netdev - Re: [RFC PATCH bpf-next 09/16] bpf: add bpf_seq_printf and bpf_seq

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Date:   Thu, 9 Apr 2020 20:26:08 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf@...r.kernel.org,
        Martin KaFai Lau <kafai@...com>, netdev@...r.kernel.org,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>, kernel-team@...com
Subject: Re: [RFC PATCH bpf-next 09/16] bpf: add bpf_seq_printf and
 bpf_seq_write helpers

On Wed, Apr 08, 2020 at 04:25:31PM -0700, Yonghong Song wrote:
>  
> +BPF_CALL_5(bpf_seq_printf, struct seq_file *, m, char *, fmt, u32, fmt_size, u64, arg1,
> +	   u64, arg2)
> +{
> +	bool buf_used = false;
> +	int i, copy_size;
> +	int mod[2] = {};
> +	int fmt_cnt = 0;
> +	u64 unsafe_addr;
> +	char buf[64];
> +
> +	/*
> +	 * bpf_check()->check_func_arg()->check_stack_boundary()
> +	 * guarantees that fmt points to bpf program stack,
> +	 * fmt_size bytes of it were initialized and fmt_size > 0
> +	 */
> +	if (fmt[--fmt_size] != 0)
> +		return -EINVAL;
...
> +/* Horrid workaround for getting va_list handling working with different
> + * argument type combinations generically for 32 and 64 bit archs.
> + */
> +#define __BPF_SP_EMIT()	__BPF_ARG2_SP()
> +#define __BPF_SP(...)							\
> +	seq_printf(m, fmt, ##__VA_ARGS__)
> +
> +#define __BPF_ARG1_SP(...)						\
> +	((mod[0] == 2 || (mod[0] == 1 && __BITS_PER_LONG == 64))	\
> +	  ? __BPF_SP(arg1, ##__VA_ARGS__)				\
> +	  : ((mod[0] == 1 || (mod[0] == 0 && __BITS_PER_LONG == 32))	\
> +	      ? __BPF_SP((long)arg1, ##__VA_ARGS__)			\
> +	      : __BPF_SP((u32)arg1, ##__VA_ARGS__)))
> +
> +#define __BPF_ARG2_SP(...)						\
> +	((mod[1] == 2 || (mod[1] == 1 && __BITS_PER_LONG == 64))	\
> +	  ? __BPF_ARG1_SP(arg2, ##__VA_ARGS__)				\
> +	  : ((mod[1] == 1 || (mod[1] == 0 && __BITS_PER_LONG == 32))	\
> +	      ? __BPF_ARG1_SP((long)arg2, ##__VA_ARGS__)		\
> +	      : __BPF_ARG1_SP((u32)arg2, ##__VA_ARGS__)))
> +
> +	__BPF_SP_EMIT();
> +	return seq_has_overflowed(m);
> +}

This function is mostly a copy-paste of bpf_trace_printk() with difference
of printing via seq_printf vs __trace_printk.
Please find a way to share the code.