Re: [PATCH v2] Add support for the --readnever command-line option (DWARF only)

[Sergio Durigan Junior <sergiodj@redhat.com>]

On Monday, November 27 2017, Pedro Alves wrote:

> On 11/24/2017 08:26 PM, Sergio Durigan Junior wrote:
>> On Friday, November 24 2017, Pedro Alves wrote:
>> 
>>> On 11/24/2017 04:54 AM, Sergio Durigan Junior wrote:
>>>> On Thursday, November 23 2017, Pedro Alves wrote:
>>>>
>>>>> On 11/23/2017 05:21 PM, Sergio Durigan Junior wrote:
>>>>>
>>>>>>>> +This option is currently limited to debug information in DWARF format.
>>>>>>>> +For all other format, this option has no effect.
>>>>>>>
>>>>>>> How hard would it be to just make it work?  There's only stabs and mdebug
>>>>>>> left, I think?  There should be a single a function somewhere that we can
>>>>>>> add an early return.  And then we don't need to document this limitation...
>>>>>>>
>>>>>>> For example, in elf_symfile_read, we could just skip the elf_locate_sections
>>>>>>> call.  In coffread.c we could skip reading stabs right after 
>>>>>>>   bfd_map_over_sections (abfd, coff_locate_sections....);
>>>>>>>
>>>>>>> Looking for:
>>>>>>>
>>>>>>>  $ grep -h "^[a-z]*_build_psymtabs" gdb/
>>>>>>>  coffstab_build_psymtabs (struct objfile *objfile,
>>>>>>>  elfstab_build_psymtabs (struct objfile *objfile, asection *stabsect,
>>>>>>>  stabsect_build_psymtabs (struct objfile *objfile, char *stab_name,
>>>>>>>  mdebug_build_psymtabs (minimal_symbol_reader &reader,
>>>>>>>  elfmdebug_build_psymtabs (struct objfile *objfile,
>>>>>>>
>>>>>>> finds all the relevant places.
>>>>>>>
>>>>>>> Maybe it wouldn't be that hard to make this be an objfile flag
>>>>>>> afterall (like OBJF_READNOW is).  That'd make it possible
>>>>>>> to add the location "-readnever" counterpart switch to add-symbol-file
>>>>>>> too, BTW:
>>>>>
>>>>> I meant "logical" instead of "location".  I was staring at
>>>>> gdb/location.c at that time.  :-P
>>>>>
>>>>>>>
>>>>>>>  symfile.c:        if (strcmp (arg, "-readnow") == 0)
>>>>>>>  symfile.c:        else if (strcmp (arg, "-readnow") == 0)
>>>>>>
>>>>>> Hm, I'll look into this.  Just to make it clear: the idea is to have
>>>>>> both a --readnever global option and also a OBJF_READNEVER specific to
>>>>>> each objfile?
>>>>>
>>>>> Sure, the idea is to do something similar to what's done for --readnow.
>>>>
>>>> Sorry, but I guess I need a few more details on this.
>>>>
>>>> The way I understand the code at elf_symfile_read, the very first thing
>>>> to do would be to check if OBJF_READNEVER is set and return early if it
>>>> is.  But it seems that you're proposing something a bit different when
>>>> you say that we should "... just skip the elf_locate_sections call."  It
>>>> doesn't seem to me that is worth continuing on that function if
>>>> OBJF_READNEVER is present.
>>>
>>> No, you can't return early the very first thing, because
>>>  --readnever is supposed to skip _debug_ info, not ELF/minimal symbols...
>>> So the "return early" would have to be _after_ the
>>> elf_read_minimal_symbols call:
>> 
>> Hm, OK, it makes sense and I confess I thought "why is Pedro mentioning
>> elf_symfile_read if the feature is about skipping DWARF, not ELF?".
>> 
>>>  static void
>>>  elf_symfile_read (struct objfile *objfile, symfile_add_flags symfile_flags)
>>>  {
>>>    bfd *abfd = objfile->obfd;
>>>    struct elfinfo ei;
>>>
>>>    memset ((char *) &ei, 0, sizeof (ei));
>>>    bfd_map_over_sections (abfd, elf_locate_sections, (void *) & ei);
>>>  
>>>    elf_read_minimal_symbols (objfile, symfile_flags, &ei);
>>>
>>> I don't know whether we can reorder that.  Maybe we can.
>>>
>>> When I looked at this quickly yesterday, I saw that elf_location_sections
>>> is what finds the stabs and mdebug sections:
>>>
>>> static void
>>> elf_locate_sections (bfd *ignore_abfd, asection *sectp, void *eip)
>>> {
>>>   struct elfinfo *ei;
>>>
>>>   ei = (struct elfinfo *) eip;
>>>   if (strcmp (sectp->name, ".stab") == 0)
>>>     {
>>>       ei->stabsect = sectp;
>>>     }
>>>   else if (strcmp (sectp->name, ".mdebug") == 0)
>>>     {
>>>       ei->mdebugsect = sectp;
>>>     }
>>> }
>>>
>>> and it seemed to be that skipping the section location would make
>>> the parts of  elf_symfile_read that actually read the symbols
>>> be no-ops, because the stabsect/mdebusect pointers would be NULL.
>>>
>>> But if returning early or something else works, that's fine.
>> 
>> OK, thanks for clarifying.
>> 
>>>> As for the *_build_psymtabs functions, I am doing exactly that: if
>>>> objfile->flags contains OBJF_READNEVER, then just return and do nothing.
>>>
>>> Sure, that should work too.  It's just the difference between
>>> skipping checking whether debug info is available (skipping before
>>> calling into those), vs letting gdb do the work to figure out whether
>>> debug info is available, but then ignore it.
>>> The grep for "*_build_psymtabs" was intended as a pointer to find
>>> what the relevant code is, including to look at the code that
>>> is calling those functions, see if there's something to be done there.
>> 
>> I think it makes more sense, logically speaking, to not mess with
>> elf_symfile_read and instead modify the *_build_psymtabs functions.
>
> Yet, somehow that logic didn't carry to the DWARF reader
> changes?  ;-)  dwarf2_has_info is the place that checks
> whether the objfile has DWARF sections, 
> see dwarf2_per_objfile::dwarf2_per_objfile.
> So for DWARF, the patch is checking for readnever _before_
> dwarf2_build_psymtabs is reached, by essentially skipping
> the "locate_sections" call.  

You're right, I never thought about making the changes for DWARF.

> A small advantage of checking before is that you can skip a little bit
> more work.  See the comment in elf_read_minimal_symbols about skipping
> work unless stabs, or this bit here:
>
>   if (info->stabsects)
>     {
>       if (!info->stabstrsect)
> 	{
> 	  error (_("The debugging information in `%s' is corrupted.\nThe "
> 		   "file has a `.stabs' section, but no `.stabstr' section."),
> 		 name);
> 	}
>       ...
>
> It's really not a bit deal, but to me it'd be more
> consistent have all readers do the same logically.

Understood.  I switched the logic and now I'm doing all the work on
elf_symfile_read (and consequently dwarf2_has_info, which now is the
right place to put the checks on).

I wasn't sure if I should modify other *_symfile_read functions to obey
the setting, so I left them alone.

> On another note: there's a symbol_file_add_separate call at the tail
> end of elf_symfile_read, where we seem to read separate info.  It seems
> like that'd end up reading debug info, with "add-symbol-file -readnever"?
> Same in read_symbols, I guess.  Or is OBJF_READNEVER etc. somehow
> propagated to the separate objfile?

"add-symbol-file -readnever" doesn't get propagated, the only thing that
does is the "--readnever" global option.  symbol_file_add_separate calls
symbol_file_add_with_addrs (responsible for created the objfile), so I
now propagate the OBJF_READNEVER flag if the main objfile has it.

I'll submit a new version soon.  Thanks,

-- 
Sergio
GPG key ID: 237A 54B1 0287 28BF 00EF  31F4 D0EB 7628 65FC 5E36
Please send encrypted e-mail if possible
http://sergiodj.net/