netdev - Re: [PATCH v3 bpf-next 4/8] printk: add type-printing %pT format specifier which uses BTF

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Date:   Tue, 23 Jun 2020 15:11:19 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Alan Maguire <alan.maguire@...cle.com>, ast@...nel.org,
        daniel@...earbox.net, yhs@...com, andriin@...com,
        arnaldo.melo@...il.com
Cc:     kafai@...com, songliubraving@...com, john.fastabend@...il.com,
        kpsingh@...omium.org, joe@...ches.com, pmladek@...e.com,
        rostedt@...dmis.org, sergey.senozhatsky@...il.com,
        andriy.shevchenko@...ux.intel.com, corbet@....net,
        bpf@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v3 bpf-next 4/8] printk: add type-printing %pT format
 specifier which uses BTF

On 23/06/2020 14.07, Alan Maguire wrote:
> diff --git a/include/linux/printk.h b/include/linux/printk.h
> index fc8f03c..8f8f5d2 100644
> --- a/include/linux/printk.h
> +++ b/include/linux/printk.h
> @@ -618,4 +618,20 @@ static inline void print_hex_dump_debug(const char *prefix_str, int prefix_type,
>  #define print_hex_dump_bytes(prefix_str, prefix_type, buf, len)	\
>  	print_hex_dump_debug(prefix_str, prefix_type, 16, 1, buf, len, true)
>  
> +/**
> + * struct btf_ptr is used for %pT (typed pointer) display; the
> + * additional type string/BTF id are used to render the pointer
> + * data as the appropriate type.
> + */
> +struct btf_ptr {
> +	void *ptr;
> +	const char *type;
> +	u32 id;
> +};
> +
> +#define	BTF_PTR_TYPE(ptrval, typeval) \
> +	(&((struct btf_ptr){.ptr = ptrval, .type = #typeval}))
> +
> +#define BTF_PTR_ID(ptrval, idval) \
> +	(&((struct btf_ptr){.ptr = ptrval, .id = idval}))

Isn't there some better place to put this than printk.h? Anyway, you
probably want the ptr member to be "const void*", to avoid "... discards
const qualifier" warnings when somebody happens to have a "const struct
foobar *".

>  #endif
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 259e558..c0d209d 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -44,6 +44,7 @@
>  #ifdef CONFIG_BLOCK
>  #include <linux/blkdev.h>
>  #endif
> +#include <linux/btf.h>
>  
>  #include "../mm/internal.h"	/* For the trace_print_flags arrays */
>  
> @@ -2092,6 +2093,87 @@ char *fwnode_string(char *buf, char *end, struct fwnode_handle *fwnode,
>  	return widen_string(buf, buf - buf_start, end, spec);
>  }
>  
> +#define btf_modifier_flag(c)	(c == 'c' ? BTF_SHOW_COMPACT :	\
> +				 c == 'N' ? BTF_SHOW_NONAME :	\
> +				 c == 'x' ? BTF_SHOW_PTR_RAW :	\
> +				 c == 'u' ? BTF_SHOW_UNSAFE : \
> +				 c == '0' ? BTF_SHOW_ZERO : 0)
> +
> +static noinline_for_stack
> +char *btf_string(char *buf, char *end, void *ptr, struct printf_spec spec,
> +		 const char *fmt)
> +{
> +	struct btf_ptr *bp = (struct btf_ptr *)ptr;
> +	u8 btf_kind = BTF_KIND_TYPEDEF;
> +	const struct btf_type *t;
> +	const struct btf *btf;
> +	char *buf_start = buf;
> +	const char *btf_type;
> +	u64 flags = 0, mod;
> +	s32 btf_id;
> +
> +	if (check_pointer(&buf, end, ptr, spec))
> +		return buf;
> +
> +	if (check_pointer(&buf, end, bp->ptr, spec))
> +		return buf;
> +
> +	while (isalnum(*fmt)) {
> +		mod = btf_modifier_flag(*fmt);
> +		if (!mod)
> +			break;
> +		flags |= mod;
> +		fmt++;
> +	}
> +
> +	btf = bpf_get_btf_vmlinux();

AFAICT, this function is only compiled if CONFIG_BPF=y and
CONFIG_BPF_SYSCALL=y, and I don't see any static inline stub defined
anywhere. Have you built the kernel with one or both of those turned off?

Rasmus