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 <p>AddressSanitizer (ASan) is a fast compiler-based tool for detecting memory bugs
 in native code. It is comparable to Valgrind (Memcheck tool), but, unlike it,
 ASan:</p>

 <ul>
   <li> + detects overflows on stack and global objects
   <li> - does not detect uninitialized reads and memory leaks
   <li> + is much faster (two-three times slowdown compared to Valgrind’s 20-100x)
   <li> + has less memory overhead
 </ul>

 <p>This document describes how to build and run parts of the Android platform with
 AddressSanitizer. If you are looking to build a standalone (i.e. SDK/NDK)
 application with AddressSanitizer, see the <a
 href="https://github.com/google/sanitizers/wiki/AddressSanitizerOnAndroid">AddressSanitizerOnAndroid</a>
 public project site instead.</p>

 <p>AddressSanitizer consists of a compiler (<code>external/clang</code>) and a runtime library
 (<code>external/compiler-rt/lib/asan</code>).</p>

 <p class="note"><strong>Note</strong>: Use the current master
 branch to gain access to the <a href="#sanitize_target">SANITIZE_TARGET</a>
 feature and the ability to build the entire Android platform with
 AddressSanitizer at once. Otherwise, you are limited to using
 <code>LOCAL_SANITIZE</code>.</p>

 <h2 id=building_with_clang>Building with Clang</h2>

 <p>As a first step to building an ASan-instrumented binary, make sure that your
 code builds with Clang. This is done by default on the master branch, so there should be nothing
 you need to do. If you believe that the module you'd like to test is being built with GCC,
 you can switch to Clang by adding <code>LOCAL_CLANG:=true</code>
 to the build rules. Clang may find bugs in your code that GCC missed.</p>

 <h2 id=building_executables_with_addresssanitizer>Building executables with AddressSanitizer</h2>

 <p>Add <code>LOCAL_SANITIZE:=address</code> to the build rule of the
 executable.</p>

 <pre>
 LOCAL_SANITIZE:=address
 </pre>

 <p>When a bug is detected, ASan prints a verbose report both to the standard
 output and to <code>logcat</code> and then crashes the process.</p>

 <h2 id=building_shared_libraries_with_addresssanitizer>Building shared libraries with AddressSanitizer</h2>

 <p>Due to the way ASan works, a library built with ASan cannot be used by an
 executable that's built without ASan.</p>

 <p class="note">Note</strong>: In runtime situations where an ASan library is
 loaded into an incorrect process, you will see unresolved symbol messages
 starting with <code>_asan</code> or <code>_sanitizer</code>.</p>

 <p>To sanitize a shared library that is used in multiple executables, not all of
 which are built with ASan, you'll need two copies of the library. The
 recommended way to do this is to add the following to <code>Android.mk</code>
 for the module in question:</p>

 <pre>
 LOCAL_SANITIZE:=address
 LOCAL_MODULE_RELATIVE_PATH := asan
 </pre>

 <p>This puts the library in <code>/system/lib/asan</code> instead of
 <code>/system/lib</code>. Then, run your executable with:
 <code>LD_LIBRARY_PATH=/system/lib/asan</code></p>

 <p>For system daemons, add the following to the appropriate section of
 <code>/init.rc</code> or <code>/init.$device$.rc</code>.</p>

 <pre>
 setenv LD_LIBRARY_PATH /system/lib/asan
 </pre>

 <p class="warning"><strong>Warning</strong>: The <code>LOCAL_MODULE_RELATIVE_PATH</code>
 setting <strong>moves</strong> your library to <code>/system/lib/asan</code>,
 meaning that clobbering and rebuilding from scratch will result in the
 library missing from <code>/system/lib</code>, and probably an unbootable
 image. That's an unfortunate limitation of the
 current build system. Don't clobber; do <code>make -j $N</code> and <code>adb
 sync</code>.</p>

 <p>Verify the process is using libraries from <code>/system/lib/asan</code>
 when present by reading <code>/proc/$PID/maps</code>. If it's not, you may need
 to disable SELinux, like so:</p>

 <pre>
 $ adb root
 $ adb shell setenforce 0
 # restart the process with adb shell kill $PID
 # if it is a system service, or may be adb shell stop; adb shell start.
 </pre>

 <h2 id=better_stack_traces>Better stack traces</h2>

 <p>AddressSanitizer uses a fast, frame-pointer-based unwinder to record a stack
 trace for every memory allocation and deallocation event in the program. Most
 of Android is built without frame pointers. As a result, you will often get
 only one or two meaningful frames. To fix this, either rebuild the library with
 ASan (recommended!), or with:</p>

 <pre>
 LOCAL_CFLAGS:=-fno-omit-frame-pointer
 LOCAL_ARM_MODE:=arm
 </pre>

 <p>Or set <code>ASAN_OPTIONS=fast_unwind_on_malloc=0</code> in the process
 environment. The latter can be very CPU-intensive, depending on
 the load.</p>

 <h2 id=symbolization>Symbolization</h2>

 <p>Initially, ASan reports contain references to offsets in binaries and shared
 libraries. There are two ways to obtain source file and line information:</p>

 <ul>
   <li>Ensure llvm-symbolizer binary is present in <code>/system/bin</code>.
 Llvm-symbolizer is built from sources in:
 <code>third_party/llvm/tools/llvm-symbolizer</code> <li>Filter the report
 through the <code>external/compiler-rt/lib/asan/scripts/symbolize.py</code>
 script.
 </ul>

 <p>The second approach can provide more data (i.e. file:line locations) because of
 the availability of symbolized libraries on the host.</p>

 <h2 id=addresssanitizer_in_the_apps>AddressSanitizer in the apps</h2>

 <p>AddressSanitizer cannot see into Java code, but it can detect bugs in the JNI
 libraries. For that, you'll need to build the executable with ASan, which in
 this case is <code>/system/bin/app_process(<em>32|64</code></em>). This will
 enable ASan in all apps on the device at the same time, which is a
 bit stressful, but nothing that a 2GB RAM device cannot handle.</p>

 <p>Add the usual <code>LOCAL_SANITIZE:=address</code> to
 the app_process build rule in <code>frameworks/base/cmds/app_process</code>. Ignore
 the <code>app_process__asan</code> target in the same file for now (if it is
 still there at the time you read
 this). Edit the Zygote record in
 <code>system/core/rootdir/init.zygote(<em>32|64</em>).rc</code> to add the
 following lines:</p>

 <pre>
 setenv LD_LIBRARY_PATH /system/lib/asan:/system/lib
 setenv ASAN_OPTIONS
 allow_user_segv_handler=true
 </pre>

 <p>Build, adb sync, fastboot flash boot, reboot.</p>

 <h2 id=using_the_wrap_property>Using the wrap property</h2>

 <p>The approach in the previous section puts AddressSanitizer into every
 application in the system (actually, into every descendant of the Zygote
 process). It is possible to run only one (or several) applications with ASan,
 trading some memory overhead for slower application startup.</p>

 <p>This can be done by starting your app with the “wrap.” property, the same one
 that’s used to run apps under Valgrind. The following example runs the Gmail app
 under ASan:</p>

 <pre>
 $ adb root
 $ adb shell setenforce 0  # disable SELinux
 $ adb shell setprop wrap.com.google.android.gm "asanwrapper"
 </pre>

 <p>In this context, asanwrapper rewrites <code>/system/bin/app_process</code>
 to <code>/system/bin/asan/app_process</code>, which is built with
 AddressSanitizer. It also adds <code>/system/lib/asan</code> at the start of
 the dynamic library search path. This way ASan-instrumented
 libraries from <code>/system/lib/asan</code> are preferred to normal libraries
 in <code>/system/lib</code> when running with asanwrapper.</p>

 <p>Again, if a bug is found, the app will crash, and the report will be printed to
 the log.</p>

 <h2 id=sanitize_target>SANITIZE_TARGET</h2>

 <p>The master branch has support for building the entire Android platform with
 AddressSanitizer at once.</p>

 <p>Run the following commands in the same build tree.</p>

 <pre>
 $ make -j42
 $ SANITIZE_TARGET=address make -j42
 </pre>

 <p>In this mode, <code>userdata.img</code> contains extra libraries and must be
 flashed to the device as well. Use the following command line:</p>

 <pre>
 $ fastboot flash userdata && fastboot flashall
 </pre>

 <p>At the moment of this writing, modern Nexus and Pixel devices boot to the UI in this mode.</p>

 <p>This works by building two sets of shared libraries: normal in
 <code>/system/lib</code> (the first make invocation), ASan-instrumented in
 <code>/data/asan/lib</code> (the second make invocation). Executables from the
 second build overwrite the ones from the first build. ASan-instrumented
 executables get a different library search path that includes
 <code>/data/asan/lib</code> before <code>/system/lib</code> through the use of
 "/system/bin/linker_asan" in PT_INTERP.</p>

 <p>The build system clobbers intermediate object directories when the
 <code>$SANITIZE_TARGET</code> value has changed. This forces a rebuild of all
 targets while preserving installed binaries under <code>/system/lib</code>.</p>

 <p>Some targets cannot be built with ASan:</p>

 <ul>
   <li>Statically linked executables.
   <li><code>LOCAL_CLANG:=false</code> targets
   <li><code>LOCAL_SANITIZE:=false</code> will not be ASan'd for <code>SANITIZE_TARGET=address</code>
 </ul>

 <p>Executables like these are skipped in the SANITIZE_TARGET build, and the
 version from the first make invocation is left in <code>/system/bin</code>.</p>

 <p>Libraries like this are simply built without ASan. They can contain some ASan
 code anyway from the static libraries they depend upon.</p>

 <h2 id=supporting_documentation>Supporting documentation</h2>

 <p><a href="https://github.com/google/sanitizers/wiki/AddressSanitizerOnAndroid">AddressSanitizerOnAndroid</a> public project site</p>
 <p><a href="https://www.chromium.org/developers/testing/addresssanitizer">AddressSanitizer and Chromium</a></p>
 <p><a href="https://github.com/google/sanitizers">Other Google Sanitizers</a></p>

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	Copyright 2017 The Android Open Source Project

	Licensed under the Apache License, Version 2.0 (the "License");
	you may not use this file except in compliance with the License.
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	Unless required by applicable law or agreed to in writing, software
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	<p>AddressSanitizer (ASan) is a fast compiler-based tool for detecting memory bugs
	in native code. It is comparable to Valgrind (Memcheck tool), but, unlike it,
	ASan:</p>

	<ul>
	<li> + detects overflows on stack and global objects
	<li> - does not detect uninitialized reads and memory leaks
	<li> + is much faster (two-three times slowdown compared to Valgrind’s 20-100x)
	<li> + has less memory overhead
	</ul>

	<p>This document describes how to build and run parts of the Android platform with
	AddressSanitizer. If you are looking to build a standalone (i.e. SDK/NDK)
	application with AddressSanitizer, see the <a
	href="https://github.com/google/sanitizers/wiki/AddressSanitizerOnAndroid">AddressSanitizerOnAndroid</a>
	public project site instead.</p>

	<p>AddressSanitizer consists of a compiler (<code>external/clang</code>) and a runtime library
	(<code>external/compiler-rt/lib/asan</code>).</p>

	<p class="note"><strong>Note</strong>: Use the current master
	branch to gain access to the <a href="#sanitize_target">SANITIZE_TARGET</a>
	feature and the ability to build the entire Android platform with
	AddressSanitizer at once. Otherwise, you are limited to using
	<code>LOCAL_SANITIZE</code>.</p>

	<h2 id=building_with_clang>Building with Clang</h2>

	<p>As a first step to building an ASan-instrumented binary, make sure that your
	code builds with Clang. This is done by default on the master branch, so there should be nothing
	you need to do. If you believe that the module you'd like to test is being built with GCC,
	you can switch to Clang by adding <code>LOCAL_CLANG:=true</code>
	to the build rules. Clang may find bugs in your code that GCC missed.</p>

	<h2 id=building_executables_with_addresssanitizer>Building executables with AddressSanitizer</h2>

	<p>Add <code>LOCAL_SANITIZE:=address</code> to the build rule of the
	executable.</p>

	<pre>
	LOCAL_SANITIZE:=address
	</pre>

	<p>When a bug is detected, ASan prints a verbose report both to the standard
	output and to <code>logcat</code> and then crashes the process.</p>

	<h2 id=building_shared_libraries_with_addresssanitizer>Building shared libraries with AddressSanitizer</h2>

	<p>Due to the way ASan works, a library built with ASan cannot be used by an
	executable that's built without ASan.</p>

	<p class="note">Note</strong>: In runtime situations where an ASan library is
	loaded into an incorrect process, you will see unresolved symbol messages
	starting with <code>_asan</code> or <code>_sanitizer</code>.</p>

	<p>To sanitize a shared library that is used in multiple executables, not all of
	which are built with ASan, you'll need two copies of the library. The
	recommended way to do this is to add the following to <code>Android.mk</code>
	for the module in question:</p>

	<pre>
	LOCAL_SANITIZE:=address
	LOCAL_MODULE_RELATIVE_PATH := asan
	</pre>

	<p>This puts the library in <code>/system/lib/asan</code> instead of
	<code>/system/lib</code>. Then, run your executable with:
	<code>LD_LIBRARY_PATH=/system/lib/asan</code></p>

	<p>For system daemons, add the following to the appropriate section of
	<code>/init.rc</code> or <code>/init.$device$.rc</code>.</p>

	<pre>
	setenv LD_LIBRARY_PATH /system/lib/asan
	</pre>

	<p class="warning"><strong>Warning</strong>: The <code>LOCAL_MODULE_RELATIVE_PATH</code>
	setting <strong>moves</strong> your library to <code>/system/lib/asan</code>,
	meaning that clobbering and rebuilding from scratch will result in the
	library missing from <code>/system/lib</code>, and probably an unbootable
	image. That's an unfortunate limitation of the
	current build system. Don't clobber; do <code>make -j $N</code> and <code>adb
	sync</code>.</p>

	<p>Verify the process is using libraries from <code>/system/lib/asan</code>
	when present by reading <code>/proc/$PID/maps</code>. If it's not, you may need
	to disable SELinux, like so:</p>

	<pre>
	$ adb root
	$ adb shell setenforce 0
	# restart the process with adb shell kill $PID
	# if it is a system service, or may be adb shell stop; adb shell start.
	</pre>

	<h2 id=better_stack_traces>Better stack traces</h2>

	<p>AddressSanitizer uses a fast, frame-pointer-based unwinder to record a stack
	trace for every memory allocation and deallocation event in the program. Most
	of Android is built without frame pointers. As a result, you will often get
	only one or two meaningful frames. To fix this, either rebuild the library with
	ASan (recommended!), or with:</p>

	<pre>
	LOCAL_CFLAGS:=-fno-omit-frame-pointer
	LOCAL_ARM_MODE:=arm
	</pre>

	<p>Or set <code>ASAN_OPTIONS=fast_unwind_on_malloc=0</code> in the process
	environment. The latter can be very CPU-intensive, depending on
	the load.</p>

	<h2 id=symbolization>Symbolization</h2>

	<p>Initially, ASan reports contain references to offsets in binaries and shared
	libraries. There are two ways to obtain source file and line information:</p>

	<ul>
	<li>Ensure llvm-symbolizer binary is present in <code>/system/bin</code>.
	Llvm-symbolizer is built from sources in:
	<code>third_party/llvm/tools/llvm-symbolizer</code> <li>Filter the report
	through the <code>external/compiler-rt/lib/asan/scripts/symbolize.py</code>
	script.
	</ul>

	<p>The second approach can provide more data (i.e. file:line locations) because of
	the availability of symbolized libraries on the host.</p>

	<h2 id=addresssanitizer_in_the_apps>AddressSanitizer in the apps</h2>

	<p>AddressSanitizer cannot see into Java code, but it can detect bugs in the JNI
	libraries. For that, you'll need to build the executable with ASan, which in
	this case is <code>/system/bin/app_process(<em>32\|64</code></em>). This will
	enable ASan in all apps on the device at the same time, which is a
	bit stressful, but nothing that a 2GB RAM device cannot handle.</p>

	<p>Add the usual <code>LOCAL_SANITIZE:=address</code> to
	the app_process build rule in <code>frameworks/base/cmds/app_process</code>. Ignore
	the <code>app_process__asan</code> target in the same file for now (if it is
	still there at the time you read
	this). Edit the Zygote record in
	<code>system/core/rootdir/init.zygote(<em>32\|64</em>).rc</code> to add the
	following lines:</p>

	<pre>
	setenv LD_LIBRARY_PATH /system/lib/asan:/system/lib
	setenv ASAN_OPTIONS
	allow_user_segv_handler=true
	</pre>

	<p>Build, adb sync, fastboot flash boot, reboot.</p>

	<h2 id=using_the_wrap_property>Using the wrap property</h2>

	<p>The approach in the previous section puts AddressSanitizer into every
	application in the system (actually, into every descendant of the Zygote
	process). It is possible to run only one (or several) applications with ASan,
	trading some memory overhead for slower application startup.</p>

	<p>This can be done by starting your app with the “wrap.” property, the same one
	that’s used to run apps under Valgrind. The following example runs the Gmail app
	under ASan:</p>

	<pre>
	$ adb root
	$ adb shell setenforce 0 # disable SELinux
	$ adb shell setprop wrap.com.google.android.gm "asanwrapper"
	</pre>

	<p>In this context, asanwrapper rewrites <code>/system/bin/app_process</code>
	to <code>/system/bin/asan/app_process</code>, which is built with
	AddressSanitizer. It also adds <code>/system/lib/asan</code> at the start of
	the dynamic library search path. This way ASan-instrumented
	libraries from <code>/system/lib/asan</code> are preferred to normal libraries
	in <code>/system/lib</code> when running with asanwrapper.</p>

	<p>Again, if a bug is found, the app will crash, and the report will be printed to
	the log.</p>

	<h2 id=sanitize_target>SANITIZE_TARGET</h2>

	<p>The master branch has support for building the entire Android platform with
	AddressSanitizer at once.</p>

	<p>Run the following commands in the same build tree.</p>

	<pre>
	$ make -j42
	$ SANITIZE_TARGET=address make -j42
	</pre>

	<p>In this mode, <code>userdata.img</code> contains extra libraries and must be
	flashed to the device as well. Use the following command line:</p>

	<pre>
	$ fastboot flash userdata && fastboot flashall
	</pre>

	<p>At the moment of this writing, modern Nexus and Pixel devices boot to the UI in this mode.</p>

	<p>This works by building two sets of shared libraries: normal in
	<code>/system/lib</code> (the first make invocation), ASan-instrumented in
	<code>/data/asan/lib</code> (the second make invocation). Executables from the
	second build overwrite the ones from the first build. ASan-instrumented
	executables get a different library search path that includes
	<code>/data/asan/lib</code> before <code>/system/lib</code> through the use of
	"/system/bin/linker_asan" in PT_INTERP.</p>

	<p>The build system clobbers intermediate object directories when the
	<code>$SANITIZE_TARGET</code> value has changed. This forces a rebuild of all
	targets while preserving installed binaries under <code>/system/lib</code>.</p>

	<p>Some targets cannot be built with ASan:</p>

	<ul>
	<li>Statically linked executables.
	<li><code>LOCAL_CLANG:=false</code> targets
	<li><code>LOCAL_SANITIZE:=false</code> will not be ASan'd for <code>SANITIZE_TARGET=address</code>
	</ul>

	<p>Executables like these are skipped in the SANITIZE_TARGET build, and the
	version from the first make invocation is left in <code>/system/bin</code>.</p>

	<p>Libraries like this are simply built without ASan. They can contain some ASan
	code anyway from the static libraries they depend upon.</p>

	<h2 id=supporting_documentation>Supporting documentation</h2>

	<p><a href="https://github.com/google/sanitizers/wiki/AddressSanitizerOnAndroid">AddressSanitizerOnAndroid</a> public project site</p>
	<p><a href="https://www.chromium.org/developers/testing/addresssanitizer">AddressSanitizer and Chromium</a></p>
	<p><a href="https://github.com/google/sanitizers">Other Google Sanitizers</a></p>

	</body>
	</html>