Add kern_openatfp(9)

[kib]

Summary

See https://github.com/openzfs/zfs/pull/15630

[pjd]

Thank you for working on this!

sys/kern/vfs_syscalls.c

1234

Personally I'd just add 'return (0)' here and eliminate 'else' below, which would reduce the diff.

I also don't see '*fpp = fp' anywhere.

[kib]

Do assign *fpp.

[kib]

Fix diff generation.

[markj]

sys/kern/vfs_syscalls.c

1116

1275

Is there a specific scenario you are guarding against here?

[kib]

sys/kern/vfs_syscalls.c

1275

Recursing into VFS, which we sort of explicitly enable there with a backdoor into filesystem driver VOP_CREATE/VOP_OPEN.

It is similar how td_fpop is handled for devfs, to avoid unlikely but hard to diagnose surprises.