Add RngRestorePolicy to SandboxConfiguration

[ludfjig]

Adds RngRestorePolicy to SandboxConfiguration to allow multiple instances of the same snapshot to have differnet rng. This can be useful if somebody expects two sandboxes craeted from the same snapshot to generate different randoms nubmer, for example a key. The default behavior is to maintain the current behavior where rng is never refreshed, and this also allows backwards compatible with old saved snapshots. However if somebody passes Refresh with an old snapshot it will be silently ignored. We should maybe consider changing this default in future versions.

RngRestorePolicy is not saved to disk as part of a snapshot because it's a property of a live sandbox and to allow maximal flexibility.

[Copilot]

Pull request overview

This PR adds a new sandbox configuration knob, RngRestorePolicy, to control whether the guest libc PRNG state is preserved from a snapshot or reseeded when restoring/resuming from a snapshot. This fits into the host/guest snapshot/restore flow by using scratch bookkeeping to pass a reseed request from host to guest at restore time.

Changes:

• Add RngRestorePolicy to SandboxConfiguration and thread it into MultiUseSandbox so restore/from-snapshot can optionally request a libc PRNG reseed.
• Add a scratch bookkeeping field (SCRATCH_TOP_LIBC_RNG_SEED_OFFSET) plus guest-side logic to consume it and call srand() before dispatching guest calls.
• Add C-guest-based regression tests validating default preservation and optional reseeding behavior across from_snapshot and restore.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/tests/c_guests/c_simpleguest/main.c	Adds a NextRandom guest export (wrap + register) for testing libc RNG behavior.
src/hyperlight_host/src/sandbox/uninitialized_evolve.rs	Threads RNG restore policy into MultiUseSandbox construction.
src/hyperlight_host/src/sandbox/snapshot/file_tests.rs	Adds snapshot/restore tests for libc RNG preservation vs reseeding using the C simple guest.
src/hyperlight_host/src/sandbox/mod.rs	Re-exports RngRestorePolicy from the sandbox module.
src/hyperlight_host/src/sandbox/initialized_multi_use.rs	Implements host-side reseed requests on from_snapshot and restore, plus stores policy in the sandbox.
src/hyperlight_host/src/sandbox/config.rs	Introduces RngRestorePolicy and getters/setters on SandboxConfiguration.
src/hyperlight_host/src/mem/mgr.rs	Adds a scratch bookkeeping write helper for the libc RNG seed and clears it during bookkeeping updates.
src/hyperlight_guest/src/layout.rs	Exposes the guest virtual address of the libc RNG seed scratch slot.
src/hyperlight_guest_bin/src/lib.rs	Adds guest-side reseed logic (refresh_libc_rng) and updates initial srand seeding via a shared fold.
src/hyperlight_guest_bin/src/guest_function/call.rs	Calls refresh_libc_rng() on each internal dispatch (libc feature).
src/hyperlight_common/src/layout.rs	Defines the new scratch bookkeeping offset constant.
CHANGELOG.md	Documents the new configuration option in the unreleased changelog.