[PATCH] Add a do_assignments hook to ldemul

[H.J. Lu]

On Fri, Apr 29, 2016 at 5:25 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Fri, Apr 29, 2016 at 6:35 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Fri, Apr 29, 2016 at 5:30 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>> 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.
>>
>> I have a fix for it.  But _bfd_elf_is_start_stop still needs fix.
>
> Here is the updated patch, which doesn't use _bfd_elf_is_start_stop.
>
> OK for master?
>

Scan over all input files for each start/stop symbol may be too
expensive.  Assuming there are fewer start/stop symbols than
input files,  this patch collects all start/stop symbols first and
only scan over all input files once.

Comments, feedbacks?


-- 
H.J.
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