namei: preserve errors from fget_cap_locked

[kevans]

Summary

Most notably, we want to make sure we don't clobber any capabilities-related errors. This is a regression from r357412 (O_SEARCH) that was picked up from the capsicum tests.

PR: 243839

[emaste]

sys/kern/vfs_lookup.c

463–466

To me a nested

if (error == 0) {
        if (dfp->f_vnode == NULL)
                error = ENOTDIR;
        else
                error = EBADF;
}

is easier to follow

[kib]

sys/kern/vfs_lookup.c

463–466

I would do it diferently:

   if (error != 0) {
      /* Preserve error */
   } else if (...f_ops == &badfileops} {
     error = EBADF;
   } else if (...f_vnode == NULL) {
     error = EINVAL;
  } else {
      dp = dfp->f_vnode;
...

Note tht I restored errors from fgetvp_rights.

[emaste]

I would do it differently

Ah yes, this is better.

(The recent OpenSMTPD vulnerability provides a good example of the need to consider this carefully and get it right from both correctness & readability.)