Re: [PATCH] Initialize _r_debug for static applications.

["Carlos O'Donell" <carlos at redhat dot com>]

On 04/10/2014 08:33 PM, Carlos O'Donell wrote:
> On 04/10/2014 08:06 PM, Carlos O'Donell wrote:
>> With Maciej's great work on reducing the difference between
>> static and dyanmic executables, and with the addition of
>> a functional main link map in static exectuables we can now
>> usefully initialize _r_debug for static applications. This
>> will allow future debuggers the ability to coalesce code
>> paths which previously had to have special cases for static
>> v.s. dyanmic.
>>
>> It also fixes this particular use case where gdb would like
>> a consistent way to find the application link map for looking
>> up TLS variables in both static and dynamic applications:
>> https://www.sourceware.org/ml/libc-help/2014-03/msg00024.html
>>
>> The discussion touches on the fact that without _r_debug the
>> debugger has to make various assupmtions while using
>> libthread_db that contrains the implementation.
>>
>> This patch initializes the debug infrastructure during
>> LIBC_START_MAIN, but only for static applications, by
>> calling `_dl_debug_initialize (0, LM_ID_BASE)'. We pass 0 as
>> the ldbase because it's a static application, and the runtime
>> address of the loader doesn't matter. We pass LM_ID_BASE because
>> we want to load, by default, the base link map, and that's what
>> really matters.
>>
>> Tested on x86_64 with no regressions.
>>
>> Jan reports this works and that gdb can be patched to use
>> _r_debug to access TLS variables in static applications
>> while still using the current libthread_db calls normally
>> used for dynamic applications.
>>
>> This removes one more difference between static and dynamic
>> applications.
>>
>> Note: This does not attempt to cleanup the -Wundef warning
>> for SHARED, which I am looking to fix in another patch so
>> we still use `#ifndef SHARED' (Makeconfig needs to be fixed
>> to use -DSHARED=0 and -DSHARED=1 IMO to make this work correctly).
>>
>> OK to checkin?
>>
>> Cheers,
>> Carlos.
>>
>> 2014-04-10  Carlos O'Donell  <carlos@redhat.com>
>>  
> 	[BZ #16831]
>> 	* csu/libc-start.c (LIBC_START_MAIN) [!SHARED]: Call
>> 	_dl_debug_initialize.
> 
> Filed 16831 for user visible change to _r_debug interface.
> 
> https://sourceware.org/bugzilla/show_bug.cgi?id=16831

Given that this only makes static binaries "better" I've gone
ahead and checked in.

commit 650362caaddf8109b47522c1113e2acbb881f693
Author: Carlos O'Donell <carlos@redhat.com>
Date:   Fri Apr 11 12:43:58 2014 -0400

    Support _r_debug for static binaries.
    
    We initialize _r_debug for static binaries to allows debug
    agents to treat static binaries a little more like dyanmic
    ones. This simplifies the work a debug agent has to do to
    access TLS in a static binary via libthread_db.
    
    Tested on x86_64.
    
    See:
    https://sourceware.org/ml/libc-alpha/2014-04/msg00183.html
    
        [BZ #16831]
        * csu/libc-start.c (LIBC_START_MAIN) [!SHARED]: Call
        _dl_debug_initialize.

Cheers,
Carlos.