[llvm-dev] [cfe-dev] RFC: Linker feature for automatically partitioning a program into multiple binaries

[Shi, Steven via llvm-dev]

Hi Peter,

>I gather from your other posts that you care most about UEFI -- I don't know what capabilities the UEFI loader has.
Yes, I care Uefi most. We can extend Uefi image loader to support new linker feature if necessary. No problem.

Actually, Uefi firmware is very sensitive on code/image size. Millions of code have to be linked into only several MB ROM space. Every KB counts for us.
Uefi only support static linking for now and there are many common library function built into each module. We use the compression on multiple modules to mitigate the common library code size issue but it is not an ideal solution. The compressed module need memory to decompress in runtime but Uefi have modules need to run (XIP) before the memory is ready. So we have the requirement to support dynamic linking.

> These binaries will all share a virtual address space so that they can refer to one another directly using PC-relative references or RELATIVE dynamic relocations as if they were all statically linked together in the first place, rather than via the GOT (or custom GOT-equivalent).
I like PC-relative relocation and don’t like GOT/PLT based relocations. We have to convert the ELF to COFF image after linking through special build tool, and it is very hard to do the conversion for GOT/PLT based relocations.
I could consider to natively support the ELF binary in the Uefi image loader, which will make everything easy. But the problem is current Uefi specification says Uefi only support PE/COFF image…

> Each exported symbol by itself imposes additional binary size costs, as it requires the name of the symbol and a dynamic symbol table entry to be stored in both the exporting and importing DSO, and on the importing side a dynamic relocation, a GOT entry and likely a PLT entry must be present. These additional costs go some way towards defeating the purpose of splitting the program into pieces in the first place, and can also impact program startup and overall performance because of the additional indirections.
I need uncompromising code size decrease, and I don’t want GOT/PLT entries. OK, your proposal is just what I want.

> I haven't thought about how this feature will interact with linker scripts. At least to start with we will likely need to forbid using this feature together with the PHDRS or SECTIONS linker script directives.
Could you help to take a look at our ELF linker script: https://github.com/tianocore/edk2/blob/master/BaseTools/Scripts/GccBase.lds, and let me know whether it can get alone with your proposed  dynamic linking feature. I’m OK to change it if it cannot. Please be aware I can fully change my loader and linking script, and let me know your suggestion on how to change them.

> A prototype/proof of concept of this feature has been implemented here: https://github.com/pcc/llvm-project/tree/lld-module-symbols
Sorry, but could you let me know the basic build steps of your lld?



Thanks
Steven

From: Peter Collingbourne [mailto:peter at pcc.me.uk]
Sent: Monday, March 4, 2019 4:27 AM
To: Shi, Steven <steven.shi at intel.com>
Cc: Rui Ueyama <ruiu at google.com>; llvm-dev <llvm-dev at lists.llvm.org>; George Rimar <grimar at accesssoftek.com>
Subject: Re: [cfe-dev] RFC: Linker feature for automatically partitioning a program into multiple binaries

It ought to be possible to implement this in the COFF linker, but the implementation would be separate and several details would be different.

- The Windows loader doesn't appear to support loading a library at a fixed address, so to be able to load the partitions on Windows you would likely need to write your own loader. I gather from your other posts that you care most about UEFI -- I don't know what capabilities the UEFI loader has.
- You would need to find some other way to reserve space in the base partition for the other partitions. A few possibilities would be to extend the base partition's bss (which was the first option that I considered on ELF before I discovered the unwinder issue), create another bss section at the end (as I'm doing on ELF) or set the base partition's SizeOfImage to cover the other partitions. However, this would all depend on the details of what the various loaders and unwinders do.
- Probably more things that I haven't thought of. To start with you'd likely need to clone everything referred to by the image's PE header and data directories.

Peter

On Sun, Mar 3, 2019 at 7:35 AM Shi, Steven <steven.shi at intel.com<mailto:steven.shi at intel.com>> wrote:
Hi Peter,
Why this feature is limited only for ELF binaries? Is it possible to support COFF DLL as well?


Thanks
Steven

From: cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org<mailto:cfe-dev-bounces at lists.llvm.org>] On Behalf Of Peter Collingbourne via cfe-dev
Sent: Saturday, March 2, 2019 9:52 AM
To: Rui Ueyama <ruiu at google.com<mailto:ruiu at google.com>>
Cc: llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>; George Rimar <grimar at accesssoftek.com<mailto:grimar at accesssoftek.com>>; cfe-dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>>
Subject: Re: [cfe-dev] RFC: Linker feature for automatically partitioning a program into multiple binaries

I covered the example of the operating system API in the original proposal, but yes, you might also be able to use this for kernel modules. The justification isn't as great for them since kernels typically require linker scripts, and I'm not planning to make this compatible with linker scripts. But if you did manage to write a kernel that didn't need a linker script, this seems like it might be useful.

Peter

On Fri, Mar 1, 2019 at 5:39 PM Rui Ueyama <ruiu at google.com<mailto:ruiu at google.com>> wrote:
Somehow I missed this email.

This is an interesting proposal. I guess that Chrome on Android is a candidate program to apply this. I wonder what are other programs you can apply this scheme. Looks like as long as you want to split a program into multiple small pieces sharing the same memory address space, this is applicable. One thing I can think of as an example of it is the kernel module. Is there anything other than that?

On Tue, Feb 26, 2019 at 5:34 PM Peter Collingbourne <peter at pcc.me.uk<mailto:peter at pcc.me.uk>> wrote:
Hi folks,

I'd like to propose adding a feature to ELF lld for automatically partitioning a program into multiple binaries. (This will also involve adding a feature to clang, so I've cc'd cfe-dev as well.)

== Problem statement ==

Embedded devices such as cell phones, especially lower end devices, are typically highly resource constrained. Users of cell phone applications must pay a cost (in terms of download size as well as storage space) for all features that the application implements, even for features that are only used by a minority of users. Therefore, there is a desire to split applications into multiple pieces that can be downloaded independently, so that the majority of users only pay the cost of the commonly used features. This can technically be achieved using traditional ELF dynamic linking: the main part of the program can be compiled as an executable or DSO that exports symbols that are then imported by a separate DSO containing the part of the program implementing the optional feature. However, this itself imposes costs:
- Each exported symbol by itself imposes additional binary size costs, as it requires the name of the symbol and a dynamic symbol table entry to be stored in both the exporting and importing DSO, and on the importing side a dynamic relocation, a GOT entry and likely a PLT entry must be present. These additional costs go some way towards defeating the purpose of splitting the program into pieces in the first place, and can also impact program startup and overall performance because of the additional indirections.
- It can result in more code needing to appear in the main part of the program than necessary. For example, imagine that both the feature and the main program make use of a common (statically linked) library, but they call different subsets of the functions in that library. With traditional ELF linking we are forced to either link and export the entire library from the main program (even the functions unused by either part of the program) or carefully maintain a list of functions that are used by the other parts of the program.
- Since the linker does not see the whole program at once and links each piece independently, a number of link-time optimizations and features stop working, such as LTO across partition boundaries, whole-program devirtualization and non-cross-DSO control flow integrity (control flow integrity has a cross-DSO mode, but that also imposes binary size costs because a significant amount of metadata needs to appear in each DSO).
There are ways around at least the first point. For example, the program could arrange to use a custom mechanism for binding references between the main program and the feature code, such as a table of entry points. However, this can impose maintenance costs (for example, the binding mechanism can be intrusive in the source code and typically has to be maintained manually), and it still does not address the last point.

== Proposed solution ==

I propose to extend lld so that it can perform the required partitioning automatically, given a set of entry points for each part of the program. The end product of linking will be a main program (which can be either an executable or a DSO) combined with a set of DSOs that must be loaded at fixed addresses relative to the base address of the main program. These binaries will all share a virtual address space so that they can refer to one another directly using PC-relative references or RELATIVE dynamic relocations as if they were all statically linked together in the first place, rather than via the GOT (or custom GOT-equivalent).

The way that it will work is that we can extend the graph reachability algorithm currently implemented by the linker for --gc-sections. The entry points for each partition are marked up with a string naming the partition, either at the source level with an attribute on the function or global variable, or by passing a flag to the compiler (this string becomes the partition's soname). These symbols will act as the GC roots for the partition and will be exported from its dynsym. Assuming that there is a single partition, let's call this set of symbols S2, while all other GC roots (e.g. non-marked-up exported symbols, sections in .init_array) we call S1. Any sections reachable from S1 are allocated to the main partition, while sections reachable only from S2 but not from S1 are allocated to S2's partition. We can extend this idea to multiple loadable partitions by defining S3, S4 and so on, but any sections reachable from multiple loadable partitions are allocated to the main partition even if they aren’t reachable from the main partition.

When assigning input sections to output sections, we take into account, in addition to the name of the input section, the partition that the input section is assigned to. The SHF_ALLOC output sections are first sorted by partition, and then by the usual sorting rules. As usual, non-SHF_ALLOC sections appear last and are not sorted by partition. In the end we are left with a collection of output sections that might look like this:

Main partition:
0x0000 ELF header, phdrs
0x1000 .rodata
0x2000 .dynsym
0x3000 .text

Loadable partition 1:
0x4000 ELF header, phdrs
0x5000 .rodata
0x6000 .dynsym
0x7000 .text

Loadable partition 2:
0x8000 ELF header, phdrs
0x9000 .rodata
0xa000 .dynsym
0xb000 .text

Non-SHF_ALLOC sections from all partitions:
.comment
.debug_info
(etc.)

Now linking proceeds mostly as usual, and we’re effectively left with a single .so that contains all of the partitions concatenated together. This isn’t very useful on its own and is likely to confuse tools (e.g. due to the presence of multiple .dynsyms); we can add a feature to llvm-objcopy that will extract the individual partitions from the output file essentially by taking a slice of the combined .so file. These slices can also be fed to tools such as debuggers provided that the non-SHF_ALLOC sections are left in place.

The envisaged usage of this feature is as follows:
$ clang -ffunction-sections -fdata-sections -c main.c # compile the main program
$ clang -ffunction-sections -fdata-sections -fsymbol-partition=libfeature.so -c feature.c # compile the feature
$ clang main.o feature.o -fuse-ld=lld -shared -o libcombined.so -Wl,-soname,libmain.so -Wl,--gc-sections
$ llvm-objcopy libcombined.so libmain.so --extract-partition=libmain.so
$ llvm-objcopy libcombined.so libfeature.so --extract-partition=libfeature.so

On Android, the loadable partitions can be loaded with the android_dlopen_ext<https://developer.android.com/ndk/reference/group/libdl> function passing ANDROID_DLEXT_RESERVED_ADDRESS to force it to be loaded at the correct address relative to the main partition. Other platforms that wish to support this feature will likely either need to add a similar feature to their dynamic loader or (in order to support loading the partitions with a regular dlopen) define a custom dynamic tag that will cause the dynamic loader to first load the main partition and then the loadable partition at the correct relative address.

== In more detail ==

Each loadable partition will require its own sections to support the dynamic loader and unwinder (namely: .ARM.exidx, .dynamic, .dynstr, .dynsym, .eh_frame_hdr, .gnu.hash, .gnu.version, .gnu.version_r, .hash, .interp, .rela.dyn, .relr.dyn), but will be able to share a GOT and PLT with the main partition. This means that all addresses associated with symbols will continue to be fixed.

In order to cause the dynamic loader to reserve address space for the loadable partitions so that they can be loaded at the correct address later, a PT_LOAD segment is added to the main partition that allocates a page of bss at the address one byte past the end of the last address in the last partition. In the Android dynamic loader at least, this is enough to cause the required space to be reserved. Other platforms would need to ensure that their dynamic loader implements similar behaviour.

I haven't thought about how this feature will interact with linker scripts. At least to start with we will likely need to forbid using this feature together with the PHDRS or SECTIONS linker script directives.

Some sections will need to be present in each partition (e.g. .interp and .note sections). Probably the most straightforward way to do this will be to cause the linker to create a clone of these sections for each partition.

== Other use cases ==

An example of another use case for this feature could be an operating system API which is exposed across multiple DSOs. Typically these DSOs will be implemented using private APIs that are not exposed to the application. This feature would allow you to create a common DSO that contains the shared code implementing the private APIs (i.e. the main partition), together with individual DSOs (i.e. the loadable partitions) that use the private APIs and expose the public ones, but without actually exposing the private APIs in the dynamic symbol table or paying the binary size cost of doing so.

== Prototype ==

A prototype/proof of concept of this feature has been implemented here: https://github.com/pcc/llvm-project/tree/lld-module-symbols
There is a test app in the test-progs/app directory that demonstrates the feature on Android with a simple hello world app (based on https://www.hanshq.net/command-line-android.html ). I have successfully tested debugging the loadable partition with gdb (e.g. setting breakpoints and printing globals), but getting unwinding working will need a bit more work.

Note that the feature as exposed by the prototype is different from what I'm proposing here, e.g. it uses a linker flag to specify which symbols go in which partitions. I think the best place to specify this information is at either the source level or the compiler flag level, so that is what I intend to implement.

Thanks,
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Peter


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Peter


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Peter
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