Re: [RFC PATCH bpf-next 09/16] bpf: add bpf_seq_printf and bpf_seq_write helpers

[Yonghong Song <yhs@fb.com>]

On 4/9/20 8:26 PM, Alexei Starovoitov wrote:
> 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.

Will do.