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

[Rui Ueyama via cfe-dev]

On Tue, Feb 26, 2019 at 8:38 PM James Y Knight via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I've long wanted a related-but-different form of this sort of thing.
>
> What I want is to be able to statically link _independent_ binaries
> (either executables or shared libraries), which happen to depend on a lot
> of the same code. And, then, to save space by reducing the amount of
> duplicated code between them.
>
> The way I imagined it is that it if I link two programs, the linker would
> emit three resulting files: Program 1, Program 2, and a shared-library
> containing the common code.
>
> As input, I'd give the linker, separately, the lists of objects/libraries
> to go into Program 1, and those to go into Program 2. It would do its
> normal static archive search/object-file-selection process separately for
> each list. After collecting the object file lists, any objects that ended
> up used both in #1 and #2 would go into the shared-library, while the
> others would go into their own separate programs.
>
> (Note that this then causes static initializers to end up being
> run/included in the expected programs in the same way as with plain static
> linking.)
>
> Finally, gc-sections should do its graph coloring starting only from the
> entry-points of the two programs, but also coloring through into the
> automatic shared library. In the end, the shared library should only
> contain functions that are used by at least one of the programs. (As I
> described it, this mechanism would leave functions used only by one program
> that came from an object file used by both programs in the shared-lib.
> Alternatively, the graph coloring could possibly determine the target
> location of the function, too)
>
> Without any new loader hacks, you'd still have some additional overhead,
> but since the symbol names in the automatic shared library are irrelevant
> (other than needing to be unique), they can be shortened to reduce the
> string-table overhead and reduce the risk of external-usage.
>
> In the end, you'd have two completely normal binaries, which use a normal
> shared object -- but the shared lib was auto-generated, with meaningless
> private symbols, only exporting the minimum set of symbols it needs to in
> order to let the dynamic loader load it into the exact programs it was
> compiled for.
>

I wonder what is the difference of having three files (2 executables and 1
DSO) and having a single binary containing all code and data. It is not too
hard to link two programs together (which is what busybox does, IIUC), and
the resulting executable would be as compact as the sum of the three
different files. If you execute a busybox'ed executable, only the needed
part is mapped to memory, so I guess you wouldn't lose anything at runtime.
What am I missing?


>
> On Tue, Feb 26, 2019 at 8:35 PM Peter Collingbourne via cfe-dev <
> cfe-dev at lists.llvm.org> 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,
>> --
>> --
>> Peter
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