netdev - Re: [PATCH v2 bpf-next 4/7] printk: add type-printing %pT format specifier which uses BTF

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Date:   Wed, 13 May 2020 16:05:41 -0700
From:   Joe Perches <joe@...ches.com>
To:     Alan Maguire <alan.maguire@...cle.com>, ast@...nel.org,
        daniel@...earbox.net, bpf@...r.kernel.org
Cc:     linux@...musvillemoes.dk, arnaldo.melo@...il.com, yhs@...com,
        kafai@...com, songliubraving@...com, andriin@...com,
        john.fastabend@...il.com, kpsingh@...omium.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v2 bpf-next 4/7] printk: add type-printing %pT format
 specifier which uses BTF

On Tue, 2020-05-12 at 06:56 +0100, Alan Maguire wrote:
> printk supports multiple pointer object type specifiers (printing
> netdev features etc).  Extend this support using BTF to cover
> arbitrary types.  "%pT" specifies the typed format, and the pointer
> argument is a "struct btf_ptr *" where struct btf_ptr is as follows:
> 
> struct btf_ptr {
> 	void *ptr;
> 	const char *type;
> 	u32 id;
> };
> 
> Either the "type" string ("struct sk_buff") or the BTF "id" can be
> used to identify the type to use in displaying the associated "ptr"
> value.  A convenience function to create and point at the struct
> is provided:
> 
> 	printk(KERN_INFO "%pT", BTF_PTR_TYPE(skb, struct sk_buff));
> 
> When invoked, BTF information is used to traverse the sk_buff *
> and display it.  Support is present for structs, unions, enums,
> typedefs and core types (though in the latter case there's not
> much value in using this feature of course).
> 
> Default output is indented, but compact output can be specified
> via the 'c' option.  Type names/member values can be suppressed
> using the 'N' option.  Zero values are not displayed by default
> but can be using the '0' option.  Pointer values are obfuscated
> unless the 'x' option is specified.  As an example:
> 
>   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 *)000000006b71155a,
>  .data = (unsigned char *)000000006b71155a,
>  .truesize = (unsigned int)768,
>  .users = (refcount_t){
>   .refs = (atomic_t){
>    .counter = (int)1,

Given

  #define BTF_INT_ENCODING(VAL)   (((VAL) & 0x0f000000) >> 24)

Maybe

  #define BTF_INT_SIGNED  (1 << 0)
  #define BTF_INT_CHAR    (1 << 1)
  #define BTF_INT_BOOL    (1 << 2)

could be extended to include

  #define BTF_INT_HEX     (1 << 3)

So hex values can be appropriately pretty-printed.