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Push the root seed version to userspace through the VDSO page, if the RANDOM_FENESTRASX algorithm is enabled. Otherwise, there is no functional change. The mechanism can be disabled with debug.fxrng_vdso_enable=0. arc4random(3) obtains a pointer to the root seed version published by the kernel in the shared page at allocation time. Like arc4random(9), it maintains its own per-process copy of the seed version corresponding to the root seed version at the time it last rekeyed. On read requests, the process seed version is compared with the version published in the shared page; if they do not match, arc4random(3) reseeds from the kernel before providing generated output. This change does not implement the FenestrasX concept of PCPU userspace generators seeded from a per-process base generator. That change is left for future discussion/work. Reviewed by: kib (previous version) Approved by: csprng (me -- only touching FXRNG here) Differential Revision: https://reviews.freebsd.org/D22839
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