Similar to the previous patch, barrier_var() is used to prevent the
LLVM23 compiler from optimizing away the return value of subprogs that
issue a bpf_tail_call. Where the caller does not already consume the
return value, store it into a global variable as well, so the compiler
cannot change the subprog return type from 'int' to 'void'.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 .../selftests/bpf/progs/tailcall_bpf2bpf2.c    |  5 ++++-
 .../bpf/progs/tailcall_bpf2bpf_hierarchy2.c    | 18 +++++++++++++-----
 .../bpf/progs/tailcall_bpf2bpf_hierarchy3.c    | 11 ++++++++---
 .../selftests/bpf/progs/verifier_sock.c        |  9 +++++++--
 4 files changed, 32 insertions(+), 11 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_hierarchy2.c b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy2.c
index ae94c9c70ab7..8f217bef3d62 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 +44,21 @@ 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;
 }
 
+int ret1, ret2;
+
 __auxiliary
 SEC("tc")
 int classifier_1(struct __sk_buff *skb)
 {
 	count1++;
-	subprog_tail1(skb);
+	ret1 = subprog_tail1(skb);
 	return 0;
 }
 
@@ -63,8 +71,8 @@ int tailcall_bpf2bpf_hierarchy_2(struct __sk_buff *skb)
 
 	clobber_regs_stack();
 
-	subprog_tail0(skb);
-	subprog_tail1(skb);
+	ret1 = subprog_tail0(skb);
+	ret2 = subprog_tail1(skb);
 
 	__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..4d89eabe1f70 100644
--- a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy3.c
+++ b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy3.c
@@ -33,17 +33,22 @@ 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;
 }
 
+int ret1, ret2;
+
 __auxiliary
 SEC("tc")
 int classifier_0(struct __sk_buff *skb)
 {
 	count++;
-	subprog_tail(skb, &jmp_table0);
-	subprog_tail(skb, &jmp_table1);
+	ret1 = subprog_tail(skb, &jmp_table0);
+	ret2 = subprog_tail(skb, &jmp_table1);
 	return count;
 }
 
diff --git a/tools/testing/selftests/bpf/progs/verifier_sock.c b/tools/testing/selftests/bpf/progs/verifier_sock.c
index 9f680cf44512..bd9fa6b2a2f9 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. */
@@ -1138,6 +1141,8 @@ int invalidate_pkt_pointers_by_global_tail_call(struct __sk_buff *sk)
 	return TCX_PASS;
 }
 
+int ret1;
+
 /* Tail calls in static sub-programs invalidate packet pointers. */
 SEC("tc")
 __failure __msg("invalid mem access")
@@ -1147,7 +1152,7 @@ int invalidate_pkt_pointers_by_static_tail_call(struct __sk_buff *sk)
 
 	if ((void *)(p + 1) > (void *)(long)sk->data_end)
 		return TCX_DROP;
-	static_tail_call(sk);
+	ret1 = static_tail_call(sk);
 	*p = 42; /* this is unsafe */
 	return TCX_PASS;
 }
-- 
2.53.0-Meta