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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ *
+ * Copyright 2021 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "wasm/WasmMemory.h"
+
+#include "mozilla/MathAlgorithms.h"
+
+#include "js/Conversions.h"
+#include "js/ErrorReport.h"
+#include "vm/ArrayBufferObject.h"
+#include "wasm/WasmCodegenTypes.h"
+#include "wasm/WasmProcess.h"
+
+using mozilla::IsPowerOfTwo;
+
+using namespace js;
+using namespace js::wasm;
+
+const char* wasm::ToString(IndexType indexType) {
+ switch (indexType) {
+ case IndexType::I32:
+ return "i32";
+ case IndexType::I64:
+ return "i64";
+ default:
+ MOZ_CRASH();
+ }
+}
+
+bool wasm::ToIndexType(JSContext* cx, HandleValue value, IndexType* indexType) {
+ RootedString typeStr(cx, ToString(cx, value));
+ if (!typeStr) {
+ return false;
+ }
+
+ Rooted<JSLinearString*> typeLinearStr(cx, typeStr->ensureLinear(cx));
+ if (!typeLinearStr) {
+ return false;
+ }
+
+ if (StringEqualsLiteral(typeLinearStr, "i32")) {
+ *indexType = IndexType::I32;
+ } else if (StringEqualsLiteral(typeLinearStr, "i64")) {
+ *indexType = IndexType::I64;
+ } else {
+ JS_ReportErrorNumberUTF8(cx, GetErrorMessage, nullptr,
+ JSMSG_WASM_BAD_STRING_IDX_TYPE);
+ return false;
+ }
+ return true;
+}
+
+/*
+ * [SMDOC] Linear memory addresses and bounds checking
+ *
+ * (Also see "WASM Linear Memory structure" in vm/ArrayBufferObject.cpp)
+ *
+ *
+ * Memory addresses, bounds check avoidance, and the huge memory trick.
+ *
+ * A memory address in an access instruction has three components, the "memory
+ * base", the "pointer", and the "offset". The "memory base" - the HeapReg on
+ * most platforms and a value loaded from the instance on x86 - is a native
+ * pointer that points to the start of the linear memory array; we'll ignore the
+ * memory base in the following. The "pointer" is the i32 or i64 index supplied
+ * by the program as a separate value argument to the access instruction; it is
+ * usually variable but can be constant. The "offset" is a constant encoded in
+ * the access instruction.
+ *
+ * The "effective address" (EA) is the non-overflowed sum of the pointer and the
+ * offset (if the sum overflows the program traps); the pointer, offset, and EA
+ * all have the same type, i32 or i64.
+ *
+ * An access has an "access size", which is the number of bytes that are
+ * accessed - currently up to 16 (for V128). The highest-addressed byte to be
+ * accessed by an access is thus the byte at (pointer+offset+access_size-1),
+ * where offset+access_size-1 is compile-time evaluable.
+ *
+ * Bounds checking ensures that the entire access is in bounds, ie, that the
+ * highest-addressed byte is at an offset in the linear memory below that of the
+ * memory's current byteLength.
+ *
+ * To avoid performing an addition with overflow check and a compare-and-branch
+ * bounds check for every memory access, we use some tricks:
+ *
+ * - An access-protected guard region of size R at the end of each memory is
+ * used to trap accesses to out-of-bounds offsets in the range
+ * 0..R-access_size. Thus the offset and the access size need not be added
+ * into the pointer before the bounds check, saving the add and overflow
+ * check. The offset is added into the pointer without an overflow check
+ * either directly before the access or in the access instruction itself
+ * (depending on the ISA). The pointer must still be explicitly
+ * bounds-checked.
+ *
+ * - On 64-bit systems where we determine there is plenty of virtual memory
+ * space (and ideally we determine that the VM system uses overcommit), a
+ * 32-bit memory is implemented as a 4GB + R reservation, where the memory
+ * from the current heap length through the end of the reservation is
+ * access-protected. The protected area R allows offsets up to R-access_size
+ * to be encoded in the access instruction. The pointer need not be bounds
+ * checked explicitly, since it has only a 4GB range and thus points into the
+ * 4GB part of the reservation. The offset can be added into the pointer
+ * (using 64-bit arithmetic) either directly before the access or in the
+ * access instruction.
+ *
+ * The value of R differs in the two situations; in the first case it tends to
+ * be small, currently 64KB; in the second case it is large, currently 2GB+64KB.
+ * The difference is due to explicit bounds checking tending to be used on
+ * 32-bit systems where memory and address space are scarce, while the implicit
+ * bounds check is used only on 64-bit systems after ensuring that sufficient
+ * address space is available in the process. (2GB is really overkill, and
+ * there's nothing magic about it; we could use something much smaller.)
+ *
+ * The implicit bounds checking strategy with the large reservation is known
+ * below and elsewhere as the "huge memory trick" or just "huge memory".
+ *
+ * All memories in a process use the same strategy, selected at process startup.
+ * The immediate reason for that is that the machine code embeds the strategy
+ * it's been compiled with, and may later be exposed to memories originating
+ * from different modules or directly from JS. If the memories did not all use
+ * the same strategy, and the same strategy as the code, linking would fail or
+ * we would have to recompile the code.
+ *
+ *
+ * The boundsCheckLimit.
+ *
+ * The bounds check limit that is stored in the instance is always valid and is
+ * always a 64-bit datum, and it is always correct to load it and use it as a
+ * 64-bit value. However, in situations when the 32 upper bits are known to be
+ * zero, it is also correct to load just the low 32 bits from the address of the
+ * limit (which is always little-endian when a JIT is enabled), and use that
+ * value as the limit.
+ *
+ * On x86 and arm32 (and on any other 32-bit platform, should there ever be
+ * one), there is explicit bounds checking and the heap, whether memory32 or
+ * memory64, is limited to 2GB; the bounds check limit can be treated as a
+ * 32-bit quantity.
+ *
+ * On all 64-bit platforms, we may use explicit bounds checking or the huge
+ * memory trick for memory32, but must always use explicit bounds checking for
+ * memory64. If the heap does not have a known maximum size or the known
+ * maximum is greater than or equal to 4GB, then the bounds check limit must be
+ * treated as a 64-bit quantity; otherwise it can be treated as a 32-bit
+ * quantity.
+ *
+ * On x64 and arm64 with Baseline and Ion, we allow 32-bit memories up to 4GB,
+ * and 64-bit memories can be larger.
+ *
+ * On mips64, memories are limited to 2GB, for now.
+ *
+ * Asm.js memories are limited to 2GB even on 64-bit platforms, and we can
+ * always assume a 32-bit bounds check limit for asm.js.
+ *
+ *
+ * Constant pointers.
+ *
+ * If the pointer is constant then the EA can be computed at compile time, and
+ * if the EA is below the initial memory size then the bounds check can be
+ * elided.
+ *
+ *
+ * Alignment checks.
+ *
+ * On all platforms, some accesses (currently atomics) require an alignment
+ * check: the EA must be naturally aligned for the datum being accessed.
+ * However, we do not need to compute the EA properly, we care only about the
+ * low bits - a cheap, overflowing add is fine, and if the offset is known
+ * to be aligned, only the pointer need be checked.
+ */
+
+// Bounds checks always compare the base of the memory access with the bounds
+// check limit. If the memory access is unaligned, this means that, even if the
+// bounds check succeeds, a few bytes of the access can extend past the end of
+// memory. To guard against this, extra space is included in the guard region to
+// catch the overflow. MaxMemoryAccessSize is a conservative approximation of
+// the maximum guard space needed to catch all unaligned overflows.
+//
+// Also see "Linear memory addresses and bounds checking" above.
+
+static const unsigned MaxMemoryAccessSize = LitVal::sizeofLargestValue();
+
+// All plausible targets must be able to do at least IEEE754 double
+// loads/stores, hence the lower limit of 8. Some Intel processors support
+// AVX-512 loads/stores, hence the upper limit of 64.
+static_assert(MaxMemoryAccessSize >= 8, "MaxMemoryAccessSize too low");
+static_assert(MaxMemoryAccessSize <= 64, "MaxMemoryAccessSize too high");
+static_assert((MaxMemoryAccessSize & (MaxMemoryAccessSize - 1)) == 0,
+ "MaxMemoryAccessSize is not a power of two");
+
+#ifdef WASM_SUPPORTS_HUGE_MEMORY
+
+static_assert(MaxMemoryAccessSize <= HugeUnalignedGuardPage,
+ "rounded up to static page size");
+static_assert(HugeOffsetGuardLimit < UINT32_MAX,
+ "checking for overflow against OffsetGuardLimit is enough.");
+
+// We have only tested huge memory on x64 and arm64.
+# if !(defined(JS_CODEGEN_X64) || defined(JS_CODEGEN_ARM64))
+# error "Not an expected configuration"
+# endif
+
+#endif
+
+// On !WASM_SUPPORTS_HUGE_MEMORY platforms:
+// - To avoid OOM in ArrayBuffer::prepareForAsmJS, asm.js continues to use the
+// original ArrayBuffer allocation which has no guard region at all.
+// - For WebAssembly memories, an additional GuardSize is mapped after the
+// accessible region of the memory to catch folded (base+offset) accesses
+// where `offset < OffsetGuardLimit` as well as the overflow from unaligned
+// accesses, as described above for MaxMemoryAccessSize.
+
+static const size_t OffsetGuardLimit = PageSize - MaxMemoryAccessSize;
+
+static_assert(MaxMemoryAccessSize < GuardSize,
+ "Guard page handles partial out-of-bounds");
+static_assert(OffsetGuardLimit < UINT32_MAX,
+ "checking for overflow against OffsetGuardLimit is enough.");
+
+size_t wasm::GetMaxOffsetGuardLimit(bool hugeMemory) {
+#ifdef WASM_SUPPORTS_HUGE_MEMORY
+ return hugeMemory ? HugeOffsetGuardLimit : OffsetGuardLimit;
+#else
+ return OffsetGuardLimit;
+#endif
+}
+
+// Assert that our minimum offset guard limit covers our inline
+// memory.copy/fill optimizations.
+static const size_t MinOffsetGuardLimit = OffsetGuardLimit;
+static_assert(MaxInlineMemoryCopyLength < MinOffsetGuardLimit, "precondition");
+static_assert(MaxInlineMemoryFillLength < MinOffsetGuardLimit, "precondition");
+
+#ifdef JS_64BIT
+wasm::Pages wasm::MaxMemoryPages(IndexType t) {
+ MOZ_ASSERT_IF(t == IndexType::I64, !IsHugeMemoryEnabled(t));
+ size_t desired = MaxMemoryLimitField(t);
+ constexpr size_t actual = ArrayBufferObject::MaxByteLength / PageSize;
+ return wasm::Pages(std::min(desired, actual));
+}
+
+size_t wasm::MaxMemoryBoundsCheckLimit(IndexType t) {
+ return MaxMemoryPages(t).byteLength();
+}
+
+#else
+// On 32-bit systems, the heap limit must be representable in the nonnegative
+// range of an int32_t, which means the maximum heap size as observed by wasm
+// code is one wasm page less than 2GB.
+wasm::Pages wasm::MaxMemoryPages(IndexType t) {
+ static_assert(ArrayBufferObject::MaxByteLength >= INT32_MAX / PageSize);
+ return wasm::Pages(INT32_MAX / PageSize);
+}
+
+// The max bounds check limit can be larger than the MaxMemoryPages because it
+// is really MaxMemoryPages rounded up to the next valid bounds check immediate,
+// see ComputeMappedSize().
+size_t wasm::MaxMemoryBoundsCheckLimit(IndexType t) {
+ size_t boundsCheckLimit = size_t(INT32_MAX) + 1;
+ MOZ_ASSERT(IsValidBoundsCheckImmediate(boundsCheckLimit));
+ return boundsCheckLimit;
+}
+#endif
+
+// Because ARM has a fixed-width instruction encoding, ARM can only express a
+// limited subset of immediates (in a single instruction).
+
+static const uint64_t HighestValidARMImmediate = 0xff000000;
+
+// Heap length on ARM should fit in an ARM immediate. We approximate the set
+// of valid ARM immediates with the predicate:
+// 2^n for n in [16, 24)
+// or
+// 2^24 * n for n >= 1.
+bool wasm::IsValidARMImmediate(uint32_t i) {
+ bool valid = (IsPowerOfTwo(i) || (i & 0x00ffffff) == 0);
+
+ MOZ_ASSERT_IF(valid, i % PageSize == 0);
+
+ return valid;
+}
+
+uint64_t wasm::RoundUpToNextValidARMImmediate(uint64_t i) {
+ MOZ_ASSERT(i <= HighestValidARMImmediate);
+ static_assert(HighestValidARMImmediate == 0xff000000,
+ "algorithm relies on specific constant");
+
+ if (i <= 16 * 1024 * 1024) {
+ i = i ? mozilla::RoundUpPow2(i) : 0;
+ } else {
+ i = (i + 0x00ffffff) & ~0x00ffffff;
+ }
+
+ MOZ_ASSERT(IsValidARMImmediate(i));
+
+ return i;
+}
+
+Pages wasm::ClampedMaxPages(IndexType t, Pages initialPages,
+ const Maybe<Pages>& sourceMaxPages,
+ bool useHugeMemory) {
+ Pages clampedMaxPages;
+
+ if (sourceMaxPages.isSome()) {
+ // There is a specified maximum, clamp it to the implementation limit of
+ // maximum pages
+ clampedMaxPages = std::min(*sourceMaxPages, wasm::MaxMemoryPages(t));
+
+#ifndef JS_64BIT
+ static_assert(sizeof(uintptr_t) == 4, "assuming not 64 bit implies 32 bit");
+
+ // On 32-bit platforms, prevent applications specifying a large max (like
+ // MaxMemoryPages()) from unintentially OOMing the browser: they just want
+ // "a lot of memory". Maintain the invariant that initialPages <=
+ // clampedMaxPages.
+ static const uint64_t OneGib = 1 << 30;
+ static const Pages OneGibPages = Pages(OneGib >> wasm::PageBits);
+ static_assert(HighestValidARMImmediate > OneGib,
+ "computing mapped size on ARM requires clamped max size");
+
+ Pages clampedPages = std::max(OneGibPages, initialPages);
+ clampedMaxPages = std::min(clampedPages, clampedMaxPages);
+#endif
+ } else {
+ // There is not a specified maximum, fill it in with the implementation
+ // limit of maximum pages
+ clampedMaxPages = wasm::MaxMemoryPages(t);
+ }
+
+ // Double-check our invariants
+ MOZ_RELEASE_ASSERT(sourceMaxPages.isNothing() ||
+ clampedMaxPages <= *sourceMaxPages);
+ MOZ_RELEASE_ASSERT(clampedMaxPages <= wasm::MaxMemoryPages(t));
+ MOZ_RELEASE_ASSERT(initialPages <= clampedMaxPages);
+
+ return clampedMaxPages;
+}
+
+size_t wasm::ComputeMappedSize(wasm::Pages clampedMaxPages) {
+ // Caller is responsible to ensure that clampedMaxPages has been clamped to
+ // implementation limits.
+ size_t maxSize = clampedMaxPages.byteLength();
+
+ // It is the bounds-check limit, not the mapped size, that gets baked into
+ // code. Thus round up the maxSize to the next valid immediate value
+ // *before* adding in the guard page.
+ //
+ // Also see "Wasm Linear Memory Structure" in vm/ArrayBufferObject.cpp.
+ uint64_t boundsCheckLimit = RoundUpToNextValidBoundsCheckImmediate(maxSize);
+ MOZ_ASSERT(IsValidBoundsCheckImmediate(boundsCheckLimit));
+
+ MOZ_ASSERT(boundsCheckLimit % gc::SystemPageSize() == 0);
+ MOZ_ASSERT(GuardSize % gc::SystemPageSize() == 0);
+ return boundsCheckLimit + GuardSize;
+}
+
+bool wasm::IsValidBoundsCheckImmediate(uint32_t i) {
+#ifdef JS_CODEGEN_ARM
+ return IsValidARMImmediate(i);
+#else
+ return true;
+#endif
+}
+
+uint64_t wasm::RoundUpToNextValidBoundsCheckImmediate(uint64_t i) {
+#ifdef JS_CODEGEN_ARM
+ return RoundUpToNextValidARMImmediate(i);
+#else
+ return i;
+#endif
+}
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