Subject: Re: [PATCH bpf-next 3/3] selftests/bpf: Add various tests to check helper access into ptr_to_btf_id.

From: David Vernet <[email-masked]>
Date: Mon, 13 Mar 2023 19:15:12 -0500

On Mon, Mar 13, 2023 at 04:58:45PM -0700, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <[email-masked]>
> 
> Add various tests to check helper access into ptr_to_btf_id.
> 
> Signed-off-by: Alexei Starovoitov <[email-masked]>

Thanks a lot for the quick turnaround on this.

LGTM, just left one small nit below.

Acked-by: David Vernet <[email-masked]>

> ---
>  .../selftests/bpf/progs/task_kfunc_failure.c  | 36 +++++++++++++++++++
>  .../selftests/bpf/progs/task_kfunc_success.c  |  4 +++
>  2 files changed, 40 insertions(+)
> 
> diff --git a/tools/testing/selftests/bpf/progs/task_kfunc_failure.c b/tools/testing/selftests/bpf/progs/task_kfunc_failure.c
> index 002c7f69e47f..27994d6b2914 100644
> --- a/tools/testing/selftests/bpf/progs/task_kfunc_failure.c
> +++ b/tools/testing/selftests/bpf/progs/task_kfunc_failure.c
> @@ -301,3 +301,39 @@ int BPF_PROG(task_kfunc_from_lsm_task_free, struct task_struct *task)
>  	bpf_task_release(acquired);
>  	return 0;
>  }
> +
> +SEC("tp_btf/task_newtask")
> +__failure __msg("access beyond the end of member comm")
> +int BPF_PROG(task_access_comm1, struct task_struct *task, u64 clone_flags)
> +{
> +	bpf_strncmp(task->comm, 17, "foo");

Instead of 17, can you do either TASK_COMM_LEN + 1, or
sizeof(task->comm) + 1, to make the test a bit less brittle? Applies to
the other testcases as well.

> +	return 0;
> +}
> +
> +SEC("tp_btf/task_newtask")
> +__failure __msg("access beyond the end of member comm")
> +int BPF_PROG(task_access_comm2, struct task_struct *task, u64 clone_flags)
> +{
> +	bpf_strncmp(task->comm + 1, 16, "foo");
> +	return 0;
> +}
> +
> +SEC("tp_btf/task_newtask")
> +__failure __msg("write into memory")
> +int BPF_PROG(task_access_comm3, struct task_struct *task, u64 clone_flags)
> +{
> +	bpf_probe_read_kernel(task->comm, 16, task->comm);
> +	return 0;
> +}
> +
> +SEC("fentry/__set_task_comm")
> +__failure __msg("R1 type=ptr_ expected")
> +int BPF_PROG(task_access_comm4, struct task_struct *task, const char *buf, bool exec)
> +{
> +	/*
> +	 * task->comm is a legacy ptr_to_btf_id. The verifier cannot guarantee
> +	 * its safety. Hence it cannot be accessed with normal load insns.
> +	 */
> +	bpf_strncmp(task->comm, 16, "foo");
> +	return 0;
> +}
> diff --git a/tools/testing/selftests/bpf/progs/task_kfunc_success.c b/tools/testing/selftests/bpf/progs/task_kfunc_success.c
> index aebc4bb14e7d..4f61596b0242 100644
> --- a/tools/testing/selftests/bpf/progs/task_kfunc_success.c
> +++ b/tools/testing/selftests/bpf/progs/task_kfunc_success.c
> @@ -207,6 +207,10 @@ int BPF_PROG(test_task_from_pid_invalid, struct task_struct *task, u64 clone_fla
>  	if (!is_test_kfunc_task())
>  		return 0;
>  
> +	bpf_strncmp(task->comm, 12, "foo");
> +	bpf_strncmp(task->comm, 16, "foo");
> +	bpf_strncmp(&task->comm[8], 4, "foo");
> +
>  	if (is_pid_lookup_valid(-1)) {
>  		err = 1;
>  		return 0;
> -- 
> 2.34.1
>