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This allocator is adapted from the GuardedPageAllocator in tcmalloc.
The allocator is intended to be a 'debug' allocator to help catch memory
errors like buffer overflows and use-after-frees. It does so by giving
allocations an entire page and left- or right- aligning them between
guard pages (to catch both underflows and overflows). It also sets pages
inaccessible once they have been freed to detect use-after-frees and can
also detect double frees.

Forthcoming changes add an allocator shim to sample allocations to this
allocator and to add additional debugging context when crashes due to
GWP-ASan occur.

GWP-ASan is going to be launched on Windows first; however, if it’s
successful it will be ported to other platforms (and therefore also be
used by different embedders.)

Bug: 896019
Change-Id: I358e1b6f91569d1d448fc7ee16d5df27ed2467aa
Reviewed-on: https://chromium-review.googlesource.com/c/1284699
Commit-Queue: Vlad Tsyrklevich <vtsyrklevich@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Reviewed-by: Cait Phillips <caitkp@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603616}