Re: [PATCH v3 bpf-next 0/8] bpf, printk: add BTF-based type printing

[Sergey Senozhatsky]

On (20/06/23 13:07), Alan Maguire wrote:
>   struct sk_buff *skb = alloc_skb(64, GFP_KERNEL);
> 
>   pr_info("%pT", BTF_PTR_TYPE(skb, "struct sk_buff"));
> 
> ...gives us:
> 
> (struct sk_buff){
>  .transport_header = (__u16)65535,
>  .mac_header = (__u16)65535,
>  .end = (sk_buff_data_t)192,
>  .head = (unsigned char *)000000007524fd8b,
>  .data = (unsigned char *)000000007524fd8b,
>  .truesize = (unsigned int)768,
>  .users = (refcount_t){
>   .refs = (atomic_t){
>    .counter = (int)1,
>   },
>  },
> }

Hmm. So this can expose the kernel memory layout (IOW do you print out real
%px pointers and so on)? If so, then I'd suggest not to use printk.
Unprivileged /dev/kmsg or /proc/kmsg reads are really OK thing for printk()
log buffer. And if you are going to print pointer hashes instead,

  .transport_header = (__u16)65535,
  .mac_header = (__u16)65535,
  .end = (sk_buff_data_t)192,
  .head = (unsigned char *)34897918740,   // pointer_hash
  .data = (unsigned char *)23942384983,   // pointer hash
  .truesize = (unsigned int)768,
  .users = (refcount_t){
   .refs = (atomic_t){
    .counter = (int)1,
   },
  },

then the value of such printouts becomes a bit unclear to me, sorry.

Probably, something like a seq print into a file somewhere in
/sys/kernel/debug/foo, from which only privileged processes can
read, would be a better approach? My 5 cents.

	-ss