virtiofsd: passthrough_ll: add fallback for racy ops

We have two operations that cannot be done race-free on a symlink in
certain cases: utimes and link.

Add racy fallback for these if the race-free method doesn't work.  We do
our best to avoid races even in this case:

  - get absolute path by reading /proc/self/fd/NN symlink

  - lookup parent directory: after this we are safe against renames in
    ancestors

  - lookup name in parent directory, and verify that we got to the original
    inode,  if not retry the whole thing

Both utimes(2) and link(2) hold i_lock on the inode across the operation,
so a racing rename/delete by this fuse instance is not possible, only from
other entities changing the filesystem.

If the "norace" option is given, then disable the racy fallbacks.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

tools/virtiofsd/helper.c

@@ -142,7 +142,10 @@ void fuse_cmdline_help(void)
            "    --daemonize                run in background\n"
            "    -o max_idle_threads        the maximum number of idle worker "
            "threads\n"
-           "                               allowed (default: 10)\n");
+           "                               allowed (default: 10)\n"
+           "    -o norace                  disable racy fallback\n"
+           "                               default: false\n"
+          );
 }
 
 static int fuse_helper_opt_proc(void *data, const char *arg, int key,