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

Pedro Alves
2017-11-27T19:13:20Z

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.

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.

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?

Thanks,
Pedro Alves