Open source system call tests.

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+# gVisor system call test suite
+
+This is a test suite for Linux system calls. It runs under both gVisor and
+Linux, and ensures compatability between the two.
+
+When adding support for a new syscall (or syscall argument) to gVisor, a
+corresponding syscall test should be added. It's usually recommended to write
+the test first and make sure that it passes on Linux before making changes to
+gVisor.
+
+This document outlines the general guidelines for tests and specific rules that
+must be followed for new tests.
+
+## Running the tests
+
+Each test file generates three different test targets that run in different
+environments:
+
+* a `native` target that runs directly on the host machine,
+* a `runsc_ptrace` target that runs inside runsc using the ptrace platform, and
+* a `runsc_kvm` target that runs inside runsc using the KVM platform.
+
+For example, the test in `access_test.cc` generates the following targets:
+
+* `//test/syscalls:access_test_native`
+* `//test/syscalls:access_test_runsc_ptrace`
+* `//test/syscalls:access_test_runsc_kvm`
+
+Any of these targets can be run directly via `bazel test`.
+
+```bash
+$ bazel test //test/syscalls:access_test_native
+$ bazel test //test/syscalls:access_test_runsc_ptrace
+$ bazel test //test/syscalls:access_test_runsc_kvm
+```
+
+To run all the tests on a particular platform, you can filter by the platform
+tag:
+
+```bash
+# Run all tests in native environment:
+$ bazel test --test_tag_filter=native //test/syscalls:*
+
+# Run all tests in runsc with ptrace:
+$ bazel test --test_tag_filter=runsc_ptrace //test/syscalls:*
+
+# Run all tests in runsc with kvm:
+$ bazel test --test_tag_filter=runsc_kvm //test/syscalls:*
+```
+
+You can also run all the tests on every platform. (Warning, this may take a
+while to run.)
+
+```bash
+# Run all tests on every platform:
+$ bazel test //test/syscalls:*
+```
+
+## Writing new tests
+
+Whenever we add support for a new syscall, or add support for a new argument or
+option for a syscall, we should always add a new test (perhaps many new tests).
+
+In general, it is best to write the test first and make sure it passes on Linux
+by running the test on the `native` platform on a Linux machine. This ensures
+that the gVisor implementation matches actual Linux behavior. Sometimes man
+pages contain errors, so always check the actual Linux behavior.
+
+gVisor uses the [Google Test][googletest] test framework, with a few custom
+matchers and guidelines, described below.
+
+### Syscall matchers
+
+When testing an individual system call, use the following syscall matchers,
+which will match the value returned by the syscall and the errno.
+
+```cc
+SyscallSucceeds()
+SyscallSucceedsWithValue(...)
+SyscallFails()
+SyscallFailsWithErrno(...)
+```
+
+### Use test utilities (RAII classes)
+
+The test utilties are written as RAII classes. These utilities should be
+preferred over custom test harnesses.
+
+Local class instances should be preferred, whereever possible, over full test
+fixtures.
+
+A test utility should be created when there is more than one test that requires
+that same functionality, otherwise the class should be test local.
+
+
+## Save/Restore support in tests
+gVisor supports save/restore, and our syscall tests are written in a way to
+enable saving/restoring at certain points. Hence, there are calls to
+`MaybeSave`, and certain tests that should not trigger saves are named with
+`NoSave`.
+
+However, the current open-source test runner does not yet support triggering
+save/restore, so these functions and annotations have no effect on the
+open-source tests.
+
+We plan on extending our open-source test runner to trigger save/restore. Until
+then, these functions and annotations should be ignored.
+
+
+[googletest]: https://github.com/abseil/googletest