[gold, strip] Question about the changed offset when stripping

[H.J. Lu]

On Tue, Dec 10, 2013 at 5:10 AM, Alexander Ivchenko <aivchenk@gmail.com> wrote:
> 2013/12/9 Ian Lance Taylor <iant@google.com>:
>> On Mon, Dec 9, 2013 at 4:33 AM, Alexander Ivchenko <aivchenk@gmail.com> wrote:
>>> Indeed my problem with debugging was due to those missing 4 bytes
>>> between .got.plt and .bss.
>>> that lead to differences between stripped/not_stripped program headers
>>> of the same binary:
>>>
>>>
>>> not_stripped:
>>> LOAD           0x003d60 0x00004d60 0x00004d60 *0x002a0* 0x002b0 RW  0x1000
>>>
>>> stripped:
>>> LOAD           0x003d60 0x00004d60 0x00004d60 *0x0029c* 0x002b0 RW  0x1000
>>
>>
>> I assume this is readelf -l output on your binary.  It looks like the
>> file size of the data segment has been changed by strip.  That does
>> seem odd, though since the memory size is unchanged it's not
>> necessarily a problem.  It depends on whether any initialized symbol
>> is defined in those missing 4 bytes.
>>
>>
>>> Still, one thing I still worry about. I see that gold is intentionally
>>> making this padding.
>>> I see in the code:
>>>
>>> Output_segment::set_section_addresses:
>>>
>>>      // Pad the total relro size to a multiple of the maximum
>>>       // section alignment seen.
>>>       uint64_t aligned_size = align_address(relro_size, max_align);
>>>       // Note the amount of padding added after the last relro section.
>>>       last_relro_pad = aligned_size - relro_size;
>>>       *has_relro = true
>>>
>>> ... and then:
>>>
>>>           *poff += last_relro_pad;
>>>           addr += last_relro_pad;
>>>           if (this->output_lists_[i].empty())
>>>             {
>>>               // If there is nothing in the ORDER_RELRO_LAST list,
>>>               // the padding will occur at the end of the relro
>>>               // segment, and we need to add it to *INCREASE_RELRO.
>>>               *increase_relro += last_relro_pad;
>>>             }
>>>         }
>>>
>>> Since all stripped binaries in e.g. Android (at least that is true for
>>> x86, may be other arch's are also affected), that were linked by gold,
>>> are missing that padding, how critical that is?
>>
>> The relro data area must end at a page boundary, or the dynamic linker
>> will not be able to mark it as read-only.  Are you sure that is the
>> problem, though?  What does the GNU_RELRO program segment look like?
>
> Between the stripped\unstripped versions of the binary that was the
> only difference in segments from "readelf -l". The GNU_RELRO looks
> like this:
>   GNU_RELRO      0x003d60 0x00004d60 0x00004d60 0x002a0 0x002a0 RW  0x20
>
>
>> That padding code in gold was added here:
>> https://sourceware.org/ml/binutils/2010-10/msg00234.html .  Part of
>> the code is clearly required.  I'm not sure it is essential to pad the
>> relro size to a maximum of the section alignment, though in general it
>> can't hurt.
>>
>> If the only change that strip introduces is a change in the file size
>> of the data segment, and if no symbol refers to those bytes, and if
>> the symbol values are unchanged, and if the RELRO segment is
>> unchanged, then I would expect the resulting executable to work
>> correctly.
>
> I also noticed that after strip the SHF_INFO_LINK flag for .rel.plt disappeared:
>
> before strip:
> <   [ 6] .rel.plt          REL             000011c4 0011c4 000138 08
> AI  2   7  4
> after strip:
>>   [ 6] .rel.plt          REL             000011c4 0011c4 000138 08   A  2   7  4
>
> Again, I'm not sure how critical that is..

This is a bug and should be fixed.  Please open a bug report
with a testcase.

>
>> But I agree it is odd for strip to be changing something here.
>>
>> Ian
>
> Since gdb/gdbserver 7.6 is able to load debug info for those stripped
> binaries, I don't know whether the issue that is solved in pr11786 has
> something to do with that problem. However it is indeed looks pretty
> similar.
>
> H.J., I will try to come up with a smaller testcase.
>
> Ideally, strip should not touch the padding after got.plt and any flags.
>


-- 
H.J.