[llvm-dev] [RFC] Debug sections for hot-patching LLD's ELF output
bd1976 llvm via llvm-dev
llvm-dev at lists.llvm.org
Thu Sep 23 17:21:12 PDT 2021
It is very important to resist features that add needless complexity :)
I will reply to the (great) points you have raised tomorrow. The
hot-patching feature is proprietary technology and I need to check how much
I can disclose about it - sorry! I will also put up a prototype
implementation so that the complexity of the implementation can be judged.
I have not attempted to describe all GOT/PLTs only the ones that are
structured "normally". x86-64's IBT PLT would need an extension to the
binary format to describe. I am not convinced we need to describe every
variation to add value. If the binary format can describe the commonly used
GOT/PLT structures then I believe that is sufficient. We can design the
binary format to be flexible so that it can be extended in the future if
support for a GOT/PLT structure that cannot be described currently is
required.
Do you have an opinion on the other sections? In particular the linkmap
section? That section is the most important information for our
hot-patching implementation and it also has clear benefits over the current
-Map file option.
Thanks.
On Fri, Sep 24, 2021 at 12:40 AM Fangrui Song <maskray at google.com> wrote:
>
> On 2021-09-22, bd1976 llvm wrote:
> > Thanks for looking at this proposal.
> >
> >On Tue, Sep 21, 2021 at 3:43 AM Fangrui Song <maskray at google.com> wrote:
> >
> >> On 2021-09-21, bd1976 llvm via llvm-dev wrote:
> >> >As mentioned Sony would like LLD to optionally emit sections that
> describe
> >> >the GOT and PLT.
> >> >
> >> >The proposed binary format of these sections is as follows:
> >> >
> >> >.debug_lld_got
> >> >==============
> >> >
> >> >The .debug_lld_got section contains a GOT description. The GOT
> description
> >> >begins with a header composed of the following fields:
> >> >
> >> >length (uleb)
> >> >- The length in bytes of the GOT description not including the length
> >> field
> >> >itself.
> >> >- This allows for padding to be added to the section, useful for
> purposes
> >> >such as slop for incremental linking.
> >>
> >> I am dubivious whether people will find incremental linking useful:)
> >> https://news.ycombinator.com/item?id=26233244 from Rui Ueyama
> >> and
> >> https://sourceware.org/pipermail/binutils/2021-September/117828.html
> >> from Cary Coutant:
> >> "Do you think you'd ever want incremental linking on powerpc? Frankly,
> >> the effort for just the one target platform was pretty high, the
> >> maintenance on it is burdensome, and I'm tempted to deprecate it and
> >> rip it out at some point in the future."
> >>
> >
> >I generally tend to agree w.r.t incremental linking. However, supporting
> >the ability to include extra space in a section could have many uses and
> >therefore I think that it is something that section formats should support
> >as long as it is cheap to do so. Having said that we don't actually have a
> >need right now for this so I'm happy to drop it from the specification.
> >
> >
> >>
> >> >- The value cannot exceed Elf_Off.
> >> >
> >> >version (uleb)
> >> >- The version of the description information.
> >> >- Currently, 0.
> >> >- The value cannot exceed Elf_Word.
> >> >
> >> >The header is then followed by list of entry descriptions.
> >> >Each entry description describes the GOT entry with the same index.
> >> >Each entry description starts with three ulebs:
> >> >
> >> >- The first uleb gives the number of ulebs used by this description (so
> >> >that the description can be skipped if the category isn't understood).
> The
> >> >value cannot exceed Elf_Word.
> >> >- The second uleb gives the number of GOT slots* used by this GOT
> entry.
> >> >The value cannot exceed Elf_Word.
> >> >- The third uleb encodes the category of the GOT entry. The value
> cannot
> >> >exceed Elf_Word.
> >> >
> >> >* Except for GOT_CAT_PADDING entries where this field gives the number
> of
> >> >bytes of padding (the value cannot exceed Elf_Off) not the number of
> GOT
> >> >slots.
> >> >
> >> >A category encoding can specify multiple associated arguments. Argument
> >> >interpretation is specified by the encoding. If an encoding requires
> >> >arguments, the bytes for those follow the bytes for the second uleb in
> the
> >> >entry description.
> >> >
> >> >Categories are:
> >> >
> >> >Encoding Argument * Size (slots)
> >> > Notes
> >> >GOT_CAT_UNKNOWN none 1
> >> >Unknown area of the GOT.
> >> >GOT_CAT_PADDING none <variable>
> >> > Padding between GOT regions.
> >> >
> >> The
> >> >size field gives the padding size in bytes not the number of GOT slots.
> >> >GOT_CAT_GOTPLT_HEADER none <target dependent>
> >> The
> >> >.got.plt header. x86_64 size = 3 slots.
> >> >GOT_CAT_GOT symbol index 1
> >> >Normal entry for a symbol.
> >> >GOT_CAT_PLTGOT symbol index 1
> >> >.got.plt Entry for a PLT reference to a symbol.
> >> >GOT_CAT_IGOTPLT symbol index 1
> >> >.igot.plt entry for an ifunc.
> >> >GOT_CAT_IGOTCANONICAL symbol index 1
> >> GOT
> >> >entry for canonical PLT entry for non-preemptible ifunc case.
> >> >GOT_CAT_TLSDESC symbol index 2
> >> GOT
> >> >entry for a TLSDESC slot.
> >> >GOT_CAT_TLS_GD symbol index 2
> >> GOT
> >> >entry for a GD TLS reference.
> >> >GOT_CAT_TLS_LD none 2
> >> GOT
> >> >entry for tls_index structure for an LD TLS reference.
> >> >GOT_CAT_TLS_IE symbol index 1
> >> GOT
> >> >entry for a IE TLS reference.
> >> >GOT_CAT_PPC64_V2_ABI_TLSLD_GOT_OFF symbol index 1
> >> >PPC64 specific TLSLD GOT slot.
> >> >
> >> >.debug_lld_plt
> >> >==============
> >> >
> >> >The .debug_lld_plt section contains a PLT description. A PLT
> description
> >> >begins with a generic header composed of the following 3 ulebs:
> >> >
> >> >length (uleb)
> >> >- The length in bytes of this PLT description not including the length
> >> >field itself.
> >> >- This allows for padding to be added to the section, useful for
> purposes
> >> >such as slop for incremental linking.
> >> >- The value cannot exceed Elf_Off.
> >> >
> >> >version (uleb)
> >> >- The version of this description information. Currently, 0. The value
> >> >cannot exceed Elf_Word.
> >> >
> >> >type (uleb)
> >> >- The type of the PLT being described.
> >> >- This affects the interpretation of the remaining description.
> >> >- Currently, only PLT_FIXSZ_ENT(value = 0) is defined for describing
> PLT
> >> >sections composed of a header and N fixed size entries.
> >> >- The value cannot exceed Elf_Word; although, currently as there is
> only
> >> >one value specified a smaller representation is sufficient.
> >> >
> >> >PLT_FIXSZ_ENT interpretation
> >> >Following the generic header is the PLT_FIXSZ_ENT description header
> which
> >> >is composed of the following 2 ulebs:
> >> >
> >> >PLT header size (uleb)
> >> >- The size of the PLT header in bytes.
> >> >- The value cannot exceed Elf_Off.
> >>
> >> >PLT entry size (uleb)
> >> >- The size of a PLT entry.
> >> >- The value cannot exceed Elf_Word.
> >>
> >> The PLT header size and PLT entry size are hard coded depending on the
> >> architecture and a few security related options like -z retpolineplt,
> >> ibt, bti. Is a generic description scheme useful?
> >>
> >
> >It's useful because the description is emitted by the linker rather than
> >requiring the consuming tools to be adapted to the linker's output. For
> >example, llvm-objdump can generate <symbol>@plt labels for PLT entries
> when
> >disassembling but this doesn't work if -z retpolineplt is used as the
> code
> >doesn't support that newer type of PLT (
> >
> https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp#L494
> >).
>
> I am concerned that this would add a significant complexity to LLD.
>
> Except canonical PLT entries (normal function and STT_GNU_IFUNC
> converted STT_FUNC), PLT entries have insignificant addresses and the
> linker can generate multiple instances.
> For example, the PowerPC64 port PLT is coupled with range extension
> thunks and there can be multiple instances.
> Each architecture's PLT may have a different shape.
> I am not sure how a generic format can describe a stub.
> Some architectures can do micro optimization like: if we know the hi
> part of a pair of hi/lo values is zero, we may save one instruction.
> Such choice is easy to represent in code but difficult to describe
> in a serialized format.
>
> AArch64's BTI PLT is also interesting: some PLT entries may have a
> leading `bti c` while some don't.
>
> x86-64's IBT PLT is worse: there are two sections: .plt and .plt.sec .
> How to describe it?
> (Multiple folks were against .plt.sec ; I subscribed to x86-64-abi after
> this event in case I missed such over-engineering designs in the
> future.)
>
> Describing PLT/GOT gives me a sense like support GNU ld --verbose style
> linker script dump (https://bugs.llvm.org/show_bug.cgi?id=51309).
> Yes, it can make some applications happy but the implementation complexity
> would be huge.
>
> Perhaps something I really want to ask is whether we ran into an XY
> problem (https://xyproblem.info/). What did the hot-patching feature
> actually need? FWIW such a feature is also implemented in the Linux
> kernel, called live-patching, which is related to dynamic ftrace.
> So far we haven't heard that they need anything from the linker side.
>
> Well, a GNU contributor added -z unique-symbol very quickly while the
> needs appear to have disappeared :)
> https://bugs.llvm.org/show_bug.cgi?id=50745
> I am sold that this option is misdesigned :)
> (
> https://maskray.me/blog/2020-11-15-explain-gnu-linker-options#z-unique-symbol
> )
>
> >
> >> If the new format is to describe dynamic relocations in a compact way, I
> >> am wondering whether this has over-engineered and can achieve the design
> >> goal.
> >> A program doesn't typically have many GLOB_DAT, TLSDESC, and TLS
> GD/LD/IE
> >> relocations.
> >>
> >
> >The purpose is to describe the GOT/PLT in a consistent and simple manner
> >for consuming tools. Over the years there have been a number of changes to
> >how the GOT is optimised. GOT entries can be patched statically, patched
> >with relocations that don't reference dynamic symbols, or patched with
> >relocations that reference a dynamic symbol etc.. using this section
> allows
> >each GOT entry to be consistently described. If we can design a more
> >compact format for the same information that would be great.
>
> Does --emit-relocs help here?
>
> >> MIPS folks invented DT_MIPS_LOCAL_GOTNO and
> >> DT_MIPS_SYMTABNO-DT_MIPS_GOTSYM, but the scheme rarely saves much space
> >> and turns out to cause more problems with .gnu.hash
> >> https://sourceware.org/pipermail/binutils/2019-December/109330.html
> >
> >
> >The .debug_lld_got section doesn't currently handle the MIPS GOT as it is
> >much more complicated than other GOTs and there already seemed to be code
> >in place to be able to parse and dump it.
> >
> >>The header is then followed by list of entry descriptions.
> >> >- Each entry description is a single uleb and describes the PLT entry
> with
> >> >the same index.
> >> >- The value of the uleb gives the index of the associated GOT entry.
> >> >- The value cannot exceed Elf_Off.
> >>
> >> Is disassembling .plt more convenient? The linker uses a predictable way
> >> to generate it so its content is not that hard to parse.
> >> It can be quick because the shape of a PLT entry is known and many bytes
> >> can be skipped.
> >> With this in mind, this information is just easy to infer from
> >> R_*_JUMP_SLOT relocations.
> >>
> >> >In addition to allowing hot-patching tools to work with the GOT and PLT
> >> the
> >> >information in these sections is of use to any tool that needs to
> display
> >> >information on the GOT and PLT sections. For example, debuggers and
> binary
> >> >tools synthesize labels of the form <symbol>@plt to label the PLT
> >> sections.
> >> >The information in these sections could be used to simplify such tasks.
> >>
> >> How is this format more suitable than existing Elf64_Rel/Elf64_Rela for
> >> hot-patching? The GOT and PLT information can be inferred from .rela.plt
> >> and .rela.dyn easily. The scheme appears to be more complex than the
> >> relocation format.
> >>
> >
> >It's a scheme that describes the GOT and PLT without the consumer needing
> >knowledge of other aspects of the dynamic file format such as the dynamic
> >relocations and symbols. Referencing static symbols directly avoids any
> >ambiguity as to which references caused a GOT entry to be created
> (matching
> >by address may find multiple aliases).
> >
> >
> >> >On Wed, Sep 15, 2021 at 3:51 AM bd1976 llvm <bd1976llvm at gmail.com>
> wrote:
> >> >
> >> >> Hi All,
> >> >>
> >> >> Sony maintains a downstream patchset to optionally emit additional
> >> >> informational sections to the ELF output file created by LLD. These
> >> >> sections describe LLD's output and the transformations applied during
> >> >> Linking. These additional sections are used with the static symbol
> >> >> table (.symtab) to facilitate the operation of hot-patching tools.
> >> >>
> >> >> Our preferences are that:
> >> >>
> >> >> - The information required for hot-patching is stored in the ELF
> >> >> output file as ELF sections, as opposed to being emitted into
> >> >> auxiliary files. Otherwise, customers have to adjust their
> processes
> >> >> to keep the ELF output file and auxiliary files together when
> >> >> packing/moving the ELF output file and ensure they are correctly
> >> >> matched.
> >> >>
> >> >> - These metadata sections are created by LLD, rather than derived via
> >> >> a post-link procedure. Performance is important, as customers want
> >> >> to be able to enable the emission of hot-patching metadata by
> >> >> default, and having LLD directly emit the required sections is more
> >> >> efficient and a simpler work-flow.
> >> >>
> >> >> The contents of these sections could be seen as debugging information
> >> >> for the linking process. Certainly, we would want to handle these
> >> >> sections with the same rules that apply to debugging sections when
> >> >> manipulating a linked ELF with binary utility tools. For that reason
> >> >> the sections are all named .debug_lld_* e.g. .debug_lld_linkmap.
> >> >>
> >> >> Currently, Sony would like to emit the following sections and we
> >> >> believe that they are generally useful:
> >> >>
> >> >> - A linkmap section that contains a subset of the information
> contained
> >> >> in a linker -Map file. This section specifies the linked address
> for
> >> >> each input section.
> >> >>
> >> >> - A section which specifies the list of wrapped symbols.
> >> >>
> >> >> - A section that describes the GOT. This provides:
> >> >> -- A category for each entry, examples: GOT entry, PLTGOT entry, TLS
> GD
> >> >> entry, LD TLS tls_index structure entry etc..
> >> >> -- A slot index at which the entry starts.
> >> >> -- A size for the entry, as GOT entries may take more than one GOT
> >> >> slot (e.g. a TLS GD entry takes two slots).
> >> >> -- An optional static symbol index to which the GOT entry is
> associated
> >> >> (some entries e.g. the LD TLS tls_index structure are not
> associated
> >> >> with a particular symbol).
> >> >>
> >> >> - A section describing the PLT. This section needs to be somewhat
> >> >> flexible to deal with the many different PLT's that exist on ELF
> >> >> toolchains. However, for a fixed size entry PLT description the
> >> section
> >> >> will supply:
> >> >> -- Which range of bytes comprises the PLT header.
> >> >> -- The size of a PLT entry.
> >> >> -- For each PLT entry, the GOT slot index of the associated GOT
> entry.
> >> >> Combined with the information on GOT entries from the GOT
> description
> >> >> section this allows for the association of a PLT entry with a
> symbol.
> >> >>
> >> >> Similar to DWARF sections these are non-alloc sections. They are
> encoded
> >> >> as sequences of ULEB128 values. As these are debugging sections, not
> >> core
> >> >> ELF sections, a compact representation is justifiable, even if the
> >> encoding
> >> >> is more complex.
> >> >>
> >> >> In order to anchor this discussion I have created
> >> >> https://reviews.llvm.org/D109804
> >> >> which contains a prototype implementation of the linkmap section
> >> referenced
> >> >> above.
> >> >>
> >> >> I would like to ascertain whether the LLVM community would be
> >> >> supportive of adding the ability to generate such sections to LLD?
> >> >>
> >> >> Thanks.
> >> >>
> >>
> >> >_______________________________________________
> >> >LLVM Developers mailing list
> >> >llvm-dev at lists.llvm.org
> >> >https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >>
> >>
>
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