[llvm-dev] [RFC] Debug sections for hot-patching LLD's ELF output

bd1976 llvm via llvm-dev llvm-dev at lists.llvm.org
Mon Sep 20 18:05:57 PDT 2021 

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.
- 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 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.

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.

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.
>
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