[PATCH 3/3] selftest/bpf: Adapt inline asm operand constraint for GCC support

[Cupertino Miranda <cupertino.miranda@xxxxxxxxxx>]

GCC errors when compiling tailcall_bpf2bpf_hierarchy2.c and
tailcall_bpf2bpf_hierarchy3.c with the following error:

progs/tailcall_bpf2bpf_hierarchy2.c: In function 'tailcall_bpf2bpf_hierarchy_2':
progs/tailcall_bpf2bpf_hierarchy2.c:66:9: error: input operand constraint contains '+'
   66 |         asm volatile (""::"r+"(ret));
      |         ^~~

Changed implementation to make use of __sink macro that abstracts the
desired behaviour.

The proposed change seems valid for both GCC and CLANG.

Signed-off-by: Cupertino Miranda <cupertino.miranda@xxxxxxxxxx>
Cc: Leon Hwang <hffilwlqm@xxxxxxxxx>
Cc: Eduard Zingerman <eddyz87@xxxxxxxxx>
Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
Cc: Yonghong Song <yonghong.song@xxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Jose Marchesi <jose.marchesi@xxxxxxxxxx>
Cc: David Faust <david.faust@xxxxxxxxxx>
---
 .../testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy2.c | 4 ++--
 .../testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy3.c | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy2.c b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy2.c
index 37604b0b97af..72fd0d577506 100644
--- a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy2.c
+++ b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy2.c
@@ -58,12 +58,12 @@ __retval(33)
 SEC("tc")
 int tailcall_bpf2bpf_hierarchy_2(struct __sk_buff *skb)
 {
-	volatile int ret = 0;
+	int ret = 0;
 
 	subprog_tail0(skb);
 	subprog_tail1(skb);
 
-	asm volatile (""::"r+"(ret));
+	__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 0cdbb781fcbc..a7fb91cb05b7 100644
--- a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy3.c
+++ b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy3.c
@@ -51,11 +51,11 @@ __retval(33)
 SEC("tc")
 int tailcall_bpf2bpf_hierarchy_3(struct __sk_buff *skb)
 {
-	volatile int ret = 0;
+	int ret = 0;
 
 	bpf_tail_call_static(skb, &jmp_table0, 0);
 
-	asm volatile (""::"r+"(ret));
+	__sink(ret);
 	return ret;
 }
 
-- 
2.30.2