问__latent_entropy在C中是用来做什么的？

EN

config GCC_PLUGIN_LATENT_ENTROPY
    bool "Generate some entropy during boot and runtime"
    help
      By saying Y here the kernel will instrument some kernel code to
      extract some entropy from both original and artificially created
      program state.  This will help especially embedded systems where
      there is little 'natural' source of entropy normally.  The cost
      is some slowdown of the boot process (about 0.5%) and fork and
      irq processing.

      Note that entropy extracted this way is not cryptographically
      secure!

      This plugin was ported from grsecurity/PaX. More information at:
       * https://grsecurity.net/
       * https://pax.grsecurity.net/

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this is where the new gcc plugin comes in: we can instrument the kernel's
boot code to do some hash-like computation and extract some entropy from
whatever program state we decide to mix into that computation. a similar
idea has in fact been implemented by Larry Highsmith of Subreption fame
in http://www.phrack.org/issues.html?issue=66&id=15 where he (manually)
instrumented the kernel's boot code to extract entropy from a few kernel
variables such as time (jiffies) and context switch counts.

the latent entropy plugin takes this extraction to a whole new level. first,
we define a new global variable that we mix into the kernel's entropy pools
on each initcall. second, each initcall function (and all other boot-only
functions they call) gets instrumented to compute a 'random' number that
gets mixed into this global variable at the end of the function (you can
think of it as an artificially created return value that each instrumented
function computes for our purposes). the computation is a mix of add/xor/rol
(the happy recovery Halvar mix :) with compile-time chosen random constants
and the sequence of these operations follows the instrumented functions's
control flow graph. for the rest of the gory details see the source code ;).

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