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

[Kaylee Blake]

On 9/3/20 11:36 pm, Florian Weimer wrote:
> * 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-)

Ah, yes, sorry. It does enable that; that was my primary motivation for
my part in it.

>> 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.

I was thinking of using the PT_NOTE program header, as that's already
used by the dynamic linker to check some things.

>> 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.
-- 
Kaylee Blake <klkblake@gmail.com>
C is the worst language, except for all the others.