Hi Tao, Sorry to hijack this old email thread :) On Mon, Dec 25, 2023 at 5:39 PM Hou Tao <houtao@xxxxxxxxxxxxxxx> wrote: > > > > On 12/25/2023 10:24 PM, Hou Tao wrote: > > Hi, > > > > On 12/22/2023 8:05 PM, Dominic wrote: > >> Can BPF_MAP_TYPE_LPM_TRIE be used for string matching? I tried it but > >> the matching doesn't work as expected. > > Yes. LPM_TRIE will work for string matching. Did you setup the key size > > of the map and the prefixlen field of bpf_lpm_trie_key correctly ? > > Because the unit of key_size is in-bytes and it should be the maximal > > length of these strings, but the unit of prefixlen is in-bits and it is > > the length of string expressed in bits. And could you share the steps on > > how you used it ? > > Forgot to mention the trick when using LPM_TRIE for string matching. > Because LPM_TRIE uses longest prefix matching to find the target > element, so using the string abcd as key to lookup LPM_TRIE will match > the string abc saved in LPM_TRIE and it is not we wanted. To fix that, > we need to add the terminated null byte of the string to the key, so the > string abc\0 will not be the prefix of the string abcd\0. The code > snippet looks as follows. > > map_fd = bpf_map_create(BPF_MAP_TYPE_LPM_TRIE, name, > sizeof(bpf_lpm_trie_key) + max_string_length + 1, value_size, > max_entries, &opts); > > key.prefixlen = (strlen(str) + 1) * 8; > memcpy(key.data, str, strlen(str) + 1); > bpf_map_update_elem(map_fd, &key, &value, BPF_NOEXIST); > I have a use case where I want to run an XDP program to parse the DNS packet to get the query domain name, then lookup the domain name in bpf map to decide if the query is allowed or not. For example if the LPM_TRIE map is pre-populated with "example.com", then queries like "foo.example.com", "bar.example.com" should be matched, right? I haven't gone that far yet because I haven't got a string match in hash map working, but that LPM_TRIE map would be my end goal. Here is the hash map string not matching problem https://github.com/vincentmli/xdp-tools/issues/2, am I missing something about string terminator '\0'? > > > >> Thanks & Regards, > >> Dominic > >> > >> . > > > > . > >
