Intermittent unexec failures on Linux >= 2.6.25

[Ulrich Mueller]

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