[PATCH] D120045: [lld][ELF] support nested special directives in output sections

Thu Feb 17 14:55:57 PST 2022

MaskRay added a comment.

In D120045#3330924 <https://reviews.llvm.org/D120045#3330924>, @bluca wrote:

> In D120045#3330921 <https://reviews.llvm.org/D120045#3330921>, @MaskRay wrote:
>
>> In D120045#3330873 <https://reviews.llvm.org/D120045#3330873>, @bluca wrote:
>>
>>> In D120045#3330562 <https://reviews.llvm.org/D120045#3330562>, @MaskRay wrote:
>>>
>>>> I added the `(TYPE=...)` syntax for the sole purpose that Fedora or systemd (https://github.com/systemd/package-notes) has a proper way to mark `.note*` sections `SHT_NOTE`.
>>>
>>> And unless this fix or something along those lines is included too, it will not be used and it will serve no purpose whatsoever. Note how this patch doesn't mention _any specific directive_ but it simply skips any unknown ones. See it as future proofing, and compatibility with the documented bfd format. It's not going to get any closer to what you want - take it or leave it.
>>
>> Some keywords have been supported. A new keyword likely introduces an interesting enough semantic difference (e.g. changing sh_flags, sh_link, etc) we should not silently ignore.
>> Ignore the unknown keyword like `FOOBAR` may cause a silently broken binary.
>> Catching the error is more important than the compatibility with older linkers.
>>
>> In addition, the `(type)` syntax is actually ambiguous: it is overloaded with the output section address syntax. Ignoring the keyword will cause trouble if the user intends to use a non-keyword variable in a pair of parentheses for the address.
>
> How would something that is not supported and was never supported before produce a broken binary? Anyway, I don't really care _how_ you fix it, feel free to come up with a different solution to fix the same incompatibility if you don't like this one

For example, if we introduce new syntax to change `sh_flags`. Since dynamic loaders don't really read the section header table, mistakes there will not be detected.
If the program does introspection or inspects a relocatable output, the wrong `sh_flags` may cause a problem.
We have syntax for SHT_NOBITS and arbitrary sh_type now, I am comfortably to state that all new syntax will make interesting semantic differences we should not silently ignore.

The solution Michael Matz, I (likely also Cary Coutant (gold)/Rui Ueyama (mold)) prefer most is to use an assembler. binutils ships GNU assembler and let your tool link the relocatable object file.
I am willing to help you if you can point me to a package which has a problem.

  .section .note.package,"a",%note
  ....

If it really doesn't work (then gold and mold will be excluded):

  .note.package (TYPE=SHT_NOTE) : { BYTE(...) BYTE(...) ... }

The `READONLY` thing is unneeded to drop the SHF_WRITE flag.

Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D120045/new/

https://reviews.llvm.org/D120045