Add syscall tests and so harness

Add two tests performing syscalls, to show the running syscalls
themselves from within a restricted DDC environment works. I think this
is useful to have for future potential work (e.g., could we provide a
compartment-safe printing function, instead of crashing with `printf`,
if making `printf` be compatible proves hard).

Adds a harness to be able to run `so` files natively, without having to
go through the compartment library. Useful for debugging / differential
testing.

tests/CMakeLists.txt

@@ -111,6 +111,7 @@ endfunction()
 
 # Library tests
 set(func_binaries
+    "so_harness"
     "test_map"
     #"test_args_near_unmapped"
     #"test_two_comps"
@@ -123,6 +124,8 @@ set(comp_binaries
     "simple_call_internal"
     "simple_call_external"
     "simple_external"
+    "simple_syscall_getpid"
+    "simple_syscall_write"
     #"time"
     #"lua_simple"
     #"lua_script"
@@ -136,6 +139,8 @@ set(tests
     "simple_libc"
     "simple_call_internal"
     "simple_call_external"
+    "simple_syscall_getpid"
+    "simple_syscall_write"
     #"time"
     #"lua_simple"
     #"lua_script"

tests/simple_syscall_getpid.c

@@ -0,0 +1,11 @@
+#include <assert.h>
+#include <unistd.h>
+
+int
+main(void)
+{
+    long int sc_pid = syscall(20);
+    pid_t pid = getpid();
+    assert(pid == sc_pid);
+    return 0;
+}

tests/simple_syscall_write.c

@@ -0,0 +1,16 @@
+#include <unistd.h>
+#include <string.h>
+#include <err.h>
+
+int
+main(void)
+{
+    char* buf = "Hello World!\n";
+    long int sc_write = syscall(4, STDOUT_FILENO, buf, strlen(buf));
+    if (sc_write == -1)
+    {
+        err(1, "Error calling `syscall`:");
+    }
+    return 0;
+}
+

tests/so_harness.c

@@ -0,0 +1,32 @@
+#include <dlfcn.h>
+#include <err.h>
+
+#include <assert.h>
+#include <stdio.h>
+
+int
+main(int argc, char **argv)
+{
+    if (argc != 2)
+    {
+        errx(1, "Expected one argument - path to `so` file to wrap.\n");
+    }
+    void *handle = dlopen(argv[1], RTLD_LAZY);
+    if (!handle)
+    {
+        errx(1, "`dlopen` error: %s\n", dlerror());
+    }
+
+    dlerror();
+    void (*handle_main)() = (void (*)(void)) dlsym(handle, "main");
+    char *sym_err = dlerror();
+    if (sym_err)
+    {
+        errx(1, "`dlsym` error: %s\n", sym_err);
+    }
+
+    handle_main();
+
+    dlclose(handle);
+    return 0;
+}