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

Peter Collingbourne via llvm-dev
Wed, 27 Feb 2019 02:36:39 -0000

On Tue, Feb 26, 2019 at 6:23 PM Jake Ehrlich <jakehehrlich at google.com>
wrote:

> Neat Idea.
>
> >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.
>
> Maybe all partitions can contain the same PT_LOAD segments? or the main
> partition contains all PT_LOAD segments for the entire address space?
>

I don't think this will work. When the main partition is loaded, the
dynamic loader will try to map all of the PT_LOADs, and with this approach
since they won't exist in the main partition (due to the p_offset being out
of bounds), the dynamic loader will likely raise an error or crash. The
per-partition PT_LOADs also give a convenient way for llvm-objcopy to
figure out which section headers need to be copied into each partition.

Another possibility that I considered for reserving the address space was
to extend the p_memsz of the last segment of the main partition to cover
the address space of the other partitions. But this prevents tools such as
unwinders from easily mapping an address onto a DSO by examining PT_LOAD
segments. For example, this is what libunwind does:
https://github.com/llvm/llvm-project/blob/e739ac0e255597d818c907223034ddf3bc18a593/libunwind/src/AddressSpace.hpp#L523

Peter


>
> On Tue, Feb 26, 2019 at 5:35 PM Peter Collingbourne via llvm-dev <
> llvm-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
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>

--
--
Peter
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