linux-kernel - Re: [PATCH bpf-next 2/2] selftests/bpf: Add test for non-NULLable PTR_TO_BTF

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Date:   Fri, 2 Jun 2023 18:26:53 -0700
From:   Stanislav Fomichev <sdf@...gle.com>
To:     David Vernet <void@...ifault.com>
Cc:     bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, martin.lau@...ux.dev, song@...nel.org,
        yhs@...com, john.fastabend@...il.com, kpsingh@...nel.org,
        haoluo@...gle.com, jolsa@...nel.org, linux-kernel@...r.kernel.org,
        kernel-team@...a.com
Subject: Re: [PATCH bpf-next 2/2] selftests/bpf: Add test for non-NULLable PTR_TO_BTF_IDs

On 06/02, David Vernet wrote:
> In a recent patch, we taught the verifier that trusted PTR_TO_BTF_ID can
> never be NULL. This prevents the verifier from incorrectly failing to
> load certain programs where it gets confused and thinks a reference
> isn't dropped because it incorrectly assumes that a branch exists in
> which a NULL PTR_TO_BTF_ID pointer is never released.
> 
> This patch adds a testcase that verifies this cannot happen.
> 
> Signed-off-by: David Vernet <void@...ifault.com>

Acked-by: Stanislav Fomichev <sdf@...gle.com>

I hope someone else can look at the actual change. It looks good to
me conceptually, but not sure what other parts it might affect.

> ---
>  .../selftests/bpf/prog_tests/cpumask.c        |  1 +
>  .../selftests/bpf/progs/cpumask_success.c     | 24 +++++++++++++++++++
>  2 files changed, 25 insertions(+)
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/cpumask.c b/tools/testing/selftests/bpf/prog_tests/cpumask.c
> index cdf4acc18e4c..d89191440fb1 100644
> --- a/tools/testing/selftests/bpf/prog_tests/cpumask.c
> +++ b/tools/testing/selftests/bpf/prog_tests/cpumask.c
> @@ -70,5 +70,6 @@ void test_cpumask(void)
>  		verify_success(cpumask_success_testcases[i]);
>  	}
>  
> +	RUN_TESTS(cpumask_success);
>  	RUN_TESTS(cpumask_failure);
>  }
> diff --git a/tools/testing/selftests/bpf/progs/cpumask_success.c b/tools/testing/selftests/bpf/progs/cpumask_success.c
> index 2fcdd7f68ac7..602a88b03dbc 100644
> --- a/tools/testing/selftests/bpf/progs/cpumask_success.c
> +++ b/tools/testing/selftests/bpf/progs/cpumask_success.c
> @@ -5,6 +5,7 @@
>  #include <bpf/bpf_tracing.h>
>  #include <bpf/bpf_helpers.h>
>  
> +#include "bpf_misc.h"
>  #include "cpumask_common.h"
>  
>  char _license[] SEC("license") = "GPL";
> @@ -426,3 +427,26 @@ int BPF_PROG(test_global_mask_rcu, struct task_struct *task, u64 clone_flags)
>  
>  	return 0;
>  }
> +
> +SEC("tp_btf/task_newtask")
> +__success
> +int BPF_PROG(test_refcount_null_tracking, struct task_struct *task, u64 clone_flags)
> +{
> +	struct bpf_cpumask *mask1, *mask2;
> +
> +	mask1 = bpf_cpumask_create();
> +	mask2 = bpf_cpumask_create();
> +
> +	if (!mask1 || !mask2)
> +		goto free_masks_return;
> +
> +	bpf_cpumask_test_cpu(0, (const struct cpumask *)mask1);
> +	bpf_cpumask_test_cpu(0, (const struct cpumask *)mask2);
> +
> +free_masks_return:
> +	if (mask1)
> +		bpf_cpumask_release(mask1);
> +	if (mask2)
> +		bpf_cpumask_release(mask2);
> +	return 0;
> +}
> -- 
> 2.40.1
>