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

[Alexander Ivchenko]

Bug report 16317

--Alexander

2013/12/10 H.J. Lu <hjl.tools@gmail.com>:
> 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.