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[bpf-next] selftests/bpf: Avoid subtraction after htons() in ipip tests

Message ID 20240808075906.1849564-1-ast@fiberby.net (mailing list archive)
State Accepted
Delegated to: BPF
Headers show
Series [bpf-next] selftests/bpf: Avoid subtraction after htons() in ipip tests | expand

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netdev/tree_selection success Clearly marked for bpf-next
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Commit Message

Asbjørn Sloth Tønnesen Aug. 8, 2024, 7:59 a.m. UTC 
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(-)

Comments

Toke Høiland-Jørgensen Aug. 8, 2024, 9:24 a.m. UTC | #1 
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>
Martin KaFai Lau Aug. 14, 2024, 1:15 a.m. UTC | #2 
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 mbox series

Patch

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,