This specifically to handle the case like below:
   // ptr below is a socket ptr identified by PTR_TO_BTF_ID
   u64 param[2] = { ptr, val };
   bpf_seq_printf(seq, fmt, sizeof(fmt), param, sizeof(param));

In this case, the 16 bytes stack for "param" contains:
   8 bytes for ptr with spilled PTR_TO_BTF_ID
   8 bytes for val as STACK_MISC

The current verifier will complain the ptr should not be visible
to the helper.
   ...
   16: (7b) *(u64 *)(r10 -64) = r2
   18: (7b) *(u64 *)(r10 -56) = r1
   19: (bf) r4 = r10
   ;
   20: (07) r4 += -64
   ; BPF_SEQ_PRINTF(seq, fmt1, (long)s, s->sk_protocol);
   21: (bf) r1 = r6
   22: (18) r2 = 0xffffa8d00018605a
   24: (b4) w3 = 10
   25: (b4) w5 = 16
   26: (85) call bpf_seq_printf#125
    R0=inv(id=0) R1_w=ptr_seq_file(id=0,off=0,imm=0)
    R2_w=map_value(id=0,off=90,ks=4,vs=144,imm=0) R3_w=inv10
    R4_w=fp-64 R5_w=inv16 R6=ptr_seq_file(id=0,off=0,imm=0)
    R7=ptr_netlink_sock(id=0,off=0,imm=0) R10=fp0 fp-56_w=mmmmmmmm
    fp-64_w=ptr_
   last_idx 26 first_idx 13
   regs=8 stack=0 before 25: (b4) w5 = 16
   regs=8 stack=0 before 24: (b4) w3 = 10
   invalid indirect read from stack off -64+0 size 16
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200509175915.2476783-1-yhs@fb.com