Stack overflow protection

[chakaz]

Since we use Fibers, we can't utilize many tools (like Valgrind, -fstack-protector) to detect stack overflows.

Instead, we may implement a simple protection of our own (maybe in Debug builds only, or enabled with a flag):

• Instead of allocating 32kb like we do today, we'll allocate 64k
• We'll configure the last 32kb to be protected memory (mprotect(PROT_NONE))
• Before freeing the memory (upon Fiber termination) we'll remove the protection

[chakaz]

A working allocator example:

class ProtectedStdMallocResource : public PMR_NS::memory_resource {
 private:
  void* do_allocate(std::size_t size, std::size_t align) final {
    void* ptr = aligned_alloc(getpagesize(), size * 2);
    CHECK_EQ(mprotect(ptr, size, PROT_NONE), 0);
    return shift_bytes(ptr, size);
  }

  void do_deallocate(void* ptr, std::size_t size, std::size_t align) final {
    ptr = shift_bytes(ptr, -size);
    CHECK_EQ(mprotect(ptr, size, PROT_READ | PROT_WRITE), 0);
    free(ptr);
  }

  bool do_is_equal(const PMR_NS::memory_resource& o) const noexcept final {
    return this == &o;
  }

  void* shift_bytes(void* ptr, size_t bytes) {
    return static_cast<uint8_t*>(ptr) + bytes;
  }
} protected_stack_alloc;

Then in dfly_main.cc:

  fb2::SetDefaultStackResource(&protected_stack_alloc, kFiberDefaultStackSize);

The only problem is that this is not used for the dispatch fibers, which use the fixed allocator so we need to plug that in there as well

[romange]

For now, helio provides a poor man solution, though I think it's very effective because can be used in both release and debug builds. See #4349