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selftests/bpf: Adjust fexit_bpf2bpf ctx layout for llvm23 true signature

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test_pkt_access_subprog2() is defined in C as

  int test_pkt_access_subprog2(int val, volatile struct __sk_buff *skb)

but llvm optimizes away the unused 'int val' argument. Before llvm23 the
BTF signature did not match the optimized assembly, so the verifier set
attach_func_proto to NULL and fell back to MAX_BPF_FUNC_REG_ARGS (5) u64
arguments (see btf_ctx_access()). The fexit ctx struct therefore placed
the return value after args[5].

With llvm23 the 'true' signature

  int test_pkt_access_subprog2(volatile struct __sk_buff *skb)

is recorded in BTF, so nr_args becomes 1 and the return value moves to
the slot right after args[1]. Select the matching args_subprog2 layout
based on __clang_major__ so the test works with both old and new llvm.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260609233412.2712178-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Yonghong Song 2026-06-09 16:34:12 -07:00 committed by Alexei Starovoitov
parent be1d838b88
commit e775c522a4
1 changed files with 11 additions and 2 deletions
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13
tools/testing/selftests/bpf/progs/fexit_bpf2bpf.c
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@ -53,14 +53,23 @@ int BPF_PROG(test_subprog1, struct sk_buff *skb, int ret)
* r0 = *(u32 *)(r1 + 0)
* w0 <<= 1
* exit
* In such case the verifier falls back to conservative and
* Before llvm23, in such case the verifier falls back to conservative and
* tracing program can access arguments and return value as u64
* instead of accurate types.
* instead of accurate types. With llvm23, the true signature
* int test_pkt_access_subprog2(volatile struct __sk_buff *skb)
* is available in btf.
*/
#if __clang_major__ >= 23
struct args_subprog2 {
__u64 args[1];
__u64 ret;
};
#else
struct args_subprog2 {
__u64 args[5];
__u64 ret;
};
#endif
__u64 test_result_subprog2 = 0;
SEC("fexit/test_pkt_access_subprog2")
int test_subprog2(struct args_subprog2 *ctx)