那么/proc/PID/fd 是啥呢？
man proc
/proc/[pid]/fd/

          This is a subdirectory containing one entry for each file
          which the process has open, named by its file descriptor,
          and which is a symbolic link to the actual file.  Thus, 0
          is standard input, 1 standard output, 2 standard error,
          and so on.

          For file descriptors for pipes and sockets, the entries
          will be symbolic links whose content is the file type with
          the inode.  A readlink(2) call on this file returns a
          string in the format:

              type:[inode]

          For example, socket:[2248868] will be a socket and its
          inode is 2248868.  For sockets, that inode can be used to
          find more information in one of the files under
          /proc/net/.

          For file descriptors that have no corresponding inode
          (e.g., file descriptors produced by bpf(2),
          epoll_create(2), eventfd(2), inotify_init(2),
          perf_event_open(2), signalfd(2), timerfd_create(2), and
          userfaultfd(2)), the entry will be a symbolic link with
          contents of the form

              anon_inode:<file-type>

          In many cases (but not all), the file-type is surrounded
          by square brackets.

          For example, an epoll file descriptor will have a symbolic
          link whose content is the string anon_inode:[eventpoll].

          In a multithreaded process, the contents of this directory
          are not available if the main thread has already
          terminated (typically by calling pthread_exit(3)).

          Programs that take a filename as a command-line argument,
          but don't take input from standard input if no argument is
          supplied, and programs that write to a file named as a
          command-line argument, but don't send their output to
          standard output if no argument is supplied, can
          nevertheless be made to use standard input or standard
          output by using /proc/[pid]/fd files as command-line
          arguments.  For example, assuming that -i is the flag
          designating an input file and -o is the flag designating
          an output file:

              $ foobar -i /proc/self/fd/0 -o /proc/self/fd/1 ...

          and you have a working filter.

          /proc/self/fd/N is approximately the same as /dev/fd/N in
          some UNIX and UNIX-like systems.  Most Linux MAKEDEV
          scripts symbolically link /dev/fd to /proc/self/fd, in
          fact.

          Most systems provide symbolic links /dev/stdin,
          /dev/stdout, and /dev/stderr, which respectively link to
          the files 0, 1, and 2 in /proc/self/fd.  Thus the example
          command above could be written as:

              $ foobar -i /dev/stdin -o /dev/stdout ...

          Permission to dereference or read (readlink(2)) the
          symbolic links in this directory is governed by a ptrace
          access mode PTRACE_MODE_READ_FSCREDS check; see ptrace(2).

          Note that for file descriptors referring to inodes (pipes
          and sockets, see above), those inodes still have
          permission bits and ownership information distinct from
          those of the /proc/[pid]/fd entry, and that the owner may
          differ from the user and group IDs of the process.  An
          unprivileged process may lack permissions to open them, as
          in this example:

              $ echo test | sudo -u nobody cat
              test
              $ echo test | sudo -u nobody cat /proc/self/fd/0
              cat: /proc/self/fd/0: Permission denied

          File descriptor 0 refers to the pipe created by the shell
          and owned by that shell's user, which is not nobody, so
          cat does not have permission to create a new file
          descriptor to read from that inode, even though it can
          still read from its existing file descriptor 0.