Re: [PATCH bpf-next] selftests/bpf: Add test for struct_ops __ref argument in any position

[bot+bpf-ci]

> diff --git a/tools/testing/selftests/bpf/progs/struct_ops_multi_args.c b/tools/testing/selftests/bpf/progs/struct_ops_multi_args.c
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/struct_ops_multi_args.c

[ ... ]

> +SEC("struct_ops/test_refcounted_multi")
> +__failure __msg("program with __ref argument cannot tail call")
> +int test_refcounted_multi(unsigned long long *ctx)
> +{

Will this test pass without Keisuke's fix [0] being applied first?

The stub function has task__nullable as the second argument and
task__ref as the third.  prepare_arg_info() populates the arg_info
array in order, so info[0] corresponds to the __nullable arg
(refcounted = false) and info[1] to the __ref arg (refcounted = true).

In check_struct_ops_btf_id(), the loop that sets has_refcounted_arg
currently reads:

    for (i = 0; i < st_ops_desc->arg_info[member_idx].cnt; i++) {
        if (st_ops_desc->arg_info[member_idx].info->refcounted) {

This always checks info[0].refcounted (the __nullable entry, which is
false), never info[i].refcounted.  The __ref argument at info[1] is
never examined, so has_refcounted_arg stays false and the tail call
is permitted.

Since the test expects __failure, but the program loads successfully
without the fix, the test will always fail when run on a tree that
does not include the companion verifier fix.

This patch may need to be folded into (or applied after) Keisuke's
fix so the two land together.

> +	/* ctx[2] is used because the refcounted variable is the third argument */
> +	struct task_struct *refcounted_task = (struct task_struct *)ctx[2];
> +
> +	bpf_task_release(refcounted_task);
> +	bpf_tail_call(ctx, &prog_array, 0);
> +
> +	return 0;
> +}

[ ... ]


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CI run summary: https://github.com/kernel-patches/bpf/actions/runs/23389590355