[llvm-dev] [RFC] Debug sections for hot-patching LLD's ELF output
bd1976 llvm via llvm-dev
llvm-dev at lists.llvm.org
Tue Sep 21 17:41:23 PDT 2021
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
).
> 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.
> 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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