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

Kaylee Blake klkblake@gmail.com
Mon Mar 9 13:28:59 GMT 2020 
On 9/3/20 11:46 pm, Florian Weimer wrote:
> * 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.
> 
> I think that's conceptually the wrong thing to do for ELF, sorry.  If
> there is no section header, the object should be unlinkable.  The
> linker should not use the dynamic segment to locate the symbol
> information, only the dynamic section (in case the link ABI and
> run-time ABI are different).

I'm confused by your comment about link and run-time ABIs differing;
surely if the ABI at runtime differs from the ABI at link time, you are
just going to crash at runtime?

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