In 2.4.*, mlockall(MCL_FUTURE) is erroneously inherited by child processes
across fork() and exec():
1. across a fork(), the inherited memory is not locked, but any new memory
allocations by the child will be VM_LOCKED.
2. across a subsequent exec(), *all* of the exec'd program's memory except
for its stack pages will be VM_LOCKED.
The problem is:
1. if MCL_FUTURE, mm->def_flags gets set to VM_LOCKED in do_mlockall().
2. mm->def_flags is not cleared during subsequent forks and execs.
3 mm->def_flags, with the leftover VM_LOCKED flag set, is subsequently
utilized in calc_vm_flags() when called by do_brk() to extend the
address space of a forked process, and by do_mmap_pgoff() when
building the non-stack address space of an exec'd process.
The proposed patch puts the fix in mm_init(), which seems to be the most
appropriate place since it's called by copy_mm(), and by mm_alloc() on behalf
of exec_mmap():
# diff -u linux/kernel/fork.c linux-2.4.17/kernel/fork.c
--- linux/kernel/fork.c Tue Jan 8 15:11:13 2002
+++ linux-2.4.17/kernel/fork.c Tue Jan 8 15:12:26 2002
@@ -219,6 +219,7 @@
init_rwsem(&mm->mmap_sem);
mm->page_table_lock = SPIN_LOCK_UNLOCKED;
mm->pgd = pgd_alloc(mm);
+ mm->def_flags = 0;
if (mm->pgd)
return mm;
free_mm(mm);
Note that it worked OK in 2.2 because mm->def_flags was explicitly cleared in
mm_alloc(), which was called by both copy_mm() and exec_mmap(). But things
were shuffled around a bit in 2.4, and it must have gotten lost in the
translation...
Dave Anderson
==============================================================================
David Anderson anderson@mclinux.com
Mission Critical Linux, Inc. http://www.mclinux.com
100 Foot of John St. Work: 978-606-0225
Lowell, MA 01852 Fax: 978-446-9470
==============================================================================
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