Users have discovered [*] that when CONFIG_ARCH_MMAP_RND_BITS == 32, it will frequently conflict with ASan's allocator, because the PIE program segment base address of 0x555555555554 plus an ASLR shift of up to ((2**32) * 4K == 0x100000000000) will sometimes exceed ASan's hardcoded base address of 0x600000000000. We fix this by simply moving the allocator base to 0x500000000000, which is below the PIE program segment base address. This is cleaner than trying to move it to another location that is sandwiched between the PIE program and library segments, because if either of those grow too large, it will crash into the allocator region. Note that we will never need to change this base address again (unless we want to increase the size of the allocator), because ASLR cannot be set above 32-bits (the PIE program segment and library segments would crash into each other; see also ARCH_MMAP_RND_BITS_MAX in https://github.com/torvalds/linux/blob/master/arch/x86/Kconfig). [*] see https://b.corp.google.com/issues/276925478 and https://groups.google.com/a/google.com/g/chrome-os-gardeners/c/BbfzCP3dEeo/m/h3C_vVUxCQAJ

Users have discovered [*] that when CONFIG_ARCH_MMAP_RND_BITS == 32, it will frequently conflict with ASan's allocator on x86-64 Linux, because the PIE program segment base address of 0x555555555554 plus an ASLR shift of up to ((2**32) * 4K == 0x100000000000) will sometimes exceed ASan's hardcoded base address of 0x600000000000. We fix this by simply moving the allocator base to 0x500000000000, which is below the PIE program segment base address. This is cleaner than trying to move it to another location that is sandwiched between the PIE program and library segments, because if either of those grow too large, it will collide with the allocator region. Note that we will never need to change this base address again (unless we want to increase the size of the allocator), because ASLR cannot be set above 32-bits for x86-64 Linux (the PIE program segment and library segments would collide with each other; see also ARCH_MMAP_RND_BITS_MAX in https://github.com/torvalds/linux/blob/master/arch/x86/Kconfig). [*] see https://b.corp.google.com/issues/276925478 and https://groups.google.com/a/google.com/g/chrome-os-gardeners/c/BbfzCP3dEeo/m/h3C_vVUxCQAJ

Users have discovered [*] that when CONFIG_ARCH_MMAP_RND_BITS == 32, it will frequently conflict with ASan's allocator on x86-64 Linux, because the PIE program segment base address of 0x555555555554 plus an ASLR shift of up to ((2**32) * 4K == 0x100000000000) will sometimes exceed ASan's hardcoded base address of 0x600000000000. We fix this by simply moving the allocator base to 0x500000000000, which is below the PIE program segment base address. This is cleaner than trying to move it to another location that is sandwiched between the PIE program and library segments, because if either of those grow too large, it will crash intoollide with the allocator region. Note that we will never need to change this base address again (unless we want to increase the size of the allocator), because ASLR cannot be set above 32-bits (the PIE program segment and library segments would crashfor x86-64 Linux (the intoPIE program segment and library segments would collide with each other; see also ARCH_MMAP_RND_BITS_MAX in https://github.com/torvalds/linux/blob/master/arch/x86/Kconfig). [*] see https://b.corp.google.com/issues/276925478 and https://groups.google.com/a/google.com/g/chrome-os-gardeners/c/BbfzCP3dEeo/m/h3C_vVUxCQAJ