Re: [PATCH bpf-next 3/3] selftests/bpf: Fix test dynptr/test_dynptr_memset_xdp_chunks failure

[Martin KaFai Lau <martin.lau@linux.dev>]

On 7/24/25 9:34 PM, Yonghong Song wrote:
> For arm64 64K page size, the xdp data size was set to be more than 64K
> in one of previous patches. This will cause failure for bpf_dynptr_memset().
> Since the failure of bpf_dynptr_memset() is expected with 64K page size,
> return success.
> 
> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
> ---
>   tools/testing/selftests/bpf/progs/dynptr_success.c | 13 ++++++++++++-
>   1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/bpf/progs/dynptr_success.c b/tools/testing/selftests/bpf/progs/dynptr_success.c
> index 3094a1e4ee91..8315273cb900 100644
> --- a/tools/testing/selftests/bpf/progs/dynptr_success.c
> +++ b/tools/testing/selftests/bpf/progs/dynptr_success.c
> @@ -9,6 +9,8 @@
>   #include "bpf_misc.h"
>   #include "errno.h"
>   
> +#define PAGE_SIZE_64K 65536
> +
>   char _license[] SEC("license") = "GPL";
>   
>   int pid, err, val;
> @@ -821,8 +823,17 @@ int test_dynptr_memset_xdp_chunks(struct xdp_md *xdp)
>   	data_sz = bpf_dynptr_size(&ptr_xdp);
>   
>   	err = bpf_dynptr_memset(&ptr_xdp, 0, data_sz, DYNPTR_MEMSET_VAL);
> -	if (err)
> +	if (err) {
> +		/* bpf_dynptr_memset() eventually called bpf_xdp_pointer()

I don't think I understand why the test fixed in patch 1 (e.g. 
test_probe_read_user_dynptr) can pass the bpf_xdp_pointer test on 0xffff. I 
thought the bpf_probe_read_user_str_dynptr will eventually call the 
__bpf_xdp_store_bytes which also does a bpf_xdp_pointer?

> +		 * where if data_sz is greater than 0xffff, -EFAULT will be
> +		 * returned. For 64K page size, data_sz is greater than
> +		 * 64K, so error is expected and let us zero out error and
> +		 * return success.
> +		 */
> +		if (data_sz >= PAGE_SIZE_64K)
> +			err = 0;
>   		goto out;
> +	}
>   
>   	bpf_for(i, 0, max_chunks) {
>   		offset = i * sizeof(buf);