llvm dev - Sep 2021 - [RFC] Debug sections for hot-patching LLD's ELF output

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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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
>>
>>