mem, yes, definitely. I'm not sure how you can protect yourself from that (or root user using ptrace or equivalent debugging tool) though...

Oh, memfd_secret?

       The memory areas backing the file created with memfd_secret(2) are visible only to the processes that  have  ac‐
       cess  to the file descriptor.  The memory region is removed from the kernel page tables and only the page tables
       of the processes holding the file descriptor map the corresponding physical memory.  (Thus, the pages in the re‐
       gion can't be accessed by the kernel itself, so that, for example, pointers to the region  can't  be  passed  to
       system calls.)