In the Linux kernel, the following vulnerability has been resolved: bpf: reject BPF_PSEUDO_FUNC reference to the main program fixups.c:jit

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

bpf: reject BPF_PSEUDO_FUNC reference to the main program

fixups.c:jit_subprogs() rewrites BPF_PSEUDO_FUNC loads to contain real
function addresses. This function is invoked from bpf_jit_subprogs()
only when env->subprog_cnt > 1. Meaning that for any program like
below:

int main(void *ctx) {
void *ptr = main;
...
bpf_timer_set_callback(..., ptr);
...
}

The 'ptr' won't be ever converted to contain an address.
In combination with e.g. bpf_timer_set_callback() this would lead to a
function call at a bogus address.

Instead of complicating the implementation, just assume that no useful
program needs main to be a sync or async callback and reject
BPF_PSEUDO_FUNC loads for the main subprogram.