start address.
Normally the .rodata section is directly placed in the same segment after where .text and optional .plt are. For shared object (sobjs) targeting ppc-amigaos
this is not acceptable. Any segment containing the .rodata section will be treated differently, than other segments. Instead of loading the complete the segment
into a single memory block, each section is individually loaded into separate memory blocks. This will break .plt_if the section is in the same segment as ._rodata.
Thus it is desired to place the .rodata section a segment of its own so that the current available dynamic linker (elf.library V53.30) can load the executable, without
resulting in crash during execution. The .rodata section can have additional section in its segment, as long the segment is flagged as read only and not executable.
The reason for this seems to be the support of 68k cross calls from/to the sobj. It seems that the .rodata section contains 68k code.
There are two marker symbols used. One for executable, object files and the other for sobjs.
Executable, object files
The marker symbol __amigaos4__ for executable is necessary because there is no distinct value defined for identifying the ppc-amigaos target in the ELF Header. I even think this is ignored by the dynamic linker, this marker symbol is required by the linker, so that the linker can identify the target from the file. Thus __amigaos4__ symbol must be protected against stripping.
Sobjs
The marker symbol DT_AMIGAOS_DYNVERSION for sobjs files has a twofold functionality. First it is used to identify that the sobj file targets ppc-amigaos, because of the same reason as the __amigaos4__ symbol for executable. A sobj file might even have an __amigaos4__ symbol, to make sure that it is targeting ppc-amigaos, but per se the __amigaos4__ is not needed for sobj files. The other proposes of the DT_AMIGAOS_DYNVERSION symbol is to tell the dynamic linker to which version of the sobj support for ppc-amigaos is adheres to. Currently it is version 2, the previous version was only used during development and isn’t used today in real life systems.
The symbol _start symbol is used to determine the entry point of the executable. That is needed because on ppc-amigaos, all executable shares the same address space, thus the assumed entry point from the ELF header might be taken by another executable. At the same time this means that all executables are relocated during the loading by the dynamic linker. Furthermore, the strip command isn’t allowed to strip relocations and symbols which are needed by ppc-amigaos.
The symbol _SDA_BASE_ defines the small data area base. An address relative to which all data is accessed with 16bit offset. The address is stored in register r13. This symbol is only needed if the executable is compiled to use the small data model, which is enabled with -msdata for the gcc. It has the limitation that the data must fit within 64k. On the ppc-amigaos target even exits another addressing schema, named shortened baserel, which works in a similar way like the small data model, but doesn’t have the limitation of that data must fit within 64k. Probably the small data model even work with sobjs file, and the baserel model might not.
_SDA_BASE_ should be defined by the linker script, so there shouldn’t be any problem. Indeed, you don’t need it when there are no small-data addressing modes in the whole executable. But usually you have that somewhere, for example in the startup-code, which always sets up small-data even when the main program doesn’t use it.
There is even the symbol __amigappc_ used. But I think this is a relict and can be removed, because that PPC is the target can easily be detected by the ELF Header
Because strip by default removes all symbols from executables and object files, the marker symbols for these kinds of files must survive a strip, else they cannot be processed further and/or not started by the dynamic linker. This requirement isn’t needed for sobjs, because the default strip don’t remove symbols from sobjs files. TODO: Verify that this is correct?
For the ppc-amigaos specific address model baserel, target specific relocation are needed, which are based upon the .data section, see Base Relative Address Model fro more information:
BREL relocation = Symbol + Addend - .data| Name | Value | FIELD | Calculation of Addend |
|---|---|---|---|
| R_PPC_AMIGAOS_BREL | 210 |
word32 |
val |
| R_PPC_AMIGAOS_BREL_LO | 211 |
half16 |
val & 0xFFFF |
| R_PPC_AMIGAOS_BREL_HI | 212 |
half16 |
(val >> 16) & 0xFFFF |
| R_PPC_AMIGAOS_BREL_HA | 213 |
half16 |
{(val >> 16) & 0xFFFF) + {{val & 0x8000) >> 15 ) |
Surprisingly it uses the base address of the .data section and not the symbol _DATA_BASE_, which should be identical.
There is also a function in elf.library (CopyDataSegment()), which can be used to make new copies for BREL-addressing. This was meant to be useful in reentrant programs or shared libraries.
Like the small data address model but uses the register r2 to store the base address, and uses 32-bit offsets. The offsets are calculated relative to the start of the data segment, or symbol _DATA_BASE_.
It seems that this address model is broken, because the register r2 has been abused for other features, which interferes with the base relative address model. Rumors are curculating that r2 has been used for tasks stuff and/or sobjs support.
The strip operations provided by binutils ld linker, like --strip-all, does remove all symbols from the output file. As mentioned in the chapter Marker Symbols this needs to adjust for the ppc-amigaos target. The mention symbols from the chapter must survive a strip run. Additional during a --strip-all run undefined symbols are kept too.
Furthermore binutils ld provides a ppc-amigaos target specific strip operation named --strip_unneeded_rel_relocs. This option can be explicitly called, which requires a --strip-all (adjusted for ppc-amigaos) run. If enabled as the name states, it removes unneeded relative relocations. Disabled by default.
From a comment:
If the symbol refers to a stripped section, we still want to keep it, e.g., SDA_BASE TODO: We should perhaps output a warning or add another option to trigger this behavior. FIXME: The section to which symbol refers must be adjusted as well.
The other present linker for ppc-amigaos is vlink, which offers the follwoing strip operations:
-S strip all debugger symbols
-s strip all symbols (except protected ones)
-X strip local symbols that start with the letters ‘L’ or ‘l’, or with a dot
-x strip all local symbols
The pointer equality between executables and sobjs is not working with current public dynamic linker (elf.library v53.30). The case for that is twofold. Firstly, the binutils, for the ppc-amigaos target <= v2.40, explicitly hinder this with the modification that the value of the function pointer symbol is not allowed to be zero. It must have a value calculated by the linker. Secondly, the dynamic linker cannot handle function pointer symbol having a value zero. It just uses the value found in the symbol entry. And a value of zero leads to a crash. Using a found non-zero value follows the ABI. But not being able to handle a zero value don’t comply with the ABI. A zero value indicates that the dynamic linker must resolve the function pointer. Doing so by the dynamic linker guaranties that function pointer values are equal between executables and sobjs files.
The newer version on binutils and elf.library will comply more with the ABI and thus offer pointer equality.
The define TARGET_KEEP_UNUSED_SECTION_SYMBOL has been introduced by the new binutils and is defined to be true. The comment above states the line in elf32-ppc.c:25 that it used by the Linux kernel and tells the assembler to generated even symbols which seems unused. If enabled it works for the ppc-amigaos target, but I don’t understand if it has possible side effects.
The GOT is marked to be executable for the ppc-amigaos target. That is in common with the ppc-vxworks target, thus something which even is required by other targets. The explanation for the pcc-vxworks target is that the GOT can contained the blrl instruction. The only difference to pcc-vxworks target is, that its additional flag the section with SEC_CODE. Which somehow makes sense. I don’t know if that is really the same reason for ppc-amigaos having it that way, or if there is another reason to have executable.
Frank Wille confirms that the GOT contains blrl instructions, but he isn’t sure why, or whether it is ever called.
If the target is ppc-amigaos all dynamic section in the out file are flagged being read only. It probably wouldn’t hurt to make it read-only, as there is no reason to write to it. But writing to it would be an error anyway. It doesn’t matter that much. NetBSD defines it writable.
Copy relocations are needed for the following scenario:
An executable is linked with a shared object and accesses (which also means writing!) data variables from this shared object. It cannot modify this data directly, because there will be conflicts with multiple processes linked to the same shared object in memory, all manipulating the shared object’s data.
The technical solution is:
The linker allocates space in the executable’s .bss (or .sbss) section for a copy of that data. It also generates an R_COPY relocation, so that initialized data is copied (by the dynamic linker) from the shared object on startup into the executable’s .bss space.
For the target ppc-amigaos the elimination of copy relocations is disabled, to support that use case. It probably not wide used, or even used at all. So theoretically it could be enabled.
The ldctor_build_sets method in ldctor.c:198 is used by the linker to build the constructor list. For the target ppc-amigaos it is always performed to build the list, even for already defined symbols. Which might happen if it is called from collect, and thus the sets may already have been built.
Constructors must be relocatable for the ppc-amigaos target.
The default for linking for target ppc-amigaos is to link statically. Additional even relocatable files are marked as being executable, not just executables.