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[compiler-rt][asan] decommit shadow memory for unmaps in fuchsia.
This CL allows asan allocator in fuchsia to decommit shadow memory for memory allocated using mmap. Big allocations in asan end up being allocated via `mmap` and freed with `munmap`. However, when that memory is freed, asan returns the corresponding shadow memory back to the OS via a call to `ReleaseMemoryPagesToOs`. In fuchsia, `ReleaseMemoryPagesToOs` is a no-op: to be able to free memory back to the OS, you have to hold a handle to the vmo you want to modify, which is tricky at the ReleaseMemoryPagesToOs level as that function is not exclusively used for shadow memory. The function `__sanitizer_fill_shadow` fills a given shadow memory range with a specific value, and if that value is 0 (unpoison) and the memory range is bigger than a threshold parameter, it will decommit that memory if it is all zeroes. This CL modifies the `FlushUnneededASanShadowMemory` function in `asan_poisoning.cpp` to add a call to `__sanitizer_fill_shadow` with value and threshold = 0. This way, all the unneeded shadow memory gets returned back to the OS. A test for this behavior can be found in fxrev.dev/391974 Differential Revision: https://reviews.llvm.org/D80355 Change-Id: Id6dd85693e78a222f0329d5b2201e0da753e01c0
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