RFR: 8345296: AArch64: VM crashes with SIGILL when prctl is disallowed

[Aleksey Shipilev]

We have caught this in some prod environments, where `prctl` is forbidden by the sandboxing mechanism. This fails the JVM, because we have the following code to check for SVE vector length:


int VM_Version::get_current_sve_vector_length() {
  assert(VM_Version::supports_sve(), "should not call this");
  return prctl(PR_SVE_GET_VL);
}


That code returns `-1` when `prctl` is disallowed, which JVM then blindly interprets as vector length, leading to `SIGILL`. I looked around other uses of `prctl` around Hotspot, and they all seem to handle the errors correctly.

Additional testing:
 - [x] Linux AArch64 server fastdebug, with seccomp reproducer
 - [ ] Linux AArch64 server fastdebug, `all`

-------------

Commit messages:
 - Do not set vector length to bad value
 - Fix

Changes: https://git.openjdk.org/jdk/pull/22479/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22479&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345296
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/22479.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22479/head:pull/22479

PR: https://git.openjdk.org/jdk/pull/22479