From: Kaylee Blake <klkblake@gmail.com>
To: binutils@sourceware.org
Subject: Re: RFC: [PATCH] ELF: Don't require section header on ELF objects
Date: Mon, 9 Mar 2020 23:44:49 +1030 [thread overview]
Message-ID: <36183da0-ed0c-13bf-2cb3-bd004e8d46f9@gmail.com> (raw)
In-Reply-To: <875zfdad9j.fsf@mid.deneb.enyo.de>
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.
next prev parent reply other threads:[~2020-03-09 13:14 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-08 17:59 H.J. Lu
2020-03-08 18:06 ` H.J. Lu
2020-03-08 23:35 ` Alan Modra
2020-03-08 23:46 ` H.J. Lu
2020-03-09 0:02 ` H.J. Lu
2020-03-09 0:02 ` Kaylee Blake
2020-03-09 0:05 ` Alan Modra
2020-03-09 1:36 ` H.J. Lu
2020-03-09 1:59 ` Kaylee Blake
2020-03-09 2:23 ` Alan Modra
2020-03-09 2:35 ` H.J. Lu
2020-03-09 4:14 ` H.J. Lu
2020-03-09 4:59 ` Kaylee Blake
2020-03-09 11:56 ` Alan Modra
2020-03-08 23:24 ` Kaylee Blake
2020-03-08 23:29 ` H.J. Lu
2020-03-08 23:38 ` Alan Modra
2020-03-08 23:45 ` H.J. Lu
2020-03-12 2:14 ` Fangrui Song
2020-03-09 8:13 ` Florian Weimer
2020-03-09 12:54 ` Kaylee Blake
2020-03-09 13:06 ` Florian Weimer
2020-03-09 13:14 ` Kaylee Blake [this message]
2020-03-09 13:16 ` Florian Weimer
2020-03-09 13:28 ` Kaylee Blake
2020-03-09 13:29 ` Florian Weimer
2020-03-09 13:45 ` Kaylee Blake
2020-03-09 13:54 ` H.J. Lu
2020-03-09 14:02 ` Kaylee Blake
2020-03-09 14:52 ` Florian Weimer
2020-03-09 15:07 ` Kaylee Blake
2020-03-09 15:29 ` Florian Weimer
2020-03-09 13:44 ` Alan Modra
2020-03-09 13:54 ` Kaylee Blake
2020-03-09 22:34 ` Alan Modra
2020-03-10 0:14 ` H.J. Lu
2020-03-09 14:34 ` Michael Matz
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