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

[Stanislav Fomichev <sdf@google.com>]

On 02/14, 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: Stanislav Fomichev <sdf@google.com>

> ---
>   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))

(thinking out loud)

Maybe time for us to put these into some common headers in the
selftests.
progs/test_ksyms_btf_null_check.c READ_ONCE as well..

> +
>   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