Re: Change in relocation of debug info

["Rodney M. Bates" <rodney_bates@lcwb.coop>]

On 5/31/21 10:08 AM, Simon Marchi via Gdb wrote:
>> When I link using gcc 5.4:
>>
>>    $gcc --version:
>>    gcc (Ubuntu 5.4.0-6ubuntu1~16.04.12) 5.4.0 20160609,
>>
>> giving executable named prog I see:
>>
>>    $objdump -g prog
>>    ...
>>    /* file ../src/SetElem.m3 line 11 addr 0x4008ee */
>>    ...
> 
>> But when linking with gcc 9.3:
>>
>>    $gcc --version:
>>    gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
>>
>> I see:
>>
>>    $objdump -g prog
>>    ...
>>    /* file ../src/SetElem.m3 line 11 addr 0x9b1 */
> 
> It just sounds like your gcc 9.3.0 is producing position-indenpendent
> executables by default, whereas your gcc 5.4.0 did not.  In the recent
> years, this has become the norm.
> 
> The executable produced by your gcc 5.4.0 is made to always be loaded at
> the same address, so the code at virtual address 0x4008ee in the
> executable will always end up at address 0x4008ee in the virtual address
> space of the process.
> 
> With the executable produced by your gcc 9.3.0, the OS / dynamic linker
> will choose a different random base address for the executable
> every time it's loaded (except when address randomization is disabled,
> which GDB tries to do by default, then it will always be the same random
> address).  So the virtual address 0x9b1 in the executable will end up at
> <load address> + 0x9b1 in the virtual address space of the process.  GDB
> has to account for that: detect the load address of the executable,
> adjust the addresses of the symbols.
> 
> This works for DWARF but doesn't seem to work for stabs for some reason
> (based on what you have shown, I didn't try it myself).  I don't know
> how difficult it would be to make it work for stabs, but it's unlikely
> that anybody working on GDB will do it, given that stabs is pretty much
> a dead format.  You can try if you want though.
> 
> An alternative is to instruct your gcc 9.3.0 to produce good old
> position-dependent executables, as your gcc 5.4.0 does.  This is done
> using -fno-pie at link time, IIRC.
> 
> Simon
> 

Thanks.  Using -f=no-pie has gotten me going for now.  My build process
has been supplying -fPIC for ages, to the older gcc, so I don't fully
understand why it wasn't happening all along, but this works for now.

-- 
Rodney Bates
rodney.m.bates@acm.org