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bug#18505: 24.3.93; intermittent unexec failures when building on Mac OS


From: Jan Djärv
Subject: bug#18505: 24.3.93; intermittent unexec failures when building on Mac OS X 10.10 beta, Xcode 6.0
Date: Sun, 21 Sep 2014 22:37:38 +0200

Hi.

21 sep 2014 kl. 20:07 skrev David Caldwell <address@hidden>:

> On 9/21/14 2:15 AM, Jan Djärv wrote:
>> Hello.
>> 
>> 20 sep 2014 kl. 20:31 skrev David Caldwell <address@hidden>:
>> 
>>> On 9/20/14 8:31 AM, Jan Djärv wrote:
>>>> Hello.
>>>> 
>>>> 19 sep 2014 kl. 06:13 skrev David Caldwell <address@hidden>:
>>>> 
>>>>> Hello,
>>>>> 
>>>>> I tried to build the latest pretest on Mac OS X Yosemite Beta with the
>>>>> new Xcode 6.0 (GM) tools and ran into this error during the unexec step:
>>>>> 
>>>>> unexec: not enough room for load commands for new __DATA segments
>>>> 
>>>> Does it happen all the time or just some times?
>>> 
>>> It depends on 2 variables: the number of load commands that need to be
>>> added (num_unexec_regions) and text_seg_lowest_offset.
>>> 
>>> num_unexec_regions jumps around a lot, doing "make clean && make" over
>>> and over it'll be different every time. Somewhere between 12 and 34.
>> 
>> What makes it do that?  Some address randomization?  Some other unknown bug?
>> I would expect num_unexec_regions to be the same for every make.
> 
> I don't know. I would also expect num_unexec_regions to be the same. If
> it changes, it seems to mean the malloc behavior is different on every
> run. But yes, perhaps address space randomization could cause that to
> happen. I don't understand that part of the code well enough to
> speculate too much.

Me neither. 

> 
>> text_seg_lowest_offset could be address randomization, but if it stays 
>> somewhat constant, that can't be it.
> 
> I just figured that out (and smacked my head because it was obvious).
> That changed when I changed headerpad. I got curious and did a binary
> search to figure out exactly how -headerpad affects
> text_seg_lowest_offset in my setup (all number hex):
> 
> -headerpad      text_seg_lowest_offset
>   0 ->  740     17a0
> 741 -> 1740     27a0
> 1741 -> ???      37a0
> 
> So, text_seg_lowest_offset directly correlate with -headerpad and ld is
> doing some sort of alignment.

Okay.

> 
>> I've seen this failure before, but usually a new make works.
>> I'm trying to decide if this is emacs 24 or trunk material.
> 
> I think it should go in both. It's really quite a low-risk change: the
> -headerpad option is well documented in ld, and the amount my patch adds
> gives an extra 1.5K of headroom on a 6M binary (.02%).
> 
> I did a bunch of 'bzr log' searches to understand the nature of the
> -headerpad setting and it appears to not have been touched since 2006
> (in the 32 bit era). I believe that is why the comment in configure is
> incorrect: load commands may have been 56 bytes on 32 bit archs, but
> they are 78 bytes on my 64 bit computer (which is all current Macs going
> forward).

I checked it in in the 24-branch.

> 
>> Is there a way to dynamically react to these changes and adjust 
>> headerpad_extra dynamically at dump time?
> 
> Unfortunately the -headerpad is specified during link time so to change
> it dynamically would require re-linking after unexec-ing. That's a large
> Makefile change to get that all working correctly.

And possibly overkill.

Thanks,

        Jan D.






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