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Subject: |
24.3.93; intermittent unexec failures when building on Mac OS X 10.10 beta, Xcode 6.0 |
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Date: |
Thu, 18 Sep 2014 21:13:41 -0700 |
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User-agent: |
Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:32.0) Gecko/20100101 Thunderbird/32.0 |
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
(Full unexec log is attached).
Poking around I discovered a couple discrepancies. The "headerpad_extra"
setting in configure.ac says entries are 56 bytes, but they are actually
78 bytes (probably because the comment was originally written in the 32
bit days). Also the number of load commands needed is variable now--it
depends on how many malloc segments were found and how many it could
merge (see find_emacs_zone_regions() and unexec_regions_merge() in
unexmacosx.c).
I printed num_unexec_regions in unexmacosx.c and observed anywhere
from about 14 to 34 regions needed. But given that it seems to be
different on every run and it's super annoying to debug when you run out
of space, I suggest bumping headerpad_extra up to 0x1000. This gives
about 1.5KB of headroom (or wasted space, depending on how you look at
it) and allows for 52 load commands, which should be future proof for
quite a while. I have attached a patch for configure.ac that does this
and changes the comment to reflect my understanding.
I've also provided a patch that makes the unexec error more helpful: it
computes the size needed for headerpad_extra given the number of load
commands that it needed for that particular unexec run.
I assume this affects the trunk code, too, though I didn't look at it.
-David
increase_headerpad_extra.patch
Description: Text document
helpful_unexec_error.patch
Description: Text document
unexec-log.txt
Description: Text document
smime.p7s
Description: S/MIME Cryptographic Signature
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--- Begin Message ---
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Subject: |
Re: bug#18505: 24.3.93; intermittent unexec failures when building on Mac OS X 10.10 beta, Xcode 6.0 |
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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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