Set F_SEAL_SHRINK on memfds we create

This prevents us from ever shrinking the file (which we never do) and lets the
wayland server skip configuring its SIGBUS handler when accessing our buffers.

Author: danieldg

src/shm/mempool.rs

@@ -250,9 +250,18 @@ fn create_shm_fd() -> io::Result<RawFd> {
     loop {
         match memfd::memfd_create(
             CStr::from_bytes_with_nul(b"smithay-client-toolkit\0").unwrap(),
-            memfd::MemFdCreateFlag::MFD_CLOEXEC,
+            memfd::MemFdCreateFlag::MFD_CLOEXEC | memfd::MemFdCreateFlag::MFD_ALLOW_SEALING,
         ) {
-            Ok(fd) => return Ok(fd),
+            Ok(fd) => {
+                // this is only an optimization, so ignore errors
+                let _ = fcntl::fcntl(
+                    fd,
+                    fcntl::F_ADD_SEALS(
+                        fcntl::SealFlag::F_SEAL_SHRINK | fcntl::SealFlag::F_SEAL_SEAL,
+                    ),
+                );
+                return Ok(fd);
+            }
             Err(nix::Error::Sys(Errno::EINTR)) => continue,
             Err(nix::Error::Sys(Errno::ENOSYS)) => break,
             Err(nix::Error::Sys(errno)) => return Err(io::Error::from(errno)),