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Message-ID: <1b63a6b193073674b6e0f9f95c62ce2af1b977cc.camel@perches.com>
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.
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