linux-kernel - 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 16:22:01 -0700
From:   Joe Perches <joe@...ches.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Alan Maguire <alan.maguire@...cle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        bpf <bpf@...r.kernel.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
        Yonghong Song <yhs@...com>, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 bpf-next 4/7] printk: add type-printing %pT format
 specifier which uses BTF

On Wed, 2020-05-13 at 16:07 -0700, Alexei Starovoitov wrote:
> On Wed, May 13, 2020 at 4:05 PM Joe Perches <joe@...ches.com> wrote:
> > 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.
> 
> Nack to that.

why?