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

Tue Feb 26 19:47:38 PST 2019

On Tue, Feb 26, 2019 at 6:41 PM Eli Friedman <efriedma at quicinc.com> wrote:

> This seems like a very complicated approach… do you have some numbers to
> give some idea how much of an improvement we’re talking about here over a
> more conventional solution involving shared libraries?  Or have you not
> gotten that far?
>

I can talk to my internal customer to see what kind of overhead they were
seeing. But I do know that at the start of the project they did evaluate
using regular dynamic linking for the feature partitions, and that was
quickly rejected in favour of other approaches due to the code size and
maintenance overhead. And with control flow integrity the binary size of
the cross-DSO metadata dwarfed the binary size savings that they were
hoping to gain by splitting their program in two.

Furthermore, there are things that you simply cannot do with a more
conventional approach, such as optimizations relying on whole-program
information (like whole-program devirtualization, which helps significantly
in my customer's program).

What’s the tradeoff involved in the specific sections you chose to split?
> It seems like it would be possible to, for example, split the GOT, or avoid
> splitting the relocation/EH/etc. sections.  Some variation would require
> different runtime support, I guess.
>

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.

I think we need to split the dynamic relocation section because otherwise
the dynamic loader will try to relocate the unreadable memory of the other
partitions and cause a SIGSEGV. Similarly, we need to split the EH sections
because unwinders will generally expect to be able to find the unwind info
for a function by enumerating PT_LOADs to map an address onto a DSO and
then using that DSO's PT_ARM_EXIDX/PT_GNU_EH_FRAME to find the unwind info.
See for example what libunwind does:
https://github.com/llvm/llvm-project/blob/e739ac0e255597d818c907223034ddf3bc18a593/libunwind/src/AddressSpace.hpp#L523

As you point out, the latter part could vary based on the runtime, but I
don't see a strong reason to do it another way.

> It looks like this doesn’t include a proposal for the corresponding LLVM
> IR extension?  I think it might be sort of complicated to define correctly…
> specifically, in terms of what it means to “use” a function or global from
> a different partition (so the program doesn’t try to speculatively access
> something which isn’t loaded).  This could come up even without LTO if you
> have C++ inline functions, since all functions with weak linkage have to be
> in the first partition.  (At least, I think they do, unless you invent a
> new kind of “partition” visibility for this.)
>

The idea here is that for code to "use" a function or global is exactly the
same thing as having a relocation pointing to it. This is the same
principle that is used to implement --gc-sections.

So for a program to end up accessing a section (speculatively or otherwise)
there needs to be a chain of relocations referring to it from the entry
points. That would force the section into either the main partition or the
same partition as the referent.

Another way to think about it is: when I load the main partition into
memory, I have loaded all code that is reachable from the main partition's
entry points. Now I dynamically load a feature partition. I've now loaded
all code that is reachable from the combination of the main partition and
the feature partition's entry points. That's pretty much the same thing as
having first loaded a conventional ELF DSO linked with --gc-sections with
just the main partition's entry points, and then replacing it with a second
DSO linked with --gc-sections with the main partition + feature partition's
entry points, except that none of the addresses in the main partition
happen to have changed. So if --gc-sections works, this should work too.

You might be wondering: what happens if I directly reference one of the
feature partition's entry points from the main partition? Well, something
interesting will happen. The feature partition's dynamic symbol table will
contain an entry for the entry point, but the entry's address will point
inside the main partition. This should work out just fine because the main
partition is guaranteed to be loaded if the feature partition is also
loaded. (Which is the same reason why direct pc-relative references from
the feature partition to the main partition will also work.)

I don't think any significant IR extensions are necessary here, except
perhaps for the part involving attaching the -fsymbol-partition names to
globals, but I think that part is mostly trivial and it would probably end
up looking like the custom section name field.

I'm not sure I understand how weak linkage is impacted here. With this
nothing special happens inside the linker until we start handling
--gc-sections, and by that time weak/strong resolution has already
happened. In ELF, dynamic loaders do not care about symbol bindings (except
for weak undefined symbols), so we get the same result whether the symbols
are weak or not.

Peter

>
>
> -Eli
>
>
>
> *From:* cfe-dev <cfe-dev-bounces at lists.llvm.org> *On Behalf Of *Peter
> Collingbourne via cfe-dev
> *Sent:* Tuesday, February 26, 2019 5:35 PM
> *To:* llvm-dev <llvm-dev at lists.llvm.org>; cfe-dev <cfe-dev at lists.llvm.org>
> *Cc:* George Rimar <grimar at accesssoftek.com>
> *Subject:* [EXT] [cfe-dev] RFC: Linker feature for automatically
> partitioning a program into multiple binaries
>
>
>
> 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
>

-- 
-- 
Peter
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