[all-commits] [llvm/llvm-project] ec4e68: [yaml2obj] - Introduce the "NoHeaders" key for "Se...

[Georgii Rymar via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: ec4e68e667bf0369c36f0e4291618ef2a614bbf0
      https://github.com/llvm/llvm-project/commit/ec4e68e667bf0369c36f0e4291618ef2a614bbf0
  Author: Georgii Rymar <grimar at accesssoftek.com>
  Date:   2020-06-15 (Mon, 15 Jun 2020)

  Changed paths:
    M llvm/include/llvm/ObjectYAML/ELFYAML.h
    M llvm/lib/ObjectYAML/ELFEmitter.cpp
    M llvm/lib/ObjectYAML/ELFYAML.cpp
    M llvm/test/tools/yaml2obj/ELF/section-headers-exclude.yaml
    M llvm/test/tools/yaml2obj/ELF/section-headers.yaml

  Log Message:
  -----------
  [yaml2obj] - Introduce the "NoHeaders" key for "SectionHeaderTable"

We have an issue currently. The following YAML piece just ignores the `Excluded` key.

```
SectionHeaderTable:
  Sections: []
  Excluded:
    - Name: .foo
```

Currently the meaning is: exclude the whole table.

The code checks that the `Sections` key is empty and doesn't catch/check
invalid/duplicated/missed `Excluded` entries.

Also there is no way to exclude all sections except the first null section,
because `Sections: []` currently just excludes the whole the sections header table.

To fix it, I suggest a change of the behavior.

1) A new `NoHeaders` key is added. It provides an explicit syntax to drop the whole table.
2) The meaning of the following is changed:

```
SectionHeaderTable:
  Sections: []
  Excluded:
    - Name: .foo

```
Assuming there are 2 sections in the object (a null section and `.foo`), with this patch it
means: exclude the `.foo` section, keep the null section. The null section is an implicit
section and I think it is reasonable to make "Sections: []" to mean it is implicitly added.
It will be consistent with the global "Sections" tag that is used to describe sections.

3) `SectionHeaderTable->Sections` is now optional. No `Sections` is the same as
   `Sections: []` (I think it avoids a confusion).
4) Using of `NoHeaders` together with `Sections`/`Excluded` is not allowed.
5) It is possible to use the `Excluded` key without the `Sections` key now (in this case
   `Excluded` must contain all sections).
6) `SectionHeaderTable:` or `SectionHeaderTable: []` is not allowed.
7) When the `SectionHeaderTable` key is present, we still require all sections to be
   present in `Sections` and `Excluded` lists. No changes here, we are still strict.

Differential revision: https://reviews.llvm.org/D81655