src/vm/SavedStacks.h

/* -*- 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 */