selftests/bpf: Keep int return type for tailcall subprogs

LLVM23 ([1]) supports 'true' function signature in BTF. The return type
of the caller of a tailcall must be an 'int'. Otherwise, verification will
fail (see check_btf_func() in check_btf.c). So with llvm23, it is possible
that the compiler may change the caller's return type from 'int' to 'void'.
To prevent this, barrier_var() and __sink() are used to avoid returning
a constant prone to be optimized.

  [1] https://github.com/llvm/llvm-project/pull/198426

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260609233407.2711577-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

6 files changed, 57 insertions, 20 deletions

diff --git a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf2.c b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf2.c
index ce97d141daee..c4fadee5aadc 100644
--- a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf2.c
+++ b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf2.c
@@ -13,11 +13,14 @@ struct {
 static __noinline
 int subprog_tail(struct __sk_buff *skb)
 {
+	int ret = 1;
+
 	if (load_byte(skb, 0))
 		bpf_tail_call_static(skb, &jmp_table, 1);
 	else
 		bpf_tail_call_static(skb, &jmp_table, 0);
-	return 1;
+	barrier_var(ret);
+	return ret;
 }
 
 int count = 0;
diff --git a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy1.c b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy1.c
index d556b19413d7..1fd07824d88a 100644
--- a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy1.c
+++ b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy1.c
@@ -16,20 +16,25 @@ int count = 0;
 static __noinline
 int subprog_tail(struct __sk_buff *skb)
 {
+	int ret = 0;
+
 	bpf_tail_call_static(skb, &jmp_table, 0);
-	return 0;
+	barrier_var(ret);
+	return ret;
 }
 
 SEC("tc")
 int entry(struct __sk_buff *skb)
 {
-	int ret = 1;
+	int ret = 1, ret1, ret2;
 
 	clobber_regs_stack();
 
 	count++;
-	subprog_tail(skb);
-	subprog_tail(skb);
+	ret1 = subprog_tail(skb);
+	ret2 = subprog_tail(skb);
+	__sink(ret1);
+	__sink(ret2);
 
 	return ret;
 }
diff --git a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy2.c b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy2.c
index ae94c9c70ab7..6fde0ab92148 100644
--- a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy2.c
+++ b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy2.c
@@ -25,8 +25,11 @@ int count1 = 0;
 static __noinline
 int subprog_tail0(struct __sk_buff *skb)
 {
+	int ret = 0;
+
 	bpf_tail_call_static(skb, &jmp_table, 0);
-	return 0;
+	barrier_var(ret);
+	return ret;
 }
 
 __auxiliary
@@ -41,16 +44,22 @@ int classifier_0(struct __sk_buff *skb)
 static __noinline
 int subprog_tail1(struct __sk_buff *skb)
 {
+	int ret = 0;
+
 	bpf_tail_call_static(skb, &jmp_table, 1);
-	return 0;
+	barrier_var(ret);
+	return ret;
 }
 
 __auxiliary
 SEC("tc")
 int classifier_1(struct __sk_buff *skb)
 {
+	int ret;
+
 	count1++;
-	subprog_tail1(skb);
+	ret = subprog_tail1(skb);
+	__sink(ret);
 	return 0;
 }
 
@@ -59,13 +68,14 @@ __retval(33)
 SEC("tc")
 int tailcall_bpf2bpf_hierarchy_2(struct __sk_buff *skb)
 {
-	int ret = 0;
+	int ret = 0, ret1, ret2;
 
 	clobber_regs_stack();
 
-	subprog_tail0(skb);
-	subprog_tail1(skb);
-
+	ret1 = subprog_tail0(skb);
+	ret2 = subprog_tail1(skb);
+	__sink(ret1);
+	__sink(ret2);
 	__sink(ret);
 	return (count1 << 16) | count0;
 }
diff --git a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy3.c b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy3.c
index 56b6b0099840..0ef9cfb2da8d 100644
--- a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy3.c
+++ b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy3.c
@@ -33,17 +33,24 @@ int count = 0;
 static __noinline
 int subprog_tail(struct __sk_buff *skb, void *jmp_table)
 {
+	int ret = 0;
+
 	bpf_tail_call_static(skb, jmp_table, 0);
-	return 0;
+	barrier_var(ret);
+	return ret;
 }
 
 __auxiliary
 SEC("tc")
 int classifier_0(struct __sk_buff *skb)
 {
+	int ret1, ret2;
+
 	count++;
-	subprog_tail(skb, &jmp_table0);
-	subprog_tail(skb, &jmp_table1);
+	ret1 = subprog_tail(skb, &jmp_table0);
+	ret2 = subprog_tail(skb, &jmp_table1);
+	__sink(ret1);
+	__sink(ret2);
 	return count;
 }
 
diff --git a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy_fentry.c b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy_fentry.c
index 5261395713cd..6db9afee2095 100644
--- a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy_fentry.c
+++ b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy_fentry.c
@@ -18,18 +18,25 @@ int count = 0;
 static __noinline
 int subprog_tail(void *ctx)
 {
+	int ret = 0;
+
 	bpf_tail_call_static(ctx, &jmp_table, 0);
-	return 0;
+	barrier_var(ret);
+	return ret;
 }
 
 SEC("fentry/dummy")
 int BPF_PROG(fentry, struct sk_buff *skb)
 {
+	int ret1, ret2;
+
 	clobber_regs_stack();
 
 	count++;
-	subprog_tail(ctx);
-	subprog_tail(ctx);
+	ret1 = subprog_tail(ctx);
+	ret2 = subprog_tail(ctx);
+	__sink(ret1);
+	__sink(ret2);
 
 	return 0;
 }
diff --git a/tools/testing/selftests/bpf/progs/verifier_sock.c b/tools/testing/selftests/bpf/progs/verifier_sock.c
index 9f680cf44512..4f2f3209eec8 100644
--- a/tools/testing/selftests/bpf/progs/verifier_sock.c
+++ b/tools/testing/selftests/bpf/progs/verifier_sock.c
@@ -1120,8 +1120,11 @@ int tail_call(struct __sk_buff *sk)
 static __noinline
 int static_tail_call(struct __sk_buff *sk)
 {
+	int ret = 0;
+
 	bpf_tail_call_static(sk, &jmp_table, 0);
-	return 0;
+	barrier_var(ret);
+	return ret;
 }
 
 /* Tail calls in sub-programs invalidate packet pointers. */
@@ -1144,10 +1147,12 @@ __failure __msg("invalid mem access")
 int invalidate_pkt_pointers_by_static_tail_call(struct __sk_buff *sk)
 {
 	int *p = (void *)(long)sk->data;
+	int ret;
 
 	if ((void *)(p + 1) > (void *)(long)sk->data_end)
 		return TCX_DROP;
-	static_tail_call(sk);
+	ret = static_tail_call(sk);
+	__sink(ret);
 	*p = 42; /* this is unsafe */
 	return TCX_PASS;
 }