netdev - Re: [PATCH bpf-next] selftest/bpf: check invalid length in test_xdp_update

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Message-ID: <15f829a2-8556-0545-7408-3fca66eb38b7@fb.com>
Date:   Fri, 4 Feb 2022 10:14:16 -0800
From:   Yonghong Song <yhs@...com>
To:     Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
CC:     Lorenzo Bianconi <lorenzo@...nel.org>, <bpf@...r.kernel.org>,
        <ast@...nel.org>, <daniel@...earbox.net>, <brouer@...hat.com>,
        <toke@...hat.com>, <andrii@...nel.org>, <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next] selftest/bpf: check invalid length in
 test_xdp_update_frags



On 2/4/22 9:52 AM, Lorenzo Bianconi wrote:
>>
>>
>> On 2/4/22 5:58 AM, Lorenzo Bianconi wrote:
>>> Update test_xdp_update_frags adding a test for a buffer size
>>> set to (MAX_SKB_FRAGS + 2) * PAGE_SIZE. The kernel is supposed
>>> to return -ENOMEM.
>>>
>>> Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
>>> ---
>>>    .../bpf/prog_tests/xdp_adjust_frags.c         | 37 ++++++++++++++++++-
>>>    1 file changed, 36 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_adjust_frags.c b/tools/testing/selftests/bpf/prog_tests/xdp_adjust_frags.c
>>> index 134d0ac32f59..61d5b585eb15 100644
>>> --- a/tools/testing/selftests/bpf/prog_tests/xdp_adjust_frags.c
>>> +++ b/tools/testing/selftests/bpf/prog_tests/xdp_adjust_frags.c
>>> @@ -5,11 +5,12 @@
>>>    void test_xdp_update_frags(void)
>>>    {
>>>    	const char *file = "./test_xdp_update_frags.o";
>>> +	int err, prog_fd, max_skb_frags, buf_size, num;
>>>    	struct bpf_program *prog;
>>>    	struct bpf_object *obj;
>>> -	int err, prog_fd;
>>>    	__u32 *offset;
>>>    	__u8 *buf;
>>> +	FILE *f;
>>>    	LIBBPF_OPTS(bpf_test_run_opts, topts);
>>>    	obj = bpf_object__open(file);
>>> @@ -99,6 +100,40 @@ void test_xdp_update_frags(void)
>>>    	ASSERT_EQ(buf[7621], 0xbb, "xdp_update_frag buf[7621]");
>>>    	free(buf);
>>> +
>>> +	/* test_xdp_update_frags: unsupported buffer size */
>>> +	f = fopen("/proc/sys/net/core/max_skb_frags", "r");
>>> +	if (!ASSERT_OK_PTR(f, "max_skb_frag file pointer"))
>>> +		goto out;
>>
>> In kernel, the nr_frags checking is against MAX_SKB_FRAGS,
>> but if /proc/sys/net/core/max_skb_flags is 2 or more less
>> than MAX_SKB_FRAGS, the test won't fail, right?
> 
> yes, you are right. Should we use the same definition used in
> include/linux/skbuff.h instead? Something like:
> 
> if (65536 / page_size + 1 < 16)
> 	max_skb_flags = 16;
> else
> 	max_skb_flags = 65536/page_size + 1;

The maximum packet size limit 64KB won't change anytime soon.
So the above should work. Some comments to explain why using
the above formula will be good.

> 
> Regards,
> Lorenzo
> 
>>
>>> +
>>> +	num = fscanf(f, "%d", &max_skb_frags);
>>> +	fclose(f);
>>> +
>>> +	if (!ASSERT_EQ(num, 1, "max_skb_frags read failed"))
>>> +		goto out;
>>> +
>>> +	/* xdp_buff linear area size is always set to 4096 in the
>>> +	 * bpf_prog_test_run_xdp routine.
>>> +	 */
>>> +	buf_size = 4096 + (max_skb_frags + 1) * sysconf(_SC_PAGE_SIZE);
>>> +	buf = malloc(buf_size);
>>> +	if (!ASSERT_OK_PTR(buf, "alloc buf"))
>>> +		goto out;
>>> +
>>> +	memset(buf, 0, buf_size);
>>> +	offset = (__u32 *)buf;
>>> +	*offset = 16;
>>> +	buf[*offset] = 0xaa;
>>> +	buf[*offset + 15] = 0xaa;
>>> +
>>> +	topts.data_in = buf;
>>> +	topts.data_out = buf;
>>> +	topts.data_size_in = buf_size;
>>> +	topts.data_size_out = buf_size;
>>> +
>>> +	err = bpf_prog_test_run_opts(prog_fd, &topts);
>>> +	ASSERT_EQ(err, -ENOMEM, "unsupported buffer size");
>>> +	free(buf);
>>>    out:
>>>    	bpf_object__close(obj);
>>>    }
>>