Re: Intermittent unexec failures on Linux >= 2.6.25

[Christian Faulhammer <opfer@gentoo.org>]

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Hi,

I want to remind you of this bug report, could you please react on this
as we are able to reproduce.

<URL:http://emacsbugs.donarmstrong.com/cgi-bin/bugreport.cgi?bug=900>

Ulrich Mueller <ulm@gentoo.org>:
> Building of Emacs 22.2.92 (also 22.2) on Linux 2.6.25 (or later)
> sometimes fails with a segmentation fault in dump-emacs / unexec.
> 
> This was reported by Jan Hrabe as Gentoo bug 236579,
> <http://bugs.gentoo.org/236579>.
> 
> I've investigated and found that indeed temacs fails in dump-emacs
> intermittently. For my test, I have run "make; rm src/emacs" 250 times
> in a loop, and in 3 cases a segmentation fault of temacs occured.
> 
> The problem seems to be that heap_bss_diff is too large for unexec
> to succeed (due to kernel heap randomisation, see
> <http://lkml.org/lkml/2007/10/23/435>).
> 
> On the other hand, it is (in case of the 3 failures) not large enough
> to fulfill the condition (heap_bss_diff > MAX_HEAP_BSS_DIFF) which
> would trigger the correct behaviour, namely setting the personality
> and calling execve of itself.
> 
> In the 247 successful cases, heap_bss_diff first had a large value
> (up to about 32 MiB), and in the exec'd temacs its value was constant,
> namely 1887 bytes.
> 
> The 3 failures had heap_bss_diff = 575327, 911199, and 268127, which
> are all smaller than MAX_HEAP_BSS_DIFF (1024*1024), so execvp was
> _not_ called.
> 
> Where does that value of MAX_HEAP_BSS_DIFF = 1 MiB come from? Could it
> be decreased, or could temacs execve itself unconditionally on Linux?
> In my opinion, a failure rate of about 1 % is too high.
> 
> (The problem doesn't exist for Linux 2.6.24, or if heap randomisation
> is turned off, i.e. with /proc/sys/kernel/randomize_va_space < 2.)
> 
> Ulrich


-- 
Christian Faulhammer, Gentoo Lisp project
<URL:http://www.gentoo.org/proj/en/lisp/>, #gentoo-lisp on FreeNode

<URL:http://www.faulhammer.org/>

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