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

[Peter Smith via cfe-dev]

Hello,

When I first read this I thought that this is kind of like overlays,
but optimising for flash size rather than memory size and with the
dynamic linker as a simplified non-evicting overlay manager. However I
think it might be more accurate to describe as a way of integrating
features implemented in shared libraries into an application?

Do you have an idea of how the development process would work for such
an application? It sounds to me like this isn't going to work well for
"here is an executable that I've already written wrote go find the
bits can be split off into partitions". I'd expect that this is more
of a, "here is an application that has been implemented with some
independent features implemented in shared libraries (natural but
heavy weight partitions) that I want to be optimised together at the
cost of (realistically) losing the shared part of the libraries.". I'm
thinking that one possible development model for you would be first
implement your independent feature as a shared library, the entry
points for the partition could then be extracted from it, with perhaps
the inputs to the shared library link step being extracted for the
main application (eliminating duplicates between the partitions would
be interesting).

My main concern with this is that this could end being fragile, with
subtle bugs that would be difficult to reason about ahead of time and
without the feature implemented on a desktop OS it could be very
difficult to test that a new feature would work with it, or whether a
change to an existing feature would break it. I think we'd need some
extensive documentation to go with it at the least.

Peter

Some other thoughts below:

> We could certainly consider having multiple GOTs which are allocated to partitions in the same way as sections are. This might be useful if for example one of the partitions references a DSO that is unused by the main program and we need to avoid having the main program depend on the DSO. But I consider > this an optimization over the proposed approach and not something that would be strictly required for correctness. I chose to omit this for now for the sake of simplicity and because my customer does not require it for now.

Introducing multiple GOTs might make it difficult to reason about GOT
relative relocations, is it always unambiguous which GOT a relocation
should refer to? Can we answer that question for all targets?

> 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.
I'd recommend not going there unless you have a really good model of
how it will work and what type of scripts are compatible with it. It
would be incredibly easy to write something to break the assumptions
that this is relying on.

An ifunc in one of the loadable partitions could be problematic as
there is only one PLT and GOT in the application, with the ifunc
dynamic relocation running eagerly at application launch but no
resolver present in memory at the time. You'll also need to be careful
to prevent any sharing of things like strings between the partitions.

On Wed, 27 Feb 2019 at 01:34, Peter Collingbourne <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 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