tor-browser

SavedStacks.h (15192B)

---

/* -*- 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/ColumnNumber.h"  // JS::TaggedColumnNumberOneOrigin
#include "js/HashTable.h"
#include "js/Stack.h"
#include "vm/SavedFrame.h"

namespace JS {
enum class SavedFrameSelfHosted;
}

namespace js {

class FrameIter;

// # 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()
     : bernoulliSeeded(false),
       bernoulli(1.0, 0x59fdad7f6b4cc573, 0x91adf38db96a9354),
       creatingSavedFrame(false) {}

 [[nodiscard]] bool saveCurrentStack(
     JSContext* cx, MutableHandle<SavedFrame*> frame,
     JS::StackCapture&& capture = JS::StackCapture(JS::AllFrames()),
     HandleObject startAt = nullptr);
 [[nodiscard]] bool copyAsyncStack(
     JSContext* cx, HandleObject asyncStack, HandleString asyncCause,
     MutableHandle<SavedFrame*> 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() = default;
   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,
                                 MutableHandle<SavedFrame*> frame,
                                 JS::StackCapture&& capture,
                                 HandleObject startAt);
 [[nodiscard]] bool adoptAsyncStack(
     JSContext* cx, MutableHandle<SavedFrame*> asyncStack,
     Handle<JSAtom*> 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) {}

   WeakHeapPtr<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) {}
   LocationValue(JSAtom* source, uint32_t sourceId, size_t line,
                 JS::TaggedColumnNumberOneOrigin 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");
   }

   WeakHeapPtr<JSAtom*> source;
   uint32_t sourceId;

   // Line number (1-origin).
   size_t line;

   // Column number in UTF-16 code units.
   JS::TaggedColumnNumberOneOrigin 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; }
 JS::TaggedColumnNumberOneOrigin 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(JS::TaggedColumnNumberOneOrigin v) { loc().column = v; }

private:
 SavedStacks::LocationValue& loc() {
   return static_cast<Wrapper*>(this)->get();
 }
};

JS::UniqueChars BuildUTF8StackString(JSContext* cx, JSPrincipals* principals,
                                    HandleObject stack);

js::SavedFrame* UnwrapSavedFrame(JSContext* cx, JSPrincipals* principals,
                                HandleObject obj,
                                JS::SavedFrameSelfHosted selfHosted,
                                bool& skippedAsync);

} /* namespace js */

#endif /* vm_SavedStacks_h */