Re: [PATCH] Reduce file size for PT_GNU_RELRO segment

["H.J. Lu" <hjl.tools@gmail.com>]

On Sat, Mar 28, 2015 at 11:30 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Fri, Mar 27, 2015 at 9:15 PM, Alan Modra <amodra@gmail.com> wrote:
>> On Fri, Mar 27, 2015 at 01:00:20PM -0700, H.J. Lu wrote:
>>> On Wed, Mar 25, 2015 at 7:22 PM, Alan Modra <amodra@gmail.com> wrote:
>>> > Isn't this just re-inventing the commonpage adjustment done for
>>> > DATA_SEGMENT_ALIGN?  Why didn't the existing code in ldexp.c work for
>>> > you?
>>>
>>> segment.  In order to properly align PT_GNU_RELRO segmnet, we pad the first
>>> section of PT_GNU_RELRO segment by
>>
>> Adjusting the start of the first section of the PT_GNU_RELRO segment
>> is exactly what DATA_SEGMENT_ALIGN is supposed to do.  You are
>> patching the wrong place.  Any adjustment to the start of the relro
>> segment belongs in ldexp.c code evaluating DATA_SEGMENT_ALIGN.
>>
>
> Since output_section_statement isn't passed to ldexp.c, I don't see how
> DATA_SEGMENT_ALIGN  in ldexp.c can check the section address and size.
>
>

lang_size_sections aligns expld.dataseg.base:

5422      /* Aligning the adjusted base guarantees the padding
5423 between sections won't change.  This is better than
5424 simply subtracting 1 << max_alignment_power which is
5425 what we used to do here.  */
5426      expld.dataseg.base &= ~((1 << max_alignment_power) - 1);
5427      lang_reset_memory_regions ();
5428      one_lang_size_sections_pass (relax, check_regions);
5429    }
5430 }
5431      link_info.relro_start = expld.dataseg.base;
(gdb) p/x expld.dataseg
$290 = {phase = 0x4, base = 0x24bd00, min_base = 0x4bcc7,
  relro_end = 0x24f000, end = 0x64eae0, pagesize = 0x1000,
  maxpagesize = 0x200000, relro = 0x0, relro_start_stat = 0x8203e0,
  relro_end_stat = 0x822b20}
(gdb)

and calls  lang_size_sections_1 to align the first section of
PT_GNU_RELRO segmnet:

(gdb) bt
#0  lang_size_sections_1 (prev=0x81f258, output_section_statement=0x8135b0,
    fill=0x0, dot=2407680, relax=0x0, check_regions=1)
    at /export/gnu/import/git/sources/binutils-gdb/ld/ldlang.c:4919
#1  0x0000000000415219 in one_lang_size_sections_pass (relax=0x0,
    check_regions=1)
    at /export/gnu/import/git/sources/binutils-gdb/ld/ldlang.c:5368
#2  0x000000000041541d in lang_size_sections (relax=0x0, check_regions=1)
    at /export/gnu/import/git/sources/binutils-gdb/ld/ldlang.c:5428
#3  0x0000000000417758 in lang_process ()
    at /export/gnu/import/git/sources/binutils-gdb/ld/ldlang.c:6766
#4  0x000000000041b588 in main (argc=74, argv=0x7fffffffd328)
    at /export/gnu/import/git/sources/binutils-gdb/ld/ldmain.c:419
(gdb) list
4914 if (section_alignment > 0)
4915  {
4916    bfd_vma savedot = newdot;
4917    newdot = align_power (newdot, section_alignment);
4918
4919    dotdelta = newdot - savedot;
4920    if (dotdelta != 0
4921 && (config.warn_section_align
4922    || os->addr_tree != NULL)
4923 && expld.phase != lang_mark_phase_enum)
(gdb)

After that, DATA_SEGMENT_ALIGN is processed:

(gdb) bt
#0  fold_binary (tree=0x820380)
    at /export/gnu/import/git/sources/binutils-gdb/ld/ldexp.c:570
#1  0x000000000042065d in exp_fold_tree_1 (tree=0x820380)
    at /export/gnu/import/git/sources/binutils-gdb/ld/ldexp.c:989
#2  0x00000000004206f2 in exp_fold_tree_1 (tree=0x8203b0)
    at /export/gnu/import/git/sources/binutils-gdb/ld/ldexp.c:1008
#3  0x0000000000420bef in exp_fold_tree (tree=0x8203b0,
    current_section=0x8078b0 <_bfd_std_section+560>, dotp=0x7fffffffcf68)
    at /export/gnu/import/git/sources/binutils-gdb/ld/ldexp.c:1166
#4  0x0000000000414e36 in lang_size_sections_1 (prev=0x820300,
    output_section_statement=0x8135b0, fill=0x0, dot=310471, relax=0x0,
    check_regions=1)
    at /export/gnu/import/git/sources/binutils-gdb/ld/ldlang.c:5211
#5  0x0000000000415219 in one_lang_size_sections_pass (relax=0x0,
    check_regions=1)
    at /export/gnu/import/git/sources/binutils-gdb/ld/ldlang.c:5368
#6  0x000000000041524f in lang_size_sections (relax=0x0, check_regions=1)
    at /export/gnu/import/git/sources/binutils-gdb/ld/ldlang.c:5378
#7  0x000000000041724a in lang_relax_sections (need_layout=1)
    at /export/gnu/import/git/sources/binutils-gdb/ld/ldlang.c:6529
#8  0x0000000000427d83 in gldelf_x86_64_map_segments (need_layout=1)
    at eelf_x86_64.c:68
#9  0x000000000042ae3f in gldelf_x86_64_after_allocation ()
---Type <return> to continue, or q <return> to quit---
    at eelf_x86_64.c:1787
#10 0x0000000000421899 in ldemul_after_allocation ()
    at /export/gnu/import/git/sources/binutils-gdb/ld/ldemul.c:70
#11 0x000000000041775d in lang_process ()
    at /export/gnu/import/git/sources/binutils-gdb/ld/ldlang.c:6770
#12 0x000000000041b588 in main (argc=74, argv=0x7fffffffd328)
    at /export/gnu/import/git/sources/binutils-gdb/ld/ldmain.c:419
(gdb) list
565  else if (expld.dataseg.phase == exp_dataseg_none)
566    {
567      expld.dataseg.phase = exp_dataseg_align_seen;
568      expld.dataseg.min_base = expld.dot;
569      expld.dataseg.base = expld.result.value;
570      expld.dataseg.pagesize = commonpage;
571      expld.dataseg.maxpagesize = maxpage;
572      expld.dataseg.relro_end = 0;
573    }
574  else
(gdb) f 11
#11 0x000000000041775d in lang_process ()
    at /export/gnu/import/git/sources/binutils-gdb/ld/ldlang.c:6770
6770  ldemul_after_allocation ();
(gdb) list
6765  /* Size up the sections.  */
6766  lang_size_sections (NULL, ! RELAXATION_ENABLED);
6767
6768  /* See if anything special should be done now we know how big
6769     everything is.  This is where relaxation is done.  */
6770  ldemul_after_allocation ();
6771
6772  /* Fix any .startof. or .sizeof. symbols.  */
6773  lang_set_startof ();
6774
(gdb)

I don't see DATA_SEGMENT_ALIGN changes section address.

-- 
H.J.