Re: [PATCH] Support gzip compressed exec and core files in gdb

Michael Eager
2015-03-12T16:14:16Z

On 03/11/15 19:50, Alan Modra wrote:
> On Wed, Mar 11, 2015 at 05:45:28PM -0700, Michael Eager wrote:
>> On 03/11/15 17:08, Alan Modra wrote:
>>> On Wed, Mar 11, 2015 at 07:56:30AM -0700, Michael Eager wrote:
>>>> On 03/11/15 01:14, Alan Modra wrote:
>>>>> On Tue, Mar 10, 2015 at 04:01:42PM -0700, Michael Eager wrote:
>>>>>> This operation cannot be done completely by BFD because BFD allows an opened
>>>>>> file to be passed to it for processing. GDB uses this functionality.
>>>>>
>>>>> I'd prefer you do this entirely outside of BFD, without adding another
>>>>> field to struct bfd. I think that can be done by simply clearing
>>>>> abfd->cacheable on files you uncompress. This prevents BFD from
>>>>> closing the file, so you won't need to open it again.
>>>>
>>>> GDB closes the exec file, then uses BFD to seek (I think when reading
>>>> syms). BFD then re-opens the file, so it needs the name of the
>>>> uncompressed file.
>>>
>>> Really? I think it quite unclean if gdb expects BFD to reopen a file
>>> that gdb has closed!
>>
>> Agreed.
>>
>> GDB doesn't expect BFD to reopen the file, per se. But it does a seek
>> on an exec file (IIRC, while reading symbols) which it previously closed
>> and when BFD notices that the file is closed, it opens it. I don't think
>> that it is feasible to remove calls to exec_close() so this doesn't happen.
>
> It looks to me that exec_close() calls bfd_close(). You won't be able
> to do anything with the bfd after bfd_close(), so I think your
> analysis is faulty and very much doubt your statement that "GDB closes
> the exec file, then uses BFD..".

The file opened in exec_file_attach() is closed in bfd_cache_close_all(),
before the function returns, not in the call to exec_close(). The bfd
is not deleted.

Later, in bfd_get_section_contents(), while sniffing the OSABI, GDB calls
bfd_seek() to do a seek on the same bfd. This notices that bfd->iostream
is zero and re-opens the file.

--
Michael Eager eager@eagercon.com
1960 Park Blvd., Palo Alto, CA 94306 650-325-8077