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
controls which components are built in which variants. It applies
automatically to every
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.
fx set core.x64 --variant=host_asan --variant=asan/cat --variant=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
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_asanmatching rule, which enables AddressSanitizer for all the executables built to run on the build host.
The second switch applies the
asanmatching rule, which enables AddressSanitizer for executables built to run on the target (i.e. the Fuchsia device). The
/catsuffix constrains this matching rule only to the binary named
The third switch is like the second, but matches the binary named
The GN code supports much more flexible matching rules than just the binary
name, but there are no shorthands for those. See the
build argument documentation for more details.
To see the list of variants available and learn more about how to define
new ones, see the
Replicating ASan failures
Our commit queue runs tests in an ASan-enabled configuration. To replicate the
build in this configuration, use the following
import("//boards/<x64-or-arm64>.gni") import("//products/core.gni") base_package_labels+=[ "//bundles/buildbot:core" ] goma_dir="<path-to-goma-dir>" is_debug=true select_variant=["asan","host_asan"] target_cpu="<x64-or-arm64>" use_goma=true
x64-or-arm64 with your desired target architecture, and replace
<path-to-goma-dir> with the path to your goma dir (for those who use goma). This
can also be generated from the command line with:
fx set core.x64 --with-base //bundles/buildbot:core --variant host_asan --variant asan --goma
Note that this will build all of the tests that are run by the commit queue and install them in the system image. This may be undesirable for two reasons:
- Building all of the tests is typically slow and unnecessary. Developers may find it more effective to limit the package labels to the tests they need.
- Installing all of the tests in the system image ahead of time means that the software deployment workflow does not get exercised.
Launching executables from within ASan-enabled binaries
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
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.
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