| Message ID | 20240808075906.1849564-1-ast@fiberby.net |
|---|---|
| State | Accepted |
| Commit | 92ec8b9367ab328cdd3d4409464137bc9775a2e5 |
| Headers | show |
| Series | [bpf-next] selftests/bpf: Avoid subtraction after htons() in ipip tests | expand |
On 8/8/24 2:24 AM, Toke Høiland-Jørgensen wrote: > Asbjørn Sloth Tønnesen <ast@fiberby.net> writes: > >> On little-endian systems, doing subtraction after htons() >> leads to interesting results: >> >> Given: >> MAGIC_BYTES = 123 = 0x007B aka. in big endian: 0x7B00 = 31488 >> sizeof(struct iphdr) = 20 >> >> Before this patch: >> __bpf_constant_htons(MAGIC_BYTES) - sizeof(struct iphdr) = 0x7AEC >> 0x7AEC = htons(0xEC7A) = htons(60538) >> >> So these were outer IP packets with a total length of 123 bytes, >> containing an inner IP packet with a total length of 60538 bytes. > > It's just using bag of holding technology! > >> After this patch: >> __bpf_constant_htons(MAGIC_BYTES - sizeof(struct iphdr)) = htons(103) >> >> Now these packets are outer IP packets with a total length of 123 bytes, >> containing an inner IP packet with a total length of 103 bytes. >> >> Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net> > > Reviewed-by: Toke Høiland-Jørgensen <toke@kernel.org> Applied to bpf-next/net. Thanks.
diff --git a/tools/testing/selftests/bpf/prog_tests/flow_dissector.c b/tools/testing/selftests/bpf/prog_tests/flow_dissector.c index 9e5f38739104..6b3078dd5645 100644 --- a/tools/testing/selftests/bpf/prog_tests/flow_dissector.c +++ b/tools/testing/selftests/bpf/prog_tests/flow_dissector.c @@ -378,8 +378,8 @@ struct test tests[] = { .iph_inner.ihl = 5, .iph_inner.protocol = IPPROTO_TCP, .iph_inner.tot_len = - __bpf_constant_htons(MAGIC_BYTES) - - sizeof(struct iphdr), + __bpf_constant_htons(MAGIC_BYTES - + sizeof(struct iphdr)), .tcp.doff = 5, .tcp.source = 80, .tcp.dest = 8080, @@ -407,8 +407,8 @@ struct test tests[] = { .iph_inner.ihl = 5, .iph_inner.protocol = IPPROTO_TCP, .iph_inner.tot_len = - __bpf_constant_htons(MAGIC_BYTES) - - sizeof(struct iphdr), + __bpf_constant_htons(MAGIC_BYTES - + sizeof(struct iphdr)), .tcp.doff = 5, .tcp.source = 80, .tcp.dest = 8080, @@ -436,8 +436,8 @@ struct test tests[] = { .iph_inner.ihl = 5, .iph_inner.protocol = IPPROTO_TCP, .iph_inner.tot_len = - __bpf_constant_htons(MAGIC_BYTES) - - sizeof(struct iphdr), + __bpf_constant_htons(MAGIC_BYTES - + sizeof(struct iphdr)), .tcp.doff = 5, .tcp.source = 99, .tcp.dest = 9090,
On little-endian systems, doing subtraction after htons() leads to interesting results: Given: MAGIC_BYTES = 123 = 0x007B aka. in big endian: 0x7B00 = 31488 sizeof(struct iphdr) = 20 Before this patch: __bpf_constant_htons(MAGIC_BYTES) - sizeof(struct iphdr) = 0x7AEC 0x7AEC = htons(0xEC7A) = htons(60538) So these were outer IP packets with a total length of 123 bytes, containing an inner IP packet with a total length of 60538 bytes. After this patch: __bpf_constant_htons(MAGIC_BYTES - sizeof(struct iphdr)) = htons(103) Now these packets are outer IP packets with a total length of 123 bytes, containing an inner IP packet with a total length of 103 bytes. Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net> --- I didn't target bpf and add a Fixes: e853ae776a58 ("selftests/bpf: support BPF_FLOW_DISSECTOR_F_STOP_AT_ENCAP"), since it only breaks when I change the BPF flow dissector to interact with tot_len. .../selftests/bpf/prog_tests/flow_dissector.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-)