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Commit 6bc53ff7 authored by Elliott Hughes's avatar Elliott Hughes
Browse files

arm64: update the "minimum maximum" comment. 

Noticed while looking at riscv64. Looks like a bug, but actually nothing
we can do about it now or for the foreseeable future.

Bug: https://github.com/google/android-riscv64/issues/45
Test: treehugger
Change-Id: I2be81b2fd7095df40958a1f641d7b89cf5a8e41d
parent 720007ba

init/security.cpp

+3 −7
Original line number Diff line number Diff line
@@ -106,13 +106,9 @@ Result<void> SetMmapRndBitsAction(const BuiltinArguments&) { 
    // uml does not support mmap_rnd_bits

    return {};

#elif defined(__aarch64__)

    // arm64 architecture supports 18 - 33 rnd bits depending on pagesize and

    // VA_SIZE. However the kernel might have been compiled with a narrower

    // range using CONFIG_ARCH_MMAP_RND_BITS_MIN/MAX. To use the maximum

    // supported number of bits, we start from the theoretical maximum of 33

    // bits and try smaller values until we reach 24 bits which is the

    // Android-specific minimum. Don't go lower even if the configured maximum

    // is smaller than 24.

    // arm64 supports 14 - 33 rnd bits depending on page size and ARM64_VA_BITS.

    // The kernel (6.5) still defaults to 39 va bits for 4KiB pages, so shipping

    // devices are only getting 24 bits of randomness in practice.

    if (SetMmapRndBitsMin(33, 24, false) && (!Has32BitAbi() || SetMmapRndBitsMin(16, 16, true))) {

        return {};

    }