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

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Joe Perches <joe@perches.com>
Cc: Alan Maguire <alan.maguire@oracle.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>, bpf <bpf@vger.kernel.org>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
	Yonghong Song <yhs@fb.com>, Martin KaFai Lau <kafai@fb.com>,
	Song Liu <songliubraving@fb.com>,
	Andrii Nakryiko <andriin@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@chromium.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Network Development <netdev@vger.kernel.org>
Subject: Re: [PATCH v2 bpf-next 4/7] printk: add type-printing %pT format specifier which uses BTF
Date: Wed, 13 May 2020 16:07:56 -0700	[thread overview]
Message-ID: <CAADnVQK8osy9W8-u-K=ucqe5q-+Uik41fBw6d-SfG-m6rgVwDQ@mail.gmail.com> (raw)
In-Reply-To: <1b63a6b193073674b6e0f9f95c62ce2af1b977cc.camel@perches.com>

On Wed, May 13, 2020 at 4:05 PM Joe Perches <joe@perches.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.