Re: [PATCH bpf-next 1/2] bpf: Fix an issue in bpf_prog_test_run_xdp when page size greater than 4K

[Yonghong Song <yonghong.song@linux.dev>]

On 6/9/25 3:45 PM, Stanislav Fomichev wrote:
> On 06/08, Yonghong Song wrote:
>> The bpf selftest xdp_adjust_tail/xdp_adjust_frags_tail_grow failed on
>> arm64 with 64KB page:
>>     xdp_adjust_tail/xdp_adjust_frags_tail_grow:FAIL
>>
>> In bpf_prog_test_run_xdp(), the xdp->frame_sz is set to 4K, but later on
>> when constructing frags, with 64K page size, the frag data_len could
>> be more than 4K. This will cause problems in bpf_xdp_frags_increase_tail().
>>
>> Limiting the data_len to be 4K for each frag fixed the above test failure.
>>
>> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
>> ---
>>   net/bpf/test_run.c | 5 +++--
>>   1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
>> index aaf13a7d58ed..5529ec007954 100644
>> --- a/net/bpf/test_run.c
>> +++ b/net/bpf/test_run.c
>> @@ -1214,6 +1214,7 @@ int bpf_prog_test_run_xdp(struct bpf_prog *prog, const union bpf_attr *kattr,
>>   	u32 repeat = kattr->test.repeat;
>>   	struct netdev_rx_queue *rxqueue;
>>   	struct skb_shared_info *sinfo;
>> +	const u32 frame_sz = 4096;
>>   	struct xdp_buff xdp = {};
>>   	int i, ret = -EINVAL;
>>   	struct xdp_md *ctx;
>> @@ -1255,7 +1256,7 @@ int bpf_prog_test_run_xdp(struct bpf_prog *prog, const union bpf_attr *kattr,
>>   		headroom -= ctx->data;
>>   	}
>>   
> [..]
>
>> -	max_data_sz = 4096 - headroom - tailroom;
>> +	max_data_sz = frame_sz - headroom - tailroom;
> I wonder whether we should do s/4096/PAGE_SIZE/ here instead. Have you
> tried that? If we are on a 64K page arch, we should not try to preserve
> 4K page limits.

The user space test_run input looks like below (in prog_tests/xdp_adjust_tail.c):

	buf = malloc(16384);
	...
	topts.data_in = buf;
	topts.data_out = buf;
	topts.data_size_in = 9000;
	topts.data_size_out = 16384;
	err = bpf_prog_test_run_opts(prog_fd, &topts);

Allowing s/4096/PAGE_SIZE (64K) will have the test failure for the above
user space input. I think I can increse buf size data_size_in/data_size_out
properly so in the kernel we can do s/4096/PAGE_SIZE.