RFC: [PATCH] ELF: Don't require section header on ELF objects

[Florian Weimer]

* Kaylee Blake:

> On 9/3/20 6:43 pm, Florian Weimer wrote:
>> * H. J. Lu:
>> 
>>> Section header isn't mandatory on ELF executable nor shared library.
>>> This patch adds a new linker option, -z nosectionheader, to omit ELF
>>> section header when building an executable or shared library, adds
>>> an objcopy and strip option, --remove-section-header, to remove ELF
>>> section header from an executable or shared library.
>>>
>>> The PT_DYNAMIC segment contains DT_HASH/DT_GNU_HASH/DT_MIPS_XHASH,
>>> DT_STRTAB, DT_SYMTAB, DT_STRSZ and DT_SYMENT, which can be used to
>>> reconstruct dynamic symbol table when section header isn't available.
>>> For DT_HASH, the number of dynamic symbol table entries equals the
>>> number of chains.  For DT_GNU_HASH/DT_MIPS_XHASH, only defined symbols
>>> with non-STB_LOCAL indings are in hash table.  Since in dynamic symbol
>>> table, all symbols with STB_LOCAL binding are placed before symbols with
>>> other bindings and all defined symbols are placed before undefined ones,
>>> the highest symbol index in DT_GNU_HASH/DT_MIPS_XHASH is the highest
>>> dynamic symbol table index.
>> 
>> Does this patch enable ld to use shared objects without a section
>> header for linking?
>> 
>> I think the NEWS and manual update should clarify this.
>> 
>> In my opinion, it should NOT be possible to link against objects
>> without section headers.  Lack of section headers clearly marks the
>> object as a run-time only object.  This is useful if you want to
>> prevent developers to create DT_NEEDED dependencies on internal
>> libraries, for example.
>
> For shared objects without debug symbols, the section header table is
> ~2kB on average of redundant data. I'm also not a fan of the
> inconsistency of having shared libraries that the dynamic linker is
> perfectly happy to load, but ld can't link against, especially since
> this seems like an oversight rather than an intended design decision.

You didn't answer my question. 8-)

> If the internal library use case is worth supporting, adding a note
> tagging said internal library as not meant to be linked against seems
> like a better (and much more efficient) approach?

The dynamic linker does not look at section headers at all.

> This could also actually result in the dynamic linker rejecting
> attempting to load through DT_NEEDED entry.

No, DT_NEEDED entries would be how the library is loaded.