Correctly align memory allocations in the case where we need to allocate memo...

Correctly align memory allocations in the case where we need to allocate memory
as far away from the stack as possible, but still as close to the VDSO as we
can.

BUG=none
TEST=run the tests in a tight loop and notice that they no longer randomly fail

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=45775

[Markus (顧孟勤), 2010-04-28 01:24:22]

Without this change and depending on the exact memory layout picked by ASR, the
sandbox would every so often fail to start up. In Chrome, we do the right thing
(i.e. fail gracefully) and this pretty much goes unnoticed. But in test runs I
would occasionally see the failure.

This is the code that Evan and I wrote a while ago to avoid infringing on the
stack's GROWTH_DOWN region. We forgot to actually align the address correctly,
and the next call to mmap() would typically fail. Often, we were able to find an
alternative location and thus didn't notice the problem.

[Evan Martin, 2010-04-28 01:36:40]

LGTM with math fix

http://codereview.chromium.org/1807002/diff/1/2
File sandbox/linux/seccomp/maps.cc (right):

http://codereview.chromium.org/1807002/diff/1/2#newcode224
sandbox/linux/seccomp/maps.cc:224: position = (addr - kMaxDistance) & ~0x3FFF;
math error