_sandboxbwrap.py: Create /dev/shm in the sandbox

Creates /dev/shm as a tmpfs in the sandbox. Before now access to
/dev/shm was only available by a plugin using `Sandbox.mark_directory()`
or adding to `Sandbox.DEVICES`, either of which would _mount_ /dev/shm
into the sandbox, allowing pollution from the host. This adds it as a
tmpfs by default, which seems sensible as it is required for POSIX
support.

Also adds a test which makes sure that we can open a shared memory
object inside the build sandbox with some (probably poor) C code.

2 files changed, 44 insertions, 0 deletions

diff --git a/tests/integration/project/elements/sandbox-bwrap/test-dev-shm.bst b/tests/integration/project/elements/sandbox-bwrap/test-dev-shm.bst
new file mode 100644
index 000000000..03dc74a35
--- /dev/null
+++ b/tests/integration/project/elements/sandbox-bwrap/test-dev-shm.bst
@@ -0,0 +1,15 @@
+kind: manual
+
+depends:
+- base.bst 
+
+config:
+  build-commands:
+  - cc test_shm.c
+
+  install-commands:
+  - ./a.out
+
+sources:
+- kind: local
+  path: files/test_shm.c
diff --git a/tests/integration/project/files/test_shm.c b/tests/integration/project/files/test_shm.c
new file mode 100644
index 000000000..4ee71cb2b
--- /dev/null
+++ b/tests/integration/project/files/test_shm.c
@@ -0,0 +1,29 @@
+#include <errno.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+
+int main ()
+{
+  int fd = shm_open ("/foo", O_RDONLY | O_CREAT, S_IRWXU);
+  if (fd < 0)
+  {
+    fprintf (stderr, "Failed to open shm: %s\n", strerror (errno));
+    exit(1);
+  }
+
+  int success = shm_unlink ("/foo");
+  if  (success < 0)
+  {
+    fprintf (stderr, "Failed to close shm: %s\n", strerror (errno));
+    exit(2);
+  }
+
+  close (fd);
+
+  return 0;
+}