netdev - Re: [PATCH net-next] bpf: add support for %s specifier to bpf_trace

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Date:	Thu, 27 Aug 2015 16:06:14 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	David Miller <davem@...emloft.net>
Cc:	mingo@...nel.org, rostedt@...dmis.org, wangnan0@...wei.com,
	hekuang@...wei.com, daniel@...earbox.net,
	brendan.d.gregg@...il.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] bpf: add support for %s specifier to
 bpf_trace_printk()

On 8/27/15 3:34 PM, David Miller wrote:
> From: Alexei Starovoitov <ast@...mgrid.com>
> Date: Wed, 26 Aug 2015 23:26:59 -0700
>
>> +/* similar to strncpy_from_user() but with extra checks */
>> +static void probe_read_string(char *buf, int size, long unsafe_ptr)
>> +{
>> +	char dst[4];
>> +	int i = 0;
>> +
>> +	size--;
>> +	for (;;) {
>> +		if (probe_kernel_read(dst, (void *) unsafe_ptr, 4))
>> +			break;
>
> I don't think this does the right thing when the string is not a multiple
> of 3 and ends at the last byte of a page that ends a valid region of
> kernel memory.
>
> Seeing this kind of error makes me skeptical to the overall value of
> optimizing this :-/

I've considered the case when first two bytes are valid, but the
other two are in a different page. In such case the probe_read_string()
will trim the string and won't be printing these two valid bytes.
I think that's very rare, so I'm picking higher performance for common
case. The strings over > 64 bytes also will be trimmed to 64.
It's a debugging facility, so I felt that's ok.
Fair or you still think it should be per byte copy?

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