CVE-2025-39990

In the Linux kernel, the following vulnerability has been resolved:

bpf: Check the helper function is valid in get_helper_proto

kernel test robot reported verifier bug [1] where the helper func pointer could be NULL due to disabled config option.

As Alexei suggested we could check on that in get_helper_proto directly. Marking tail_call helper func with BPF_PTR_POISON, because it is unused by design.

[1] https://lore.kernel.org/oe-lkp/202507160818.68358831-lkp@intel.com