Re: elf.5.html: Resolving confusion.

[Florian Weimer <fweimer@redhat.com>]

* Robin Kuzmin:

> http://man7.org/linux/man-pages/man5/elf.5.html
>
> I see the fragment:
>
>        A section header table index is a subscript into this array.  Some
>        section header table indices are reserved: the initial entry and the
>        indices between SHN_LORESERVE and SHN_HIRESERVE.  The initial entry
>        is used in ELF extensions for e_phnum, e_shnum and e_strndx; in other
>        cases, each field in the initial entry is set to zero.  An object
>        file does not have sections for these special indices:
>
>        SHN_UNDEF
>               This value marks an undefined, missing, irrelevant, or other‐
>               wise meaningless section reference.
>
> I interpret it like this:
>
>        A section header table index **(e_shstrndx)** is a subscript
> into this array.

No, e_shstrndx is just one of the possible indices.  It's just the
string table that is used for section names.

>  Some
>        section header table indices are reserved:
>        the initial entry **(index 0)**
>        and the indices **from** SHN_LORESERVE **to** SHN_HIRESERVE **,
> inclusive**.
>        **Such reserved indices, except SHN_XINDEX (0xffff), cannot be
> used in e_shstrndx.
>        If e_shstrndx is SHN_XINDEX (0xffff) then the sh_link filed of
> the initial ElfN_Shdr cannot contain such reserved indices.**
>        The **three fields in the** initial entry ** - sh_info, sh_size
> and sh_link - can be** used in ELF extensions for e_phnum, e_shnum and
> **e_shstrndx correspondingly**. **If they are not used then they are
> set to zero. All other fields of the initial entry are set to zero.**
>        **The section header table entries with the following special
> indices contain special values,         and in the ELF file there are
> no sections associated with such entries.**
>
>        SHN_UNDEF
>               This value marks an undefined, missing, irrelevant, or other‐
>               wise meaningless section reference.
>               **This index can be 0 in which case it means the initial
> ElfN_Shdr with a special meaning described above.**
>
> Is such an interpretation correct?

I'm not sure if your clarifications are correct.  I don't think the
section header extension mechanism is used for extending e_phum.

The main thing that's not clear to me in the current description is
whether the 256 reserved indices have still entries in the table
(probably of type SHT_NULL).

Cc:ing Mark, in case he has further comments.

Thanks,
Florian