Re: [PATCH bpf-next 2/2] selftests/bpf: Add tests for BPF_END bitwise tracking

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2026-02-02 at 21:35 +0800, Tianci Cao wrote:
> Now BPF_END has bitwise tracking support. This patch adds selftests to
> cover various cases of BPF_END (`bswap(16|32|64)`, `be(16|32|64)`,
> `le(16|32|64)`) with bitwise propagation.
> 
> This patch is based on existing `verifier_bswap.c`, and add several
> types of new tests:
> 
> 1. Unconditional byte swap operations:
>    - bswap16: port number routing (network byte order conversion)
>    - bswap32: IPv4 subnet routing (network byte order conversion)
>    - bswap64: IPv6 subnet routing (network byte order conversion)
> 
> 2. Endian conversion operations (architecture-aware):
>    - be16/be32/be64: convert to big-endian
>      * on little-endian: do swap
>      * on big-endian: truncation (16/32-bit) or no-op (64-bit)
>    - le16/le32/le64: convert to little-endian
>      * on big-endian: do swap
>      * on little-endian: truncation (16/32-bit) or no-op (64-bit)
> 
> Each test simulates realistic networking scenarios where a value is
> masked to a specific range (e.g., 0x3f00), then byte-swapped, and the
> verifier must prove the result stays within expected bounds.
> 
> Specifically, these selftests are based on dead code elimination:
> If the BPF verifier can precisely track bitwise through byte swap
> operations, it can prune the trap path (invalid memory access) that
> should be unreachable, allowing the program to pass verification.
> If bitwise tracking is incorrect, the verifier cannot prove the trap
> is unreachable, causing verification failure.
> 
> The tests use preprocessor conditionals (#ifdef __BYTE_ORDER__) to
> verify correct behavior on both little-endian and big-endian
> architectures, and require Clang 18+ for bswap instruction support.
> 
> Co-developed-by: Shenghao Yuan <shenghaoyuan0928@163.com>
> Signed-off-by: Shenghao Yuan <shenghaoyuan0928@163.com>
> Co-developed-by: Yazhou Tang <tangyazhou518@outlook.com>
> Signed-off-by: Yazhou Tang <tangyazhou518@outlook.com>
> Signed-off-by: Tianci Cao <ziye@zju.edu.cn>
> ---

Test cases themselves lgtm.

Essentially you have two groups of tests that follow same template:
- bswap16_port_routing, bswap32_ipv4_subnet_routing, bswap64_ipv6_subnet_routing
  (why not just bswap16/32/64 as names, by the way?);
- be16_network, be32_network, be64_network, le16_network, le32_network, le64_network.

Maybe define two macros using bswap64_ipv6_subnet_routing and
le64_network as templates to avoid the copy-paste?
E.g. like DEFINE_BAD_OFFSET_TEST in verifier_value_illegal_alu.c.
Also, could you please cut the amount of comments,
test cases are quite self explaining.

[...]