Re: [PATCH bpf-next] selftests/bpf: Fix map_kptr test.

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

On Wed, Feb 15, 2023 at 12:50:51AM CET, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.org>
>
> The compiler is optimizing out majority of unref_ptr read/writes, so the test
> wasn't testing much. For example, one could delete '__kptr' tag from
> 'struct prog_test_ref_kfunc __kptr *unref_ptr;' and the test would still "pass".
>
> Convert it to volatile stores. Confirmed by comparing bpf asm before/after.
>
> Fixes: 2cbc469a6fc3 ("selftests/bpf: Add C tests for kptr")
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> ---

Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

There's also the same test in the test_verifier suite, so there's still coverage
for this case.

>  tools/testing/selftests/bpf/progs/map_kptr.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/progs/map_kptr.c b/tools/testing/selftests/bpf/progs/map_kptr.c
> index eb8217803493..228ec45365a8 100644
> --- a/tools/testing/selftests/bpf/progs/map_kptr.c
> +++ b/tools/testing/selftests/bpf/progs/map_kptr.c
> @@ -62,21 +62,23 @@ extern struct prog_test_ref_kfunc *
>  bpf_kfunc_call_test_kptr_get(struct prog_test_ref_kfunc **p, int a, int b) __ksym;
>  extern void bpf_kfunc_call_test_release(struct prog_test_ref_kfunc *p) __ksym;
>
> +#define WRITE_ONCE(x, val) ((*(volatile typeof(x) *) &(x)) = (val))
> +
>  static void test_kptr_unref(struct map_value *v)
>  {
>  	struct prog_test_ref_kfunc *p;
>
>  	p = v->unref_ptr;
>  	/* store untrusted_ptr_or_null_ */
> -	v->unref_ptr = p;
> +	WRITE_ONCE(v->unref_ptr, p);
>  	if (!p)
>  		return;
>  	if (p->a + p->b > 100)
>  		return;
>  	/* store untrusted_ptr_ */
> -	v->unref_ptr = p;
> +	WRITE_ONCE(v->unref_ptr, p);
>  	/* store NULL */
> -	v->unref_ptr = NULL;
> +	WRITE_ONCE(v->unref_ptr, NULL);
>  }
>
>  static void test_kptr_ref(struct map_value *v)
> @@ -85,7 +87,7 @@ static void test_kptr_ref(struct map_value *v)
>
>  	p = v->ref_ptr;
>  	/* store ptr_or_null_ */
> -	v->unref_ptr = p;
> +	WRITE_ONCE(v->unref_ptr, p);
>  	if (!p)
>  		return;
>  	if (p->a + p->b > 100)
> @@ -99,7 +101,7 @@ static void test_kptr_ref(struct map_value *v)
>  		return;
>  	}
>  	/* store ptr_ */
> -	v->unref_ptr = p;
> +	WRITE_ONCE(v->unref_ptr, p);
>  	bpf_kfunc_call_test_release(p);
>
>  	p = bpf_kfunc_call_test_acquire(&(unsigned long){0});
> --
> 2.30.2
>