Mount a tmpfs at /dev/shm in the sandbox.

...To get shm_open() and friends working.

Fixes #3196.

Makefile

@@ -43,7 +43,9 @@ NODE_HEADERS=$(METEOR_DEV_BUNDLE)/include/node
 WARNINGS=-Wall -Wextra -Wglobal-constructors -Wno-sign-compare -Wno-unused-parameter
 CXXFLAGS2=-std=c++1z $(WARNINGS) $(CXXFLAGS) -DSANDSTORM_BUILD=$(BUILD) -DKJ_HAS_OPENSSL -DKJ_HAS_ZLIB -DKJ_HAS_LIBDL -pthread -fPIC -I$(NODE_HEADERS) -DKJ_STD_COMPAT
 CFLAGS2=$(CFLAGS) -pthread -fPIC -DKJ_STD_COMPAT
-LIBS2=$(LIBS) deps/libsodium/build/src/libsodium/.libs/libsodium.a deps/boringssl/build/ssl/libssl.a deps/boringssl/build/crypto/libcrypto.a -lz -ldl -pthread
+# -lrt is not used by sandstorm itself, but the test app uses it. It would be
+#  nice if we could not link everything against it.
+LIBS2=$(LIBS) deps/libsodium/build/src/libsodium/.libs/libsodium.a deps/boringssl/build/ssl/libssl.a deps/boringssl/build/crypto/libcrypto.a -lz -ldl -pthread -lrt
 
 define color
   printf '\033[0;34m==== $1 ====\033[0m\n'

src/sandstorm/supervisor.c++

@@ -954,6 +954,12 @@ void SupervisorMain::makeCharDeviceNode(
   KJ_SYSCALL(mount(kj::str("/dev/", realName).cStr(), dst.cStr(), nullptr, MS_BIND, nullptr));
 }
 
+void mountTmpFs(const char *name, const char *dest) {
+    KJ_SYSCALL(mount(name, dest, "tmpfs",
+                     MS_NOSUID | MS_NODEV,
+                     "size=16m,nr_inodes=4k,mode=770"));
+}
+
 void SupervisorMain::setupFilesystem() {
   // The root of our mount namespace will be the app package itself.  We optionally create
   // tmp, dev, and var.  tmp is an ordinary tmpfs.  dev is a read-only tmpfs that contains
@@ -986,8 +992,7 @@ void SupervisorMain::setupFilesystem() {
     // 2) When we exit, the mount namespace disappears and the tmpfs is thus automatically
     //    unmounted.  No need for careful cleanup, and no need to implement a risky recursive
     //    delete.
-    KJ_SYSCALL(mount("sandstorm-tmp", "tmp", "tmpfs", MS_NOSUID,
-                     "size=16m,nr_inodes=4k,mode=770"));
+    mountTmpFs("sandstorm-tmp", "tmp");
   }
   if (access("dev", F_OK) == 0) {
     KJ_SYSCALL(mount("sandstorm-dev", "dev", "tmpfs",
@@ -997,6 +1002,18 @@ void SupervisorMain::setupFilesystem() {
     makeCharDeviceNode("zero", "zero", 1, 5);
     makeCharDeviceNode("random", "urandom", 1, 9);
     makeCharDeviceNode("urandom", "urandom", 1, 9);
+
+    // Create /dev/shm so shm_open() and friends work. Note that even though /dev
+    // is already a tmpfs, we need to mount a separate tmpfs for /dev/shm, because
+    // the former will be read-only.
+    //
+    // TODO: it might be nice to have /dev/shm and /tmp share the same partition,
+    // so we don't have to strictly separate their storage capacity. We could mount
+    // a single tmpfs somewhere invisible, create subdirectories, and then bind-mount
+    // them to their final destinations.
+    mkdir("/dev/shm", 0700);
+    mountTmpFs("sandstorm-shm", "/dev/shm");
+
     KJ_SYSCALL(mount("dev", "dev", nullptr,
                      MS_REMOUNT | MS_BIND | MS_NOEXEC | MS_NOSUID | MS_NODEV | MS_RDONLY,
                      nullptr));

src/sandstorm/test-app/test-app.c++

@@ -23,6 +23,10 @@
 #include <map>
 
 #include <sys/time.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
 
 #include <kj/main.h>
 #include <kj/debug.h>
@@ -93,6 +97,40 @@ private:
 
 // =======================================================================================
 
+void testSystemApi() {
+  // Test that some syscalls & platform APIs work as expected. Print a success
+  // message to the console so the test suite can verify this.
+
+  std::cout << "Testing System APIs" << std::endl;
+
+  auto result = kj::runCatchingExceptions([]() {
+    // Test use of /dev/shm:
+    const char *obj_name = "/shome-shm-obj";
+    int shm_fd;
+    KJ_SYSCALL(shm_fd = shm_open(obj_name, O_RDWR|O_CREAT, 0700));
+    KJ_DEFER(KJ_SYSCALL(shm_unlink(obj_name)));
+
+    KJ_ASSERT(close(shm_fd) == 0, "Closing shm_fd failed");
+
+    // Make sure the mapping actually works:
+    int *mapped = (int *)mmap(
+      nullptr, sizeof(int), PROT_READ|PROT_WRITE, MAP_SHARED, shm_fd, 0
+    );
+    KJ_ASSERT(mapped != nullptr, "mmap() failed");
+    KJ_SYSCALL(munmap(mapped, sizeof(int)));
+  });
+
+  KJ_IF_MAYBE(exception, result) {
+    auto msg = kj::str(*exception);
+    std::cout << msg.cStr() << std::endl;
+    throw(*exception);
+  }
+
+  std::cout << "testSystemApi() passed." << std::endl;
+}
+
+// =======================================================================================
+
 class WebSessionImpl final: public sandstorm::WebSession::Server {
 public:
   WebSessionImpl(sandstorm::UserInfo::Reader userInfo,
@@ -148,6 +186,9 @@ public:
         httpResponse.setMimeType("text/plain");
         httpResponse.getBody().setBytes(response.getText().asBytes());
       });
+    } else if(path == "test-system-api") {
+      testSystemApi();
+      return kj::READY_NOW;
     } else if(path == "schedule") {
       context.getResults().initNoContent();
       // Put the extra headers in a map, so we can easily look for specific ones:

src/sandstorm/test-app/test-app.html

@@ -155,5 +155,7 @@ <h1>Test App</h1>
 
     <p><button onclick="postScheduledJob({shouldCancel: false, refStr: 'oneshot'})" id="do-schedule-oneshot">
       Schedule a one-shot job.</button></p>
+
+    <p><button onclick="doPost('/test-system-api', '')" id="do-test-system-api">Test the system api.</button></p>
   </body>
 </html>