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

On 7/25/25 5:59 PM, Yonghong Song wrote:

On 7/25/25 4:29 PM, Martin KaFai Lau wrote:

quoted

On 7/24/25 9:34 PM, Yonghong Song wrote:

quoted

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?

For example, for test_probe_read_user_dynptr, for function test_dynptr_probe_xdp(),
for this one:
    off = xdp_near_frag_end_offset();

the off = 64928. Note that xdp_near_frag_end_offset() return value depends page 
size.

__u32 xdp_near_frag_end_offset(void)
{
         const __u32 headroom = 256;
         const __u32 max_frag_size =  __PAGE_SIZE - headroom - sizeof(struct 
skb_shared_info);
         /* 32 bytes before the approximate end of the fragment */
         return max_frag_size - 32;
}

The 'len' depends on 'test_len[i]' and test_len is
    __u32 test_len[7] = {0/* placeholder */, 0, 1, 2, 255, 256, 257};

In bpf_xdp_pointer, we have the following test

         if (unlikely(offset > 0xffff || len > 0xffff))

Thanks for the explanation. Applied. Thanks.

I wonder if the 0xffff check can be removed from bpf_xdp_pointer() and depend on 
checking the xdp_get_buff_len(). The 0xffff check was also removed from the 
bpf_skb_load_bytes some time ago. [cc: Lorenzo, netdev]

Otherwise, it is not very useful to be able to create such xdp buff from the 
bpf_prog_test_run_xdp.