llvm dev - Apr 2021 - [lld] Questions regarding linker scripts

Hi,

I have two questions regarding lld behavior related to linker/version scripts.

1. It seems that lld does not strictly obey linker scripts, specifically
regarding section collapsing. For example, a directive like this:

  SECTIONS
  {
    [...]
    .rela.text      : { *(.rela.text .rela.text.* .rela.gnu.linkonce.t.*) }
    [...]

I see no collapsing of these sections in the resulting object, with many
.rela.text.<mangled_function_name> sections present. Is this expected
behavior?


2. Regarding version scripts specifically, they are currently ignored for
relocatable objects:

  https://github.com/llvm/llvm-project/blob/main/lld/ELF/Driver.cpp
  [...]
  // Apply version scripts.
  //
  // For a relocatable output, version scripts don't make sense, and
  // parsing a symbol version string (e.g. dropping "@ver1" from a
symbol
  // name "foo at ver1") rather do harm, so we don't call this if
-r is given.
  if (!config->relocatable) {
  [...]

I understand the motivation here from the perspective of actual symbol
versioning. But an anonymous version script, for example:

{
  global: foo; bar;
  local: *;
}

...would still be useful for relocatable objects, I think. Without this
capability, the only way I'm aware of to broadly apply symbol binding is to
use objcopy after linking is complete. It can be prohibitively slow. But in
fact, that's the approach that FreeBSD takes:

  https://github.com/freebsd/freebsd-src/blob/main/sys/conf/kmod.mk#L262
  https://github.com/freebsd/freebsd-src/blob/main/sys/conf/kmod_syms.awk

Is it reasonable to suggest allowing version scripts for relocatable objects, or
providing an option to do so? (FYI I also tried using a VERSION section inside a
larger linker script to no avail.)

Both of these behaviors of lld were surprising to me, and since they both relate
to linker script input I decided to bundle my questions together.

Thank you for any insight,

Justin

On 2021-04-22, Justin Cady via llvm-dev wrote:
>Hi,
>
>I have two questions regarding lld behavior related to linker/version
scripts.
>
>1. It seems that lld does not strictly obey linker scripts, specifically
regarding section collapsing. For example, a directive like this:
>
>  SECTIONS
>  {
>    [...]
>    .rela.text      : { *(.rela.text .rela.text.* .rela.gnu.linkonce.t.*) }
>    [...]
>
>I see no collapsing of these sections in the resulting object, with many
.rela.text.<mangled_function_name> sections present. Is this expected
behavior?
Are you using --emit-relocs? It is an expected behavior that input section
descriptions do not match
--emit-relocs copied relocation sections. GNU linkers behave this way, too.

If you are using -r, input section descriptions match .text* but do not work on
relocation sections.
I think GNU ld can actually match the first relocation section but cannot
combine multiple .rela*.
Combining multiple .rela* do not make sense because these .rela* have different
sh_link values (relocated section indexes).
>
>2. Regarding version scripts specifically, they are currently ignored for
relocatable objects:
>
>  https://github.com/llvm/llvm-project/blob/main/lld/ELF/Driver.cpp
>  [...]
>  // Apply version scripts.
>  //
>  // For a relocatable output, version scripts don't make sense, and
>  // parsing a symbol version string (e.g. dropping "@ver1" from a
symbol
>  // name "foo at ver1") rather do harm, so we don't call this
if -r is given.
>  if (!config->relocatable) {
>  [...]
>
>I understand the motivation here from the perspective of actual symbol
versioning. But an anonymous version script, for example:
>
>{
>  global: foo; bar;
>  local: *;
>}
>
>...would still be useful for relocatable objects, I think. Without this
capability, the only way I'm aware of to broadly apply symbol binding is to
use objcopy after linking is complete. It can be prohibitively slow. But in
fact, that's the approach that FreeBSD takes:
I have checked  ld.bfd -r --version-script=a.ver a.o.
It doesn't localize symbols either (it likely ignores --version-script).
>  https://github.com/freebsd/freebsd-src/blob/main/sys/conf/kmod.mk#L262
>  https://github.com/freebsd/freebsd-src/blob/main/sys/conf/kmod_syms.awk
>
>Is it reasonable to suggest allowing version scripts for relocatable
objects, or providing an option to do so? (FYI I also tried using a VERSION
section inside a larger linker script to no avail.)
Note that version IDs except VER_NDX_LOCAL/VER_NDX_GLOBAL do not make sence for
ET_REL files.  So
the question is whether it makes sense for `local:` to apply to -r links.  Given
that (1) our
behavior matches GNU ld (2) the --version-script usage can be easily emulated
with {,llvm-}objcopy
-G (3) this can add some code complexity to LLD, I am inclined we don't make
the extension.
>Both of these behaviors of lld were surprising to me, and since they both
relate to linker script input I decided to bundle my questions together.
There are awkward points but in general the behaviors look reasonable to me.
>Thank you for any insight,
>
>Justin
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