Re: [PATCH bpf-next v2 04/14] selftests/bpf: Test returning referenced kptr from struct_ops programs

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2024-12-20 at 11:55 -0800, Amery Hung wrote:

[...]

> diff --git a/tools/testing/selftests/bpf/progs/struct_ops_kptr_return_fail__local_kptr.c b/tools/testing/selftests/bpf/progs/struct_ops_kptr_return_fail__local_kptr.c
> new file mode 100644
> index 000000000000..b8b4f05c3d7f
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/struct_ops_kptr_return_fail__local_kptr.c
> @@ -0,0 +1,34 @@
> +#include <vmlinux.h>
> +#include <bpf/bpf_tracing.h>
> +#include "../test_kmods/bpf_testmod.h"
> +#include "bpf_experimental.h"
> +#include "bpf_misc.h"
> +
> +char _license[] SEC("license") = "GPL";
> +
> +struct cgroup *bpf_cgroup_acquire(struct cgroup *p) __ksym;
> +void bpf_task_release(struct task_struct *p) __ksym;
> +
> +/* This test struct_ops BPF programs returning referenced kptr. The verifier should
> + * reject programs returning a local kptr.
> + */
> +SEC("struct_ops/test_return_ref_kptr")
> +__failure __msg("At program exit the register R0 is not a known value (ptr_or_null_)")
> +struct task_struct *BPF_PROG(kptr_return_fail__local_kptr, int dummy,
> +			     struct task_struct *task, struct cgroup *cgrp)
> +{
> +	struct task_struct *t;
> +
> +	bpf_task_release(task);
> +
> +	t = bpf_obj_new(typeof(*task));

The return type (and btf id) of the bpf_obj_new is 'void *',
but here a function with return type (and btf id) of
'struct task_struct *' is necessary.
For example:

-       t = bpf_obj_new(typeof(*task));
+       t = bpf_task_from_pid(32);

However, if this replacement is done the test does
not signal verification error because of insufficient
checks in the verifier.c:check_return_code().

> +	if (!t)
> +		return NULL;
> +
> +	return t;
> +}
> +
> +SEC(".struct_ops.link")
> +struct bpf_testmod_ops testmod_kptr_return = {
> +	.test_return_ref_kptr = (void *)kptr_return_fail__local_kptr,
> +};

[...]