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Re: [PATCH] Add a record_link_assignments hook to ldemul


On Thu, Apr 28, 2016 at 11:35 PM, Alan Modra <amodra@gmail.com> wrote:
> On Thu, Apr 28, 2016 at 05:59:55PM -0700, H.J. Lu wrote:
>> On Thu, Apr 28, 2016 at 4:38 PM, Alan Modra <amodra@gmail.com> wrote:
>> > On Thu, Apr 28, 2016 at 02:01:47PM -0700, H.J. Lu wrote:
>> >> +      /* Symbol is defined.  Check if it is also defined in a regular
>> >> +      input file only when it is currently defined in a dynamic
>> >> +      object, since otherwise, it can't be a __start_<name> nor
>> >> +      __stop_<name> symbol.  */
>> >> +      if (!h->def_dynamic)
>> >>       return NULL;
>> > [snip]
>> >> +         if (s != NULL)
>> >> +           {
>> >> +             h->root.u.undef.section = s;
>> >> +             break;
>> >> +           }
>> >
>> > You can't set u.undef here on a defined symbol.  That's just too ugly,
>> > even if you later set it to undefined.  Better to force it
>> > bfd_link_hash_undefined here.
>>
>> Like this?
>
> No, I meant that if you want to make use of the u.undef.section cache,
> then set it to undefined first.  If you don't intend to set
> u.undef.section then set to undefined later.
>
>> > This is getting quite messy, and I'm wondering if we even need
>> > _bfd_elf_is_start_stop, except for gc-sections code.
>
> Note that when I wrote _bfd_elf_is_start_stop I thought it might come
> in useful in check_relocs, to prevent __start_* and __stop_* from
> being seen as dynamic.  Now that you're running both
> find_statement_assignment and lang_do_assignments before check_relocs,
> doing anything special with _bfd_elf_is_start_stop should not be
> necessary.

 __start_* and __stop_* aren't handled properly with shared library:

https://sourceware.org/bugzilla/show_bug.cgi?id=20022

_bfd_elf_is_start_stop needs to check symbols defined in a
shared library.

> find_statement_assignment has given the bfd backend a look at symbols,
> and you've processed the PROVIDE (__start_sec = .) linker script
> statements in lang_do_assignments.  So it ought to be possible to make
> __start_sec a def_regular normal symbol by that point.  As I said
> before, I'd expect that to work better if find_statement_assignment
> ran before lang_do_assignments.  That allows
> bfd_elf_record_link_assignment to force def_dynamic symbols to
> bfd_link_hash_undefined so that ldexp.c code handling PROVIDE doesn't
> need to know ELF specific details.
>
> However, you said that doing it that way didn't work.  What didn't
> work, and what needs to change in bfd_elf_record_link_assignment to
> make it work?

The failed testcase is ld-elf/ehdr_start-userdef.  We have

         /* Only adjust the export class if the symbol was referenced
             and not defined, otherwise leave it alone.  */
          if (h != NULL
              && (h->root.type == bfd_link_hash_new
                  || h->root.type == bfd_link_hash_undefined
                  || h->root.type == bfd_link_hash_undefweak
                  || h->root.type == bfd_link_hash_common))
            {

Since this is run before  lang_do_assignments, we don't see

__ehdr_start = 0x12345678;

in linker script and get

(gdb) p *h
$2 = {root = {root = {next = 0x0, string = 0x859bad "__ehdr_start",
      hash = 78192523}, type = bfd_link_hash_undefined, non_ir_ref = 0,
    linker_def = 0, u = {undef = {next = 0x0, abfd = 0x857100, section = 0x0},
      def = {next = 0x0, section = 0x857100, value = 0}, i = {next = 0x0,
        link = 0x857100, warning = 0x0}, c = {next = 0x0, p = 0x857100,
        size = 0}}}, indx = -1, dynindx = -1, got = {refcount = 0, offset = 0,
    glist = 0x0, plist = 0x0}, plt = {refcount = 0, offset = 0, glist = 0x0,
    plist = 0x0}, size = 0, type = 0, other = 0, target_internal = 0,
  ref_regular = 1, def_regular = 0, ref_dynamic = 0, def_dynamic = 0,
  ref_regular_nonweak = 1, dynamic_adjusted = 0, needs_copy = 0,
  needs_plt = 0, non_elf = 0, versioned = unversioned, forced_local = 0,
  dynamic = 0, mark = 0, non_got_ref = 0, dynamic_def = 0,
  ref_dynamic_nonweak = 0, pointer_equality_needed = 0, unique_global = 0,
  protected_def = 0, dynstr_index = 0, u = {weakdef = 0x0,
    elf_hash_value = 0}, verinfo = {verdef = 0x0, vertree = 0x0}, vtable = 0x0}

We get

     3: 0000000012345678     0 NOTYPE  LOCAL  DEFAULT  ABS __ehdr_start


instead of

     3: 0000000012345678     0 NOTYPE  GLOBAL  DEFAULT  ABS __ehdr_start

in output.

-- 
H.J.


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