[PATCH] Add a do_assignments hook to ldemul
H.J. Lu
hjl.tools@gmail.com
Sat Apr 30 23:11:00 GMT 2016
On Sat, Apr 30, 2016 at 7:40 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> 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?
>
An updated patch.
--
H.J.
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