platform_bionic/libc/malloc_hooks

Malloc Hooks

Malloc hooks allows a program to intercept all allocation/free calls that happen during execution. It is only available in Android P and newer versions of the OS.

There are two ways to enable these hooks, set a special system property, or set a special environment variable and run your app/program.

When malloc hooks is enabled, it works by adding a shim layer that replaces the normal allocation calls. The replaced calls are:

• malloc
• free
• calloc
• realloc
• posix_memalign
• memalign
• aligned_alloc
• malloc_usable_size

On 32 bit systems, these two deprecated functions are also replaced:

• pvalloc
• valloc

These four hooks are defined in malloc.h:

void* (*volatile __malloc_hook)(size_t, const void*);
void* (*volatile __realloc_hook)(void*, size_t, const void*);
void (*volatile __free_hook)(void*, const void*);
void* (*volatile __memalign_hook)(size_t, size_t, const void*);

When malloc is called and __malloc_hook has been set, then the hook function is called instead.

When realloc is called and __realloc_hook has been set, then the hook function is called instead.

When free is called and __free_hook has been set, then the hook function is called instead.

When memalign is called and __memalign_hook has been set, then the hook function is called instead.

For posix_memalign, if __memalign_hook has been set, then the hook is called, but only if alignment is a power of 2.

For aligned_alloc, if __memalign_hook has been set, then the hook is called, but only if alignment is a power of 2.

For calloc, if __malloc_hook has been set, then the hook function is called, then the allocated memory is set to zero.

For the two deprecated functions pvalloc and valloc, if __memalign_hook has been set, then the hook is called with an appropriate alignment value.

There is no hook for malloc_usable_size as of now.

These hooks can be set at any time, but there is no thread safety, so the caller must guarantee that it does not depend on allocations/frees occurring at the same time.

Implementation Details

When malloc hooks is enabled, then the hook pointers are set to the current default allocation functions. It is expected that if an app does intercept the allocation/free calls, it will eventually call the original hook function to do allocations. If the app does not do this, it runs the risk of crashing whenever a malloc_usable_size call is made.

Example Implementation

Below is a simple implementation intercepting only malloc/calloc calls.

void* new_malloc_hook(size_t bytes, const void* arg) {
  return orig_malloc_hook(bytes, arg);
}

auto orig_malloc_hook = __malloc_hook;
__malloc_hook = new_malloc_hook;

Enabling Examples

For platform developers

Enable the hooks for all processes:

adb shell stop
adb shell setprop libc.debug.hooks.enable 1
adb shell start

Enable malloc hooks using an environment variable:

adb shell
# export LIBC_HOOKS_ENABLE=1
# ls

Any process spawned from this shell will run with malloc hooks enabled.

For app developers

Enable malloc hooks for a specific program/application:

adb shell setprop wrap.<APP> '"LIBC_HOOKS_ENABLE=1"'

For example, to enable malloc hooks for the google search box:

adb shell setprop wrap.com.google.android.googlequicksearchbox '"LIBC_HOOKS_ENABLE=1 logwrapper"'
adb shell am force-stop com.google.android.googlequicksearchbox

NOTE: On pre-O versions of the Android OS, property names had a length limit of 32. This meant that to create a wrap property with the name of the app, it was necessary to truncate the name to fit. On O, property names can be an order of magnitude larger, so there should be no need to truncate the name at all.