This document describes how malloc / new calls are routed in the various Chrome platforms.
Bear in mind that the chromium codebase does not always just use malloc()
.
Some examples:
- Large parts of the renderer (Blink) use two home-brewed allocators, PartitionAlloc and BlinkGC (Oilpan).
- Some subsystems, such as the V8 JavaScript engine, handle memory management autonomously.
- Various parts of the codebase use abstractions such as
SharedMemory
orDiscardableMemory
which, similarly to the above, have their own page-level memory management.
The allocator
target defines at compile-time the platform-specific choice of
the allocator and extra-hooks which services calls to malloc/new. The relevant
build-time flags involved are use_allocator_shim
and
use_partition_alloc_as_malloc
.
By default, these are true on all platforms except iOS (not yet supported) and
NaCl (no plan to support).
Furthermore, when building with a sanitizer (e.g. asan
, msan
, ...) both the
allocator and the shim layer are disabled.
The allocator
target provides the linker flags required for the Windows shim
layer. The base
target is (almost) the only one depending on allocator
. No
other targets should depend on it, with the exception of the very few
executables / dynamic libraries that don't depend, either directly or
indirectly, on base
within the scope of a linker unit.
More importantly, no other place outside of /base
should depend on the
specific allocator.
If such a functional dependency is required that should be achieved using
abstractions in base
(see /base/allocator/allocator_extension.h
and
/base/memory/
)
Why base
depends on allocator
?
Because it needs to provide services that depend on the actual allocator
implementation. In the past base
used to pretend to be allocator-agnostic
and get the dependencies injected by other layers. This ended up being an
inconsistent mess.
See the allocator cleanup doc for more context.
Linker unit targets (executables and shared libraries) that depend in some way
on base
(most of the targets in the codebase) automatically get the correct
set of linker flags to pull in the Windows shim-layer (if needed).
This directory contains just the allocator (i.e. shim) layer that switches between the different underlying memory allocation implementations.
On most platforms, Chrome overrides the malloc / operator new symbols (and corresponding free / delete and other variants). This is to enforce security checks and lately to enable the memory-infra heap profiler. Historically each platform had its special logic for defining the allocator symbols in different places of the codebase. The unified allocator shim is a project aimed to unify the symbol definition and allocator routing logic in a central place.
- Full documentation: Allocator shim design doc.
- Current state: Available and enabled by default on Android, CrOS, Linux, Mac OS and Windows.
- Tracking bug: crbug.com/550886.
- Build-time flag:
use_allocator_shim
.
Overview of the unified allocator shim The allocator shim consists of three stages:
+-------------------------+ +-----------------------+ +----------------+
| malloc & friends | -> | shim layer | -> | Routing to |
| symbols definition | | implementation | | allocator |
+-------------------------+ +-----------------------+ +----------------+
| - libc symbols (malloc, | | - Security checks | | - glibc |
| calloc, free, ...) | | - Chain of dispatchers| | - Android |
| - C++ symbols (operator | | that can intercept | | bionic |
| new, delete, ...) | | and override | | - WinHeap |
| - glibc weak symbols | | allocations | | - Partition |
| (__libc_malloc, ...) | +-----------------------+ | Alloc |
+-------------------------+ +----------------+
1. malloc symbols definition
This stage takes care of overriding the symbols malloc
, free
,
operator new
, operator delete
and friends and routing those calls inside the
allocator shim (next point).
This is taken care of by the headers in allocator_shim_override_*
.
On Windows: Windows' UCRT (Universal C Runtime) exports weak symbols, that we
can override in allocator_shim_override_ucr_symbols_win.h
.
On Linux/CrOS: the allocator symbols are defined as exported global symbols
in allocator_shim_override_libc_symbols.h
(for malloc
, free
and friends)
and in allocator_shim_override_cpp_symbols.h
(for operator new
,
operator delete
and friends).
This enables proper interposition of malloc symbols referenced by the main
executable and any third party libraries. Symbol resolution on Linux is a breadth first search that starts from the root link unit, that is the executable
(see EXECUTABLE AND LINKABLE FORMAT (ELF) - Portable Formats Specification).
The Linux/CrOS shim was introduced by
crrev.com/1675143004.
On Android: load-time symbol interposition (unlike the Linux/CrOS case) is not
possible. This is because Android processes are fork()
-ed from the Android
zygote, which pre-loads libc.so and only later native code gets loaded via
dlopen()
(symbols from dlopen()
-ed libraries get a different resolution
scope).
In this case, the approach instead of wrapping symbol resolution at link time
(i.e. during the build), via the --Wl,-wrap,malloc
linker flag.
The use of this wrapping flag causes:
- All references to allocator symbols in the Chrome codebase to be rewritten as
references to
__wrap_malloc
and friends. The__wrap_malloc
symbols are defined in theallocator_shim_override_linker_wrapped_symbols.h
and route allocator calls inside the shim layer. - The reference to the original
malloc
symbols (which typically is defined by the system's libc.so) are accessible via the special__real_malloc
and friends symbols (which will be relocated, at load time, againstmalloc
).
In summary, this approach is transparent to the dynamic loader, which still sees undefined symbol references to malloc symbols. These symbols will be resolved against libc.so as usual. More details in crrev.com/1719433002.
2. Shim layer implementation This stage contains the actual shim implementation. This consists of:
- A singly linked list of dispatchers (structs with function pointers to
malloc
-like functions). Dispatchers can be dynamically inserted at runtime (using theInsertAllocatorDispatch
API). They can intercept and override allocator calls. - The security checks (suicide on malloc-failure via
std::new_handler
, etc). This happens insideallocator_shim.cc
3. Final allocator routing
The final element of the aforementioned dispatcher chain is statically defined
at build time and ultimately routes the allocator calls to the actual allocator
(as described in the Background section above). This is taken care of by the
headers in allocator_shim_default_dispatch_to_*
files.