As discussed before (see the 2021 thread "Incorrect thread TID
caching") clone() has been effectively unusable because it produces a
child process in invalid state, with wrong tid in its thread
structure, among other problems.

The attached proposed patch attempts to make clone() usable by having
it share the _Fork logic for establishing a consistent process state
after forking, and also blocks use of flags which produce invalid
state.

With CLONE_VM, the old behavior is kept, with a caveat that the child
context is extremely restrictive, ala vfork.

It was raised previously (pardon the pun) that raise() should perhaps
be modified to make a SYS_gettid syscall rather than using the thread
structure tid, so that it's possible to call raise() in a vfork
context without signaling the wrong process. It might make sense to do
that now too.

Rich