bpf verifier: not able to store an immediate number into a register memory if reg type is PTR_TO_CTX

[QiongwenXu]

bpf verifier: not able to store an immediate number into a register memory if reg type is PTR_TO_CTX but storing a register value is legal eg:

*(u32*)(r1+0) = r2 -> legal
*(u32*)(r1+0) = 0  -> illegal

where r1 is the input of BPF program

reference: https://github.com/torvalds/linux/blob/a8205e310011f09cc73cd577d7b0074c57b9bb54/kernel/bpf/verifier.c key information:

1. * At the start of BPF program the register R1 contains a pointer to bpf_context
   * and has type PTR_TO_CTX.

2.         if (is_ctx_reg(env, insn->dst_reg)) {
               verbose(env, "BPF_ST stores into R%d %s is not allowed\n",
                   insn->dst_reg,
                   reg_type_str[reg_state(env, insn->dst_reg)->type]);
               return -EACCES;
           }