[PATCH bpf-next v2] selftests/bpf: Fix string read in strncmp benchmark

[Viktor Malik <vmalik@redhat.com>]

The strncmp benchmark uses the bpf_strncmp helper and a hand-written
loop to compare two strings. The values of the strings are filled from
userspace. One of the strings is non-const (in .bss) while the other is
const (in .rodata) since that is the requirement of bpf_strncmp.

The problem is that in the hand-written loop, Clang optimizes the reads
from the const string to always return 0 which breaks the benchmark.

Use barrier_var to prevent the optimization.

The effect can be seen on the strncmp-no-helper variant.

Before this change:

    # ./bench strncmp-no-helper
    Setting up benchmark 'strncmp-no-helper'...
    Benchmark 'strncmp-no-helper' started.
    Iter   0 (112.309us): hits    0.000M/s (  0.000M/prod), drops    0.000M/s, total operations    0.000M/s
    Iter   1 (-23.238us): hits    0.000M/s (  0.000M/prod), drops    0.000M/s, total operations    0.000M/s
    Iter   2 ( 58.994us): hits    0.000M/s (  0.000M/prod), drops    0.000M/s, total operations    0.000M/s
    Iter   3 (-30.466us): hits    0.000M/s (  0.000M/prod), drops    0.000M/s, total operations    0.000M/s
    Iter   4 ( 29.996us): hits    0.000M/s (  0.000M/prod), drops    0.000M/s, total operations    0.000M/s
    Iter   5 ( 16.949us): hits    0.000M/s (  0.000M/prod), drops    0.000M/s, total operations    0.000M/s
    Iter   6 (-60.035us): hits    0.000M/s (  0.000M/prod), drops    0.000M/s, total operations    0.000M/s
    Summary: hits    0.000 ± 0.000M/s (  0.000M/prod), drops    0.000 ± 0.000M/s, total operations    0.000 ± 0.000M/s

After this change:

    # ./bench strncmp-no-helper
    Setting up benchmark 'strncmp-no-helper'...
    Benchmark 'strncmp-no-helper' started.
    Iter   0 ( 77.711us): hits    5.534M/s (  5.534M/prod), drops    0.000M/s, total operations    5.534M/s
    Iter   1 ( 11.215us): hits    6.006M/s (  6.006M/prod), drops    0.000M/s, total operations    6.006M/s
    Iter   2 (-14.253us): hits    5.931M/s (  5.931M/prod), drops    0.000M/s, total operations    5.931M/s
    Iter   3 ( 59.087us): hits    6.005M/s (  6.005M/prod), drops    0.000M/s, total operations    6.005M/s
    Iter   4 (-21.379us): hits    6.010M/s (  6.010M/prod), drops    0.000M/s, total operations    6.010M/s
    Iter   5 (-20.310us): hits    5.861M/s (  5.861M/prod), drops    0.000M/s, total operations    5.861M/s
    Iter   6 ( 53.937us): hits    6.004M/s (  6.004M/prod), drops    0.000M/s, total operations    6.004M/s
    Summary: hits    5.969 ± 0.061M/s (  5.969M/prod), drops    0.000 ± 0.000M/s, total operations    5.969 ± 0.061M/s

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Fixes: 9c42652f8be3 ("selftests/bpf: Add benchmark for bpf_strncmp() helper")
Signed-off-by: Viktor Malik <vmalik@redhat.com>
---
 tools/testing/selftests/bpf/progs/strncmp_bench.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/progs/strncmp_bench.c b/tools/testing/selftests/bpf/progs/strncmp_bench.c
index 18373a7df76e..f47bf88f8d2a 100644
--- a/tools/testing/selftests/bpf/progs/strncmp_bench.c
+++ b/tools/testing/selftests/bpf/progs/strncmp_bench.c
@@ -35,7 +35,10 @@ static __always_inline int local_strncmp(const char *s1, unsigned int sz,
 SEC("tp/syscalls/sys_enter_getpgid")
 int strncmp_no_helper(void *ctx)
 {
-	if (local_strncmp(str, cmp_str_len + 1, target) < 0)
+	const char *target_str = target;
+
+	barrier_var(target_str);
+	if (local_strncmp(str, cmp_str_len + 1, target_str) < 0)
 		__sync_add_and_fetch(&hits, 1);
 	return 0;
 }
-- 
2.48.1