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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 11:15:26 +0200

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.  
text_seg_lowest_offset could be address randomization, but if it stays somewhat 
constant, that can't be it.

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.

Is there a way to dynamically react to these changes and adjust headerpad_extra 
dynamically at dump time?

> 
> text_seg_lowest_offset seems more stable, but it still changes. When I
> first unpacked the tarball and compiled, text_seg_lowest_offset was very
> low: 0x17c0. This stayed constant for about an hour while I was
> debugging this (adding debug prints) and then mysteriously jumped up to
> 0x24f0 at which point the unexec started succeeding without the change
> to headerpad_extra.
> 
> I just unpacked a clean pretest source directory and
> text_seg_lowest_offset was 0x17a0 and unexec failed.
> 
> Doing
>  (make clean && make) 2>&1 | grep "Lowest offset of all sections"
> over and over gives me:
> 
> Lowest offset of all sections in __TEXT segment:   0x17a0
> Lowest offset of all sections in __TEXT segment:   0x17a0
> Lowest offset of all sections in __TEXT segment:   0x17a0
> Lowest offset of all sections in __TEXT segment:   0x17a0
> Lowest offset of all sections in __TEXT segment:   0x17a0
> Lowest offset of all sections in __TEXT segment:   0x17a0
> 
> Though one of those times it didn't fail (presumably because
> num_unexec_regions was low enough).

        Jan D.








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