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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:
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
#ifndef vm_SavedStacks_h
#define vm_SavedStacks_h
#include "mozilla/Attributes.h"
#include "mozilla/FastBernoulliTrial.h"
#include "mozilla/Maybe.h"
#include "js/HashTable.h"
#include "js/Wrapper.h"
#include "vm/JSContext.h"
#include "vm/SavedFrame.h"
#include "vm/Stack.h"
namespace JS {
enum class SavedFrameSelfHosted;
}
namespace js {
// # Saved Stacks
//
// The `SavedStacks` class provides a compact way to capture and save JS stacks
// as `SavedFrame` `JSObject` subclasses. A single `SavedFrame` object
// represents one frame that was on the stack, and has a strong reference to its
// parent `SavedFrame` (the next youngest frame). This reference is null when
// the `SavedFrame` object is the oldest frame that was on the stack.
//
// This comment documents implementation. For usage documentation, see the
// `js/src/doc/SavedFrame/SavedFrame.md` file and relevant `SavedFrame`
// functions in `js/src/jsapi.h`.
//
// ## Compact
//
// Older saved stack frame tails are shared via hash consing, to deduplicate
// structurally identical data. `SavedStacks` contains a hash table of weakly
// held `SavedFrame` objects, and when the owning compartment is swept, it
// removes entries from this table that aren't held alive in any other way. When
// saving new stacks, we use this table to find pre-existing `SavedFrame`
// objects. If such an object is already extant, it is reused; otherwise a new
// `SavedFrame` is allocated and inserted into the table.
//
// Naive | Hash Consing
// --------------+------------------
// c -> b -> a | c -> b -> a
// | ^
// d -> b -> a | d ---|
// | |
// e -> b -> a | e ---'
//
// This technique is effective because of the nature of the events that trigger
// capturing the stack. Currently, these events consist primarily of `JSObject`
// allocation (when an observing `Debugger` has such tracking), `Promise`
// settlement, and `Error` object creation. While these events may occur many
// times, they tend to occur only at a few locations in the JS source. For
// example, if we enable Object allocation tracking and run the esprima
// JavaScript parser on its own JavaScript source, there are approximately 54700
// total `Object` allocations, but just ~1400 unique JS stacks at allocation
// time. There's only ~200 allocation sites if we capture only the youngest
// stack frame.
//
// ## Security and Wrappers
//
// We save every frame on the stack, regardless of whether the `SavedStack`'s
// compartment's principals subsume the frame's compartment's principals or
// not. This gives us maximum flexibility down the line when accessing and
// presenting captured stacks, but at the price of some complication involved in
// preventing the leakage of privileged stack frames to unprivileged callers.
//
// When a `SavedFrame` method or accessor is called, we compare the caller's
// compartment's principals to each `SavedFrame`'s captured principals. We avoid
// using the usual `CallNonGenericMethod` and `nativeCall` machinery which
// enters the `SavedFrame` object's compartment before we can check these
// principals, because we need access to the original caller's compartment's
// principals (unlike other `CallNonGenericMethod` users) to determine what view
// of the stack to present. Instead, we take a similar approach to that used by
// DOM methods, and manually unwrap wrappers until we get the underlying
// `SavedFrame` object, find the first `SavedFrame` in its stack whose captured
// principals are subsumed by the caller's principals, access the reserved slots
// we care about, and then rewrap return values as necessary.
//
// Consider the following diagram:
//
// Content Compartment
// +---------------------------------------+
// | |
// | +------------------------+ |
// Chrome Compartment | | | |
// +--------------------+ | | SavedFrame C (content) | |
// | | | | | |
// | +--------------+ +------------------------+ |
// | | | ^ |
// | var x -----> | Xray Wrapper |-----. | |
// | | | | | |
// | +--------------+ | +------------------------+ |
// | | | | | | |
// | +--------------+ | | SavedFrame B (content) | |
// | | | | | | |
// | var y -----> | CCW (waived) |--. | +------------------------+ |
// | | | | | ^ |
// | +--------------+ | | | |
// | | | | | | |
// +--------------------+ | | | +------------------------+ |
// | | '-> | | |
// | | | SavedFrame A (chrome) | |
// | '----> | | |
// | +------------------------+ |
// | ^ |
// | | |
// | var z -----' |
// | |
// +---------------------------------------+
//
// CCW is a plain cross-compartment wrapper, yielded by waiving Xray vision. A
// is the youngest `SavedFrame` and represents a frame that was from the chrome
// compartment, while B and C are from frames from the content compartment. C is
// the oldest frame.
//
// Note that it is always safe to waive an Xray around a SavedFrame object,
// because SavedFrame objects and the SavedFrame prototype are always frozen you
// will never run untrusted code.
//
// Depending on who the caller is, the view of the stack will be different, and
// is summarized in the table below.
//
// Var | View
// -----+------------
// x | A -> B -> C
// y, z | B -> C
//
// In the case of x, the `SavedFrame` accessors are called with an Xray wrapper
// around the `SavedFrame` object as the `this` value, and the chrome
// compartment as the cx's current principals. Because the chrome compartment's
// principals subsume both itself and the content compartment's principals, x
// has the complete view of the stack.
//
// In the case of y, the cross-compartment machinery automatically enters the
// content compartment, and calls the `SavedFrame` accessors with the wrapped
// `SavedFrame` object as the `this` value. Because the cx's current compartment
// is the content compartment, and the content compartment's principals do not
// subsume the chrome compartment's principals, it can only see the B and C
// frames.
//
// In the case of z, the `SavedFrame` accessors are called with the `SavedFrame`
// object in the `this` value, and the content compartment as the cx's current
// compartment. Similar to the case of y, only the B and C frames are exposed
// because the cx's current compartment's principals do not subsume A's captured
// principals.
class SavedStacks {
friend class SavedFrame;
friend bool JS::ubi::ConstructSavedFrameStackSlow(
JSContext* cx, JS::ubi::StackFrame& ubiFrame,
MutableHandleObject outSavedFrameStack);
public:
SavedStacks()
: frames(),
bernoulliSeeded(false),
bernoulli(1.0, 0x59fdad7f6b4cc573, 0x91adf38db96a9354),
creatingSavedFrame(false) {}
[[nodiscard]] bool saveCurrentStack(
JSContext* cx, MutableHandleSavedFrame frame,
JS::StackCapture&& capture = JS::StackCapture(JS::AllFrames()));
[[nodiscard]] bool copyAsyncStack(
JSContext* cx, HandleObject asyncStack, HandleString asyncCause,
MutableHandleSavedFrame adoptedStack,
const mozilla::Maybe<size_t>& maxFrameCount);
void traceWeak(JSTracer* trc);
void trace(JSTracer* trc);
uint32_t count();
void clear();
void chooseSamplingProbability(JS::Realm* realm);
// Set the sampling random number generator's state to |state0| and
// |state1|. One or the other must be non-zero. See the comments for
// mozilla::non_crypto::XorShift128PlusRNG::setState for details.
void setRNGState(uint64_t state0, uint64_t state1) {
bernoulli.setRandomState(state0, state1);
}
size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf);
// An alloction metadata builder that marks cells with the JavaScript stack
// at which they were allocated.
struct MetadataBuilder : public AllocationMetadataBuilder {
MetadataBuilder() : AllocationMetadataBuilder() {}
virtual JSObject* build(JSContext* cx, HandleObject obj,
AutoEnterOOMUnsafeRegion& oomUnsafe) const override;
};
static const MetadataBuilder metadataBuilder;
private:
SavedFrame::Set frames;
bool bernoulliSeeded;
mozilla::FastBernoulliTrial bernoulli;
bool creatingSavedFrame;
// Similar to mozilla::ReentrancyGuard, but instead of asserting against
// reentrancy, just change the behavior of SavedStacks::saveCurrentStack to
// return a nullptr SavedFrame.
struct MOZ_RAII AutoReentrancyGuard {
SavedStacks& stacks;
explicit AutoReentrancyGuard(SavedStacks& stacks) : stacks(stacks) {
stacks.creatingSavedFrame = true;
}
~AutoReentrancyGuard() { stacks.creatingSavedFrame = false; }
};
[[nodiscard]] bool insertFrames(JSContext* cx, MutableHandleSavedFrame frame,
JS::StackCapture&& capture);
[[nodiscard]] bool adoptAsyncStack(
JSContext* cx, MutableHandleSavedFrame asyncStack, HandleAtom asyncCause,
const mozilla::Maybe<size_t>& maxFrameCount);
[[nodiscard]] bool checkForEvalInFramePrev(
JSContext* cx, MutableHandle<SavedFrame::Lookup> lookup);
SavedFrame* getOrCreateSavedFrame(JSContext* cx,
Handle<SavedFrame::Lookup> lookup);
SavedFrame* createFrameFromLookup(JSContext* cx,
Handle<SavedFrame::Lookup> lookup);
void setSamplingProbability(double probability);
// Cache for memoizing PCToLineNumber lookups.
struct PCKey {
PCKey(JSScript* script, jsbytecode* pc) : script(script), pc(pc) {}
HeapPtr<JSScript*> script;
jsbytecode* pc;
void trace(JSTracer* trc) { /* PCKey is weak. */
}
bool traceWeak(JSTracer* trc) {
return TraceWeakEdge(trc, &script, "traceWeak");
}
};
public:
struct LocationValue {
LocationValue() : source(nullptr), sourceId(0), line(0), column(0) {}
LocationValue(JSAtom* source, uint32_t sourceId, size_t line,
uint32_t column)
: source(source), sourceId(sourceId), line(line), column(column) {}
void trace(JSTracer* trc) {
TraceNullableEdge(trc, &source, "SavedStacks::LocationValue::source");
}
bool traceWeak(JSTracer* trc) {
MOZ_ASSERT(source);
// TODO: Bug 1501334: IsAboutToBeFinalized doesn't work for atoms.
// Otherwise we should assert TraceWeakEdge always returns true;
return TraceWeakEdge(trc, &source, "traceWeak");
}
HeapPtr<JSAtom*> source;
uint32_t sourceId;
size_t line;
uint32_t column;
};
private:
struct PCLocationHasher : public DefaultHasher<PCKey> {
using ScriptPtrHasher = DefaultHasher<JSScript*>;
using BytecodePtrHasher = DefaultHasher<jsbytecode*>;
static HashNumber hash(const PCKey& key) {
return mozilla::AddToHash(ScriptPtrHasher::hash(key.script),
BytecodePtrHasher::hash(key.pc));
}
static bool match(const PCKey& l, const PCKey& k) {
return ScriptPtrHasher::match(l.script, k.script) &&
BytecodePtrHasher::match(l.pc, k.pc);
}
};
// We eagerly Atomize the script source stored in LocationValue because
// wasm does not always have a JSScript and the source might not be
// available when we need it later. However, since the JSScript does not
// actually hold this atom, we have to trace it strongly to keep it alive.
// Thus, it takes two GC passes to fully clean up this table: the first GC
// removes the dead script; the second will clear out the source atom since
// it is no longer held by the table.
using PCLocationMap =
GCHashMap<PCKey, LocationValue, PCLocationHasher, SystemAllocPolicy>;
PCLocationMap pcLocationMap;
[[nodiscard]] bool getLocation(JSContext* cx, const FrameIter& iter,
MutableHandle<LocationValue> locationp);
};
template <typename Wrapper>
struct WrappedPtrOperations<SavedStacks::LocationValue, Wrapper> {
JSAtom* source() const { return loc().source; }
uint32_t sourceId() const { return loc().sourceId; }
size_t line() const { return loc().line; }
uint32_t column() const { return loc().column; }
private:
const SavedStacks::LocationValue& loc() const {
return static_cast<const Wrapper*>(this)->get();
}
};
template <typename Wrapper>
struct MutableWrappedPtrOperations<SavedStacks::LocationValue, Wrapper>
: public WrappedPtrOperations<SavedStacks::LocationValue, Wrapper> {
void setSource(JSAtom* v) { loc().source = v; }
void setSourceId(uint32_t v) { loc().sourceId = v; }
void setLine(size_t v) { loc().line = v; }
void setColumn(uint32_t v) { loc().column = v; }
private:
SavedStacks::LocationValue& loc() {
return static_cast<Wrapper*>(this)->get();
}
};
JS::UniqueChars BuildUTF8StackString(JSContext* cx, JSPrincipals* principals,
HandleObject stack);
uint32_t FixupColumnForDisplay(uint32_t column);
js::SavedFrame* UnwrapSavedFrame(JSContext* cx, JSPrincipals* principals,
HandleObject obj,
JS::SavedFrameSelfHosted selfHosted,
bool& skippedAsync);
} /* namespace js */
#endif /* vm_SavedStacks_h */