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add information on stats
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Daniel Micay
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README.md
178
README.md
@ -17,6 +17,7 @@
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* [Large allocations](#large-allocations)
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* [Memory tagging](#memory-tagging)
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* [API extensions](#api-extensions)
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* [Stats](#stats)
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* [System calls](#system-calls)
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## Introduction
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@ -258,8 +259,8 @@ The following integer configuration options are available:
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the size class regions.
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* `CONFIG_N_ARENA`: `1` (default) to control the number of arenas
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* `CONFIG_STATS`: `false` (default) to control whether stats on allocation /
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deallocation count and active allocations are tracked. This is currently only
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exposed via the mallinfo APIs on Android.
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deallocation count and active allocations are tracked. See the [section on
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stats](#stats) for more details.
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* `CONFIG_EXTENDED_SIZE_CLASSES`: `true` (default) to control whether small
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size class go up to 128kiB instead of the minimum requirement for avoiding
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memory waste of 16kiB. The option to extend it even further will be offered
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@ -743,6 +744,179 @@ this implementation, it retrieves an upper bound on the size for small memory
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allocations based on calculating the size class region. This function is safe
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to use from signal handlers already.
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## Stats
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If stats are enabled, hardened\_malloc keeps tracks allocator statistics in
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order to provide implementations of `mallinfo` and `malloc_info`.
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On Android, `mallinfo` is used for [mallinfo-based garbage collection
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triggering](https://developer.android.com/preview/features#mallinfo) so
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hardened\_malloc enables `CONFIG_STATS` by default. The `malloc_info`
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implementation on Android is the standard one in Bionic, with the information
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is provided to Bionic via Android's internal extended `mallinfo` API with
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support for arenas and size class bins. This means the `malloc_info` output is
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fully compatible, including still having `jemalloc-1` as the version of the
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data format to retain compatibility with existing tooling.
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On non-Android Linux, `mallinfo` has zeroed fields even with `CONFIG_STATS`
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enabled because glibc `mallinfo` is inherently broken. It defines the fields as
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`int` instead of `size_t`, resulting in undefined signed overflows. It also
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misuses the fields and provides a strange, idiosyncratic set of values rather
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than following the SVID/XPG `mallinfo` definition. The `malloc_info` function
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is still provided, with the same format as Android but with the version set to
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`hardened_malloc-1`. The data format may be changed in the future.
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As an example, consider the follow program from the hardened\_malloc tests:
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```c
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#include <pthread.h>
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#include <malloc.h>
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__attribute__((optimize(0)))
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void leak_memory(void) {
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(void)malloc(1024 * 1024 * 1024);
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(void)malloc(16);
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(void)malloc(32);
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(void)malloc(4096);
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}
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void *do_work(void *p) {
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leak_memory();
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return NULL;
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}
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int main(void) {
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pthread_t thread[4];
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for (int i = 0; i < 4; i++) {
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pthread_create(&thread[i], NULL, do_work, NULL);
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}
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for (int i = 0; i < 4; i++) {
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pthread_join(thread[i], NULL);
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}
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malloc_info(0, stdout);
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}
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```
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This produces the following output when piped through `xmllint --format -`:
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```xml
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<?xml version="1.0"?>
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<malloc version="hardened_malloc-1">
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<heap nr="0">
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<bin nr="2" size="32">
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<nmalloc>1</nmalloc>
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<ndalloc>0</ndalloc>
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<slab_allocated>4096</slab_allocated>
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<allocated>32</allocated>
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</bin>
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<bin nr="3" size="48">
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<nmalloc>1</nmalloc>
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<ndalloc>0</ndalloc>
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<slab_allocated>4096</slab_allocated>
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<allocated>48</allocated>
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</bin>
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<bin nr="13" size="320">
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<nmalloc>4</nmalloc>
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<ndalloc>0</ndalloc>
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<slab_allocated>20480</slab_allocated>
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<allocated>1280</allocated>
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</bin>
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<bin nr="29" size="5120">
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<nmalloc>2</nmalloc>
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<ndalloc>0</ndalloc>
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<slab_allocated>40960</slab_allocated>
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<allocated>10240</allocated>
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</bin>
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<bin nr="45" size="81920">
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<nmalloc>1</nmalloc>
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<ndalloc>0</ndalloc>
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<slab_allocated>81920</slab_allocated>
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<allocated>81920</allocated>
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</bin>
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</heap>
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<heap nr="1">
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<bin nr="2" size="32">
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<nmalloc>1</nmalloc>
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<ndalloc>0</ndalloc>
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<slab_allocated>4096</slab_allocated>
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<allocated>32</allocated>
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</bin>
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<bin nr="3" size="48">
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<nmalloc>1</nmalloc>
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<ndalloc>0</ndalloc>
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<slab_allocated>4096</slab_allocated>
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<allocated>48</allocated>
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</bin>
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<bin nr="29" size="5120">
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<nmalloc>1</nmalloc>
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<ndalloc>0</ndalloc>
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<slab_allocated>40960</slab_allocated>
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<allocated>5120</allocated>
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</bin>
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</heap>
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<heap nr="2">
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<bin nr="2" size="32">
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<nmalloc>1</nmalloc>
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<ndalloc>0</ndalloc>
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<slab_allocated>4096</slab_allocated>
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<allocated>32</allocated>
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</bin>
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<bin nr="3" size="48">
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<nmalloc>1</nmalloc>
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<ndalloc>0</ndalloc>
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<slab_allocated>4096</slab_allocated>
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<allocated>48</allocated>
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</bin>
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<bin nr="29" size="5120">
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<nmalloc>1</nmalloc>
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<ndalloc>0</ndalloc>
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<slab_allocated>40960</slab_allocated>
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<allocated>5120</allocated>
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</bin>
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</heap>
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<heap nr="3">
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<bin nr="2" size="32">
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<nmalloc>1</nmalloc>
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<ndalloc>0</ndalloc>
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<slab_allocated>4096</slab_allocated>
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<allocated>32</allocated>
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</bin>
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<bin nr="3" size="48">
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<nmalloc>1</nmalloc>
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<ndalloc>0</ndalloc>
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<slab_allocated>4096</slab_allocated>
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<allocated>48</allocated>
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</bin>
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<bin nr="29" size="5120">
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<nmalloc>1</nmalloc>
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<ndalloc>0</ndalloc>
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<slab_allocated>40960</slab_allocated>
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<allocated>5120</allocated>
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</bin>
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</heap>
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<heap nr="4">
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<allocated_large>4294967296</allocated_large>
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</heap>
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</malloc>
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```
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The heap entries correspond to the arenas. Unlike jemalloc, hardened\_malloc
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doesn't handle large allocations within the arenas, so it presents those in the
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`malloc_info` statistics as a separate arena dedicated to large allocations.
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For example, with 4 arenas enabled, there will be a 5th arena in the statistics
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for the large allocations.
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The `nmalloc` / `ndalloc` fields are 64-bit integers tracking allocation and
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deallocation count. These are defined as wrapping on overflow, per the jemalloc
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implementation.
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See the [section on size classes](#size-classes) to map the size class bin
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number to the corresponding size class. The bin index begins at 0, mapping to
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the 0 byte size class, followed by 1 for the 16 bytes, 2 for 32 bytes, etc. and
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large allocations are treated as one group.
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## System calls
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This is intended to aid with creating system call whitelists via seccomp-bpf
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