Re: SEC_NEVER_LOAD cleanup

[Kai Tietz <ktietz70@googlemail.com>]

2010/10/16 Dave Korn <dave.korn.cygwin@gmail.com>:
> On 16/10/2010 21:08, Kai Tietz wrote:
>> 2010/10/16 Alan Modra <amodra@gmail.com>:
>>> On Fri, Oct 15, 2010 at 09:54:18PM +0100, Dave Korn wrote:
>>>>>   .gnu_debuglink_overlay ALIGN(__section_alignment__) (NOLOAD):
>>>>>   {
>>>>>     BYTE(0) /* c */
>
>>> Using your script with a small tweak for ELF gave me the same error.
>>>
>>>>   I think (haven't checked yet) that this looks like a consequence of the
>>>> ldlang.c changes.  Any thoughts on how to fix it?
>>> Like this, I think, just as we discard input sections and padding in
>>> noload sections.  Can you test this out for me on cygwin?
>>>
>>>        * ldwrite.c (build_link_order <lang_data_statement_enum>): Don't
>>>        output when section has no contents.
>>>        (build_link_order <lang_reloc_statement_enum>): Likewise.
>
>> Well, this looks ok, too. But AFAICS it is touching just the surface.
>> My recent patch took care that for PE-COFF the content of the
>> debugging sections didn't got zero'ed. Maybe it is a general failure
>> to assume (at least for pe-coff, but maybe for elf, too) that NOLOAD
>> means to discard sections from linking?
>
>  I'm seeing odd behaviour.  If I build the cygwin dll with the
> map_input_to_output_sections reverted, I get this:
>
>> $ objdump -h cygwin0.dll
>>
>> cygwin0.dll:     file format pei-i386
>>
>> Sections:
>> Idx Name          Size      VMA       LMA       File off  Algn
>
>>   9 .gnu_debuglink_overlay 00000010  61239000  61239000  001fee00  2**2
>>                   CONTENTS, EXCLUDE
>
>  ... and this:
>
>> $ objdump -s -j .gnu_debuglink_overlay  cygwin0.dll
>>
>> cygwin0.dll:     file format pei-i386
>>
>> Contents of section .gnu_debuglink_overlay:
>>  61239000 00000000 00000000 00000000 00000000  ................
>>
>> $
>
>  With the build_link_order change instead, I get this:
>
>> $ objdump -h cygwin0.dll
>>
>> cygwin0.dll:     file format pei-i386
>>
>> Sections:
>> Idx Name          Size      VMA       LMA       File off  Algn
>
>>   9 .gnu_debuglink_overlay 00000010  61239000  61239000  00000000  2**2
>>                   EXCLUDE
>
>  ... and this:
>
>> $ objdump -s -j .gnu_debuglink_overlay  cygwin0.dll
>>
>> cygwin0.dll:     file format pei-i386
>>
>> objdump: section '.gnu_debuglink_overlay' mentioned in a -j option, but not
>> found in any input file
>
>  I think it really needs that CONTENTS flag.  Can look closer at it later, I
> have to go offline for a while now.
>
>    cheers,
>      DaveK
>
>

Well, the CONTENT flag is here of less use. The underlying issue is
(at least for pe-coff) that SEC_NEVER_LOAD (which indicates for
pe-coff just that this section doesn't have load and allocation flags
set and will not loaded into memory - btw different to discard flag,
which means that it is initially loaded but then discarded) is treated
similar to SEC_EXCLUDE. The SEC_EXCLUDE is in fact the PE-COFF flag
IMAGE_SCN_LNK_REMOVE.
This looks wrong to me. If you want I can prepare a patch for this.

Cheers,
Kai
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