backends/hostmem-shm: factor out allocation of "anonymous shared memory with an fd"

Let's factor it out so we can reuse it.

Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/1736967650-129648-2-git-send-email-steven.sistare@oracle.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>

util/oslib-posix.c

@@ -931,3 +931,55 @@ void qemu_close_all_open_fd(const int *skip, unsigned int nskip)
         qemu_close_all_open_fd_fallback(skip, nskip, open_max);
     }
 }
+
+int qemu_shm_alloc(size_t size, Error **errp)
+{
+    g_autoptr(GString) shm_name = g_string_new(NULL);
+    int fd, oflag, cur_sequence;
+    static int sequence;
+    mode_t mode;
+
+    cur_sequence = qatomic_fetch_inc(&sequence);
+
+    /*
+     * Let's use `mode = 0` because we don't want other processes to open our
+     * memory unless we share the file descriptor with them.
+     */
+    mode = 0;
+    oflag = O_RDWR | O_CREAT | O_EXCL;
+
+    /*
+     * Some operating systems allow creating anonymous POSIX shared memory
+     * objects (e.g. FreeBSD provides the SHM_ANON constant), but this is not
+     * defined by POSIX, so let's create a unique name.
+     *
+     * From Linux's shm_open(3) man-page:
+     *   For  portable  use,  a shared  memory  object should be identified
+     *   by a name of the form /somename;"
+     */
+    g_string_printf(shm_name, "/qemu-" FMT_pid "-shm-%d", getpid(),
+                    cur_sequence);
+
+    fd = shm_open(shm_name->str, oflag, mode);
+    if (fd < 0) {
+        error_setg_errno(errp, errno,
+                         "failed to create POSIX shared memory");
+        return -1;
+    }
+
+    /*
+     * We have the file descriptor, so we no longer need to expose the
+     * POSIX shared memory object. However it will remain allocated as long as
+     * there are file descriptors pointing to it.
+     */
+    shm_unlink(shm_name->str);
+
+    if (ftruncate(fd, size) == -1) {
+        error_setg_errno(errp, errno,
+                         "failed to resize POSIX shared memory to %zu", size);
+        close(fd);
+        return -1;
+    }
+
+    return fd;
+}