The Fuchsia GN build machinery allows for separate components to be built in different “variants”. A variant usually just means using extra compiler options, but they can do more than that if you write some more GN code. The variants defined so far enable things like sanitizers and LTO.
The GN build argument select_variant controls which components are built in which variants. It applies automatically to every executable, loadable_module, or driver_module target in GN files. It's a flexible mechanism in which you give a list of matching rules to apply to each target to decide which variant to use (if any). To support this flexibility, the value for select_variant uses a detailed GN syntax. For simple cases, this can just be a list of strings.
Using fx set:
fx set core.x64 --variant={host_asan,asan/cat,asan/ledger}
Alternatively, you can add or modify the variants on an existing build by editing the GN args (substituting your build's GN output directory for out/default as necessary):
gn args out/default
That command will bring up an editor. Append to that file:
select_variant = [ "host_asan", "asan/cat", "asan/ledger" ]
The first switch applies the host_asan matching rule, which enables AddressSanitizer for all the executables built to run on the build host.
The second switch applies the asan matching rule, which enables AddressSanitizer for executables built to run on the target (i.e. the Fuchsia device). The /cat suffix constrains this matching rule only to the binary named cat.
The third switch is like the second, but matches the binary named ledger.
The GN code supports much more flexible matching rules than just the binary name, but there are no shorthands for those. See the select_variant build argument documentation for more details.
To see the list of variants available and learn more about how to define new ones, see the known_variants build argument.
If you are trying to use the ASan variant, you may encounter an error that looks like this:
launcher: error: Launch: elf_load: handle_interp failed dlsvc: could not open 'asan/ld.so.1'
Fuchsia is structured around packages and components. Each component contains all of the shared libraries it needs to run. This helps Fuchsia avoid library versioning issues that plague other operating systems. It also means that, if you want to run a binary from within a component, you must provide the appropriate shared library loader for that binary.
There are a set of command line programs located in the /boot/ directory of Fuchsia installs that are not contained in packages, but in the boot filesystem. These programs do not have their own shared library loader, and will use whatever shared libraries the component executing them provides. This normally works, as programs like sh and ls have very minimal, very common dependencies. However, there‘s no guarantee that the component’s package will have sufficient or compatible shared libraries for the command line program's needs. ASan-enabled packages usually do not contain the right launcher for these programs, so most ASan-enabled components cannot run executables out of /boot. If an ASan-enabled component tries to do so, it gets the error above.
Fortunately, it turns out that the fix involves doing what all packages should do anyway, which is to declare their dependencies explicitly. If your package depends on a binary, it should declare it as a dependency, and then use that declared dependency instead of the one in the /boot directory. In the case of our build system, the zircon_extras_manifest rule defined in //build/config/fuchsia/zircon_images.gni will allow you to depend on any of the binaries found in the /boot directory. They will be installed in /pkg/bin/, and you should execute them from there.
