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RFC: clone with CLONE_VM behavior

Hi all,

Szabolcs has brought to my attention that the new posix_spawn is showing
some issue on his aarch64 [1], but it is not limited to aarch64.

The problem is due the fact GLIBC clone implementation resets both
THREAD_SELF pid and tid when CLONE_VM is specified. This leads to
inconsistency since the value is not restored back in parent and
thus INVALID_TD_P and INVALID_NOT_TERMINATED_TD_P (used in pthread
implementations) will bail with an error handler.

Previous posix_spawn uses vfork which only interferes with THREAD_SELF
pid field by negating it before the syscall and restoring the value
after it.

I am trying to came up with the best solution for this, since both
pid and tid is used in both pthread_{join,cancel} and also on raise
(which also have another issue somewhat related [2]) and I am inclined
to just remove the CLONE_VM changes to pid/tid fields in the syscall
itself and moving it to START_THREAD_DEFN instead.

Any better ideas, tips, advices?


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