path: root/lib/gwp_asan/guarded_pool_allocator.h

//===-- guarded_pool_allocator.h --------------------------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef GWP_ASAN_GUARDED_POOL_ALLOCATOR_H_
#define GWP_ASAN_GUARDED_POOL_ALLOCATOR_H_

#include "gwp_asan/definitions.h"
#include "gwp_asan/mutex.h"
#include "gwp_asan/options.h"
#include "gwp_asan/random.h"
#include "gwp_asan/stack_trace_compressor.h"

#include <stddef.h>
#include <stdint.h>

namespace gwp_asan {
// This class is the primary implementation of the allocator portion of GWP-
// ASan. It is the sole owner of the pool of sequentially allocated guarded
// slots. It should always be treated as a singleton.

// Functions in the public interface of this class are thread-compatible until
// init() is called, at which point they become thread-safe (unless specified
// otherwise).
class GuardedPoolAllocator {
public:
  static constexpr uint64_t kInvalidThreadID = UINT64_MAX;

  enum class Error {
    UNKNOWN,
    USE_AFTER_FREE,
    DOUBLE_FREE,
    INVALID_FREE,
    BUFFER_OVERFLOW,
    BUFFER_UNDERFLOW
  };

  struct AllocationMetadata {
    // The number of bytes used to store a compressed stack frame. On 64-bit
    // platforms, assuming a compression ratio of 50%, this should allow us to
    // store ~64 frames per trace.
    static constexpr size_t kStackFrameStorageBytes = 256;

    // Maximum number of stack frames to collect on allocation/deallocation. The
    // actual number of collected frames may be less than this as the stack
    // frames are compressed into a fixed memory range.
    static constexpr size_t kMaxTraceLengthToCollect = 128;

    // Records the given allocation metadata into this struct.
    void RecordAllocation(uintptr_t Addr, size_t Size,
                          options::Backtrace_t Backtrace);

    // Record that this allocation is now deallocated.
    void RecordDeallocation(options::Backtrace_t Backtrace);

    struct CallSiteInfo {
      // The compressed backtrace to the allocation/deallocation.
      uint8_t CompressedTrace[kStackFrameStorageBytes];
      // The thread ID for this trace, or kInvalidThreadID if not available.
      uint64_t ThreadID = kInvalidThreadID;
      // The size of the compressed trace (in bytes). Zero indicates that no
      // trace was collected.
      size_t TraceSize = 0;
    };

    // The address of this allocation.
    uintptr_t Addr = 0;
    // Represents the actual size of the allocation.
    size_t Size = 0;

    CallSiteInfo AllocationTrace;
    CallSiteInfo DeallocationTrace;

    // Whether this allocation has been deallocated yet.
    bool IsDeallocated = false;
  };

  // During program startup, we must ensure that memory allocations do not land
  // in this allocation pool if the allocator decides to runtime-disable
  // GWP-ASan. The constructor value-initialises the class such that if no
  // further initialisation takes place, calls to shouldSample() and
  // pointerIsMine() will return false.
  constexpr GuardedPoolAllocator(){};
  GuardedPoolAllocator(const GuardedPoolAllocator &) = delete;
  GuardedPoolAllocator &operator=(const GuardedPoolAllocator &) = delete;

  // Note: This class is expected to be a singleton for the lifetime of the
  // program. If this object is initialised, it will leak the guarded page pool
  // and metadata allocations during destruction. We can't clean up these areas
  // as this may cause a use-after-free on shutdown.
  ~GuardedPoolAllocator() = default;

  // Initialise the rest of the members of this class. Create the allocation
  // pool using the provided options. See options.inc for runtime configuration
  // options.
  void init(const options::Options &Opts);

  // Return whether the allocation should be randomly chosen for sampling.
  ALWAYS_INLINE bool shouldSample() {
    // NextSampleCounter == 0 means we "should regenerate the counter".
    //                   == 1 means we "should sample this allocation".
    if (UNLIKELY(ThreadLocals.NextSampleCounter == 0))
      ThreadLocals.NextSampleCounter =
          (getRandomUnsigned32() % AdjustedSampleRate) + 1;

    return UNLIKELY(--ThreadLocals.NextSampleCounter == 0);
  }

  // Returns whether the provided pointer is a current sampled allocation that
  // is owned by this pool.
  ALWAYS_INLINE bool pointerIsMine(const void *Ptr) const {
    uintptr_t P = reinterpret_cast<uintptr_t>(Ptr);
    return GuardedPagePool <= P && P < GuardedPagePoolEnd;
  }

  // Allocate memory in a guarded slot, and return a pointer to the new
  // allocation. Returns nullptr if the pool is empty, the requested size is too
  // large for this pool to handle, or the requested size is zero.
  void *allocate(size_t Size);

  // Deallocate memory in a guarded slot. The provided pointer must have been
  // allocated using this pool. This will set the guarded slot as inaccessible.
  void deallocate(void *Ptr);

  // Returns the size of the allocation at Ptr.
  size_t getSize(const void *Ptr);

  // Returns the largest allocation that is supported by this pool. Any
  // allocations larger than this should go to the regular system allocator.
  size_t maximumAllocationSize() const;

  // Dumps an error report (including allocation and deallocation stack traces).
  // An optional error may be provided if the caller knows what the error is
  // ahead of time. This is primarily a helper function to locate the static
  // singleton pointer and call the internal version of this function. This
  // method is never thread safe, and should only be called when fatal errors
  // occur.
  static void reportError(uintptr_t AccessPtr, Error E = Error::UNKNOWN);

  // Get the current thread ID, or kInvalidThreadID if failure. Note: This
  // implementation is platform-specific.
  static uint64_t getThreadID();

private:
  static constexpr size_t kInvalidSlotID = SIZE_MAX;

  // These functions anonymously map memory or change the permissions of mapped
  // memory into this process in a platform-specific way. Pointer and size
  // arguments are expected to be page-aligned. These functions will never
  // return on error, instead electing to kill the calling process on failure.
  // Note that memory is initially mapped inaccessible. In order for RW
  // mappings, call mapMemory() followed by markReadWrite() on the returned
  // pointer.
  void *mapMemory(size_t Size) const;
  void markReadWrite(void *Ptr, size_t Size) const;
  void markInaccessible(void *Ptr, size_t Size) const;

  // Get the page size from the platform-specific implementation. Only needs to
  // be called once, and the result should be cached in PageSize in this class.
  static size_t getPlatformPageSize();

  // Install the SIGSEGV crash handler for printing use-after-free and heap-
  // buffer-{under|over}flow exceptions. This is platform specific as even
  // though POSIX and Windows both support registering handlers through
  // signal(), we have to use platform-specific signal handlers to obtain the
  // address that caused the SIGSEGV exception.
  static void installSignalHandlers();

  // Returns the index of the slot that this pointer resides in. If the pointer
  // is not owned by this pool, the result is undefined.
  size_t addrToSlot(uintptr_t Ptr) const;

  // Returns the address of the N-th guarded slot.
  uintptr_t slotToAddr(size_t N) const;

  // Returns a pointer to the metadata for the owned pointer. If the pointer is
  // not owned by this pool, the result is undefined.
  AllocationMetadata *addrToMetadata(uintptr_t Ptr) const;

  // Returns the address of the page that this pointer resides in.
  uintptr_t getPageAddr(uintptr_t Ptr) const;

  // Gets the nearest slot to the provided address.
  size_t getNearestSlot(uintptr_t Ptr) const;

  // Returns whether the provided pointer is a guard page or not. The pointer
  // must be within memory owned by this pool, else the result is undefined.
  bool isGuardPage(uintptr_t Ptr) const;

  // Reserve a slot for a new guarded allocation. Returns kInvalidSlotID if no
  // slot is available to be reserved.
  size_t reserveSlot();

  // Unreserve the guarded slot.
  void freeSlot(size_t SlotIndex);

  // Returns the offset (in bytes) between the start of a guarded slot and where
  // the start of the allocation should take place. Determined using the size of
  // the allocation and the options provided at init-time.
  uintptr_t allocationSlotOffset(size_t AllocationSize) const;

  // Returns the diagnosis for an unknown error. If the diagnosis is not
  // Error::INVALID_FREE or Error::UNKNOWN, the metadata for the slot
  // responsible for the error is placed in *Meta.
  Error diagnoseUnknownError(uintptr_t AccessPtr, AllocationMetadata **Meta);

  void reportErrorInternal(uintptr_t AccessPtr, Error E);

  // Cached page size for this system in bytes.
  size_t PageSize = 0;

  // A mutex to protect the guarded slot and metadata pool for this class.
  Mutex PoolMutex;
  // The number of guarded slots that this pool holds.
  size_t MaxSimultaneousAllocations = 0;
  // Record the number allocations that we've sampled. We store this amount so
  // that we don't randomly choose to recycle a slot that previously had an
  // allocation before all the slots have been utilised.
  size_t NumSampledAllocations = 0;
  // Pointer to the pool of guarded slots. Note that this points to the start of
  // the pool (which is a guard page), not a pointer to the first guarded page.
  uintptr_t GuardedPagePool = UINTPTR_MAX;
  uintptr_t GuardedPagePoolEnd = 0;
  // Pointer to the allocation metadata (allocation/deallocation stack traces),
  // if any.
  AllocationMetadata *Metadata = nullptr;

  // Pointer to an array of free slot indexes.
  size_t *FreeSlots = nullptr;
  // The current length of the list of free slots.
  size_t FreeSlotsLength = 0;

  // See options.{h, inc} for more information.
  bool PerfectlyRightAlign = false;

  // Printf function supplied by the implementing allocator. We can't (in
  // general) use printf() from the cstdlib as it may malloc(), causing infinite
  // recursion.
  options::Printf_t Printf = nullptr;
  options::Backtrace_t Backtrace = nullptr;
  options::PrintBacktrace_t PrintBacktrace = nullptr;

  // The adjusted sample rate for allocation sampling. Default *must* be
  // nonzero, as dynamic initialisation may call malloc (e.g. from libstdc++)
  // before GPA::init() is called. This would cause an error in shouldSample(),
  // where we would calculate modulo zero. This value is set UINT32_MAX, as when
  // GWP-ASan is disabled, we wish to never spend wasted cycles recalculating
  // the sample rate.
  uint32_t AdjustedSampleRate = UINT32_MAX;

  // Pack the thread local variables into a struct to ensure that they're in
  // the same cache line for performance reasons. These are the most touched
  // variables in GWP-ASan.
  struct alignas(8) ThreadLocalPackedVariables {
    constexpr ThreadLocalPackedVariables() {}
    // Thread-local decrementing counter that indicates that a given allocation
    // should be sampled when it reaches zero.
    uint32_t NextSampleCounter = 0;
    // Guard against recursivity. Unwinders often contain complex behaviour that
    // may not be safe for the allocator (i.e. the unwinder calls dlopen(),
    // which calls malloc()). When recursive behaviour is detected, we will
    // automatically fall back to the supporting allocator to supply the
    // allocation.
    bool RecursiveGuard = false;
  };
  static TLS_INITIAL_EXEC ThreadLocalPackedVariables ThreadLocals;
};
} // namespace gwp_asan

#endif // GWP_ASAN_GUARDED_POOL_ALLOCATOR_H_