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Declaring the rseq_cs field as a union between __u64 and two __u32
allows both 32-bit and 64-bit kernels to read the full __u64, and
therefore validate that a 32-bit user-space cleared the upper 32
bits, thus ensuring a consistent behavior between native 32-bit
kernels and 32-bit compat tasks on 64-bit kernels.

Check that the rseq_cs value read is < TASK_SIZE.

The asm/byteorder.h header needs to be included by rseq.h, now
that it is not using linux/types_32_64.h anymore.

Considering that only __32 and __u64 types are declared in linux/rseq.h,
the linux/types.h header should always be included for both kernel and
user-space code: including stdint.h is just for u64 and u32, which are
not used in this header at all.

Use copy_from_user()/clear_user() to interact with a 64-bit field,
because arm32 does not implement 64-bit __get_user, and ppc32 does not
64-bit get_user. Considering that the rseq_cs pointer does not need to
be loaded/stored with si...