[Bug 247790] PIDs are no longer randomized after reaching pid_max

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=247790

            Bug ID: 247790
           Summary: PIDs are no longer randomized after reaching pid_max
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: kd-dev at pm.me

Created attachment 216234
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=216234&action=edit
Wrap `trypid' with a modulus.

Overview:

If `random' pushes `trypid' over the `max_pid' threshold then
`trypid' is reset to 2 and is no-longer randomized.

Steps to Reproduce:

# sysctl kern.randompid=10240 # Unreasonably large for a smaller test
# for x in `jot 100`; do
>       sh -c 'echo $$'
> done | awk '{print length}' | sort -n | uniq -c

Expected Results:

Longer PIDs are orders of magnitude more likely than smaller PIDs.

>   1 3
>   8 4
>  91 5

Actual Results:

Two-digit PIDs are far more likely than they should be.

>   5 2
>  10 4
>  85 5

Additional Information:

Only CURRENT is affected.

Patch:

The attached patch wraps `trypid' with a modulus rather than simply
setting it to 2.  I have successfully compiled the patch without
warnings, and am currently running the patched kernel.

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