Re: Intermittent unexec failures on Linux >= 2.6.25

[Chong Yidong <cyd@stupidchicken.com>]

Hi Jan,

>> 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.

Do you remember the rationale for setting

#define MAX_HEAP_BSS_DIFF (1024*1024)

in emacs.c?  This variable was introduced by you on 2004-10-20, and I'm
not too familiar with this part of the code.

>> 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.)