Uninitialized memory in C and C++ is a common cause of reliability problems, memory safety bugs and information leaks. To avoid these issues, Android initializes as much memory as possible.
Zero initialized userspace memory
Since Android 12, stack memory is zero initialized
in all platform native code (including JNI) and heap memory is zero
initialized in all platform native processes (such as netd)
but not in the zygote or in apps.
First and third-party apps built with the NDK are strongly
recommended to use the -ftrivial-auto-var-init=zero compiler flag to zero-initialize their stack local
variables. The compiler optimizes away any zeroing that is unnecessary.
For example, when a local variable is explicitly initialized
(such as, int x = 123; variable x is initialized only once).
If the program has a large stack buffer in a performance
hotspot, the developer can disable initialization using a compiler
attribute:
__attribute__((__uninitialized__)) char buf[BUFSIZ];
Apps can also opt in to heap zero initialization by using the
android:nativeHeapZeroInitialized manifest attribute.
Alternatively, heap zero initialization can be controlled at runtime
with:
int mallopt(M_BIONIC_ZERO_INIT, level)
Where level is 0 or 1.
Zero initialized kernel memory
The kernel stack and heap is zero initialized for GKI kernels, which is strongly recommended by the CDD.
For stack initialization, GKI uses the
CONFIG_INIT_STACK_ALL_ZERO config, which results in building the
kernel using the -ftrivial-auto-var-init=zero compiler flag.
For heap initialization, GKI uses the
CONFIG_INIT_ON_ALLOC_DEFAULT_ON, which makes all page heap, SLAB
and SLUB allocations zero-initialized when they are created. This option is
effectively similar to passing init_on_alloc=1 as a kernel
boot-time option.
Bug reports
Our tools generate insightful bug reports that contain additional information to aid with debugging. The additional allocation and deallocation stack trace help better understand the life cycle of a given allocation and lead to root-causing memory safety bugs much faster.
During development, vendors should monitor the presence of bugs by checking
/data/tombstones and
logcat for native crashes. For more information on
debugging Android native code see the information here.