[PATCH bpf-next v4 3/6] bpf: Move is_valid_bpf_tramp_flags() to the public trampoline code

[Alexei Starovoitov]

On Tue, May 17, 2022 at 03:18:35AM -0400, Xu Kuohai wrote:
>  
> +static bool is_valid_bpf_tramp_flags(unsigned int flags)
> +{
> +	if ((flags & BPF_TRAMP_F_RESTORE_REGS) &&
> +	    (flags & BPF_TRAMP_F_SKIP_FRAME))
> +		return false;
> +
> +	/* BPF_TRAMP_F_RET_FENTRY_RET is only used by bpf_struct_ops,
> +	 * and it must be used alone.
> +	 */
> +	if ((flags & BPF_TRAMP_F_RET_FENTRY_RET) &&
> +	    (flags & ~BPF_TRAMP_F_RET_FENTRY_RET))
> +		return false;
> +
> +	return true;
> +}
> +
> +int bpf_prepare_trampoline(struct bpf_tramp_image *tr, void *image,
> +			   void *image_end, const struct btf_func_model *m,
> +			   u32 flags, struct bpf_tramp_links *tlinks,
> +			   void *orig_call)
> +{
> +	if (!is_valid_bpf_tramp_flags(flags))
> +		return -EINVAL;
> +
> +	return arch_prepare_bpf_trampoline(tr, image, image_end, m, flags,
> +					   tlinks, orig_call);
> +}

It's an overkill to introduce a new helper function just to validate
flags that almost compile time constants.
The flags are not user supplied.
Please move /* BPF_TRAMP_F_RET_FENTRY_RET is only used by bpf_struct_ops ... */
comment to bpf_struct_ops.c right before it calls arch_prepare_bpf_trampoline()
And add a comment to trampoline.c saying that BPF_TRAMP_F_RESTORE_REGS
and BPF_TRAMP_F_SKIP_FRAME should not be set together.
We could add a warn_on there or in arch code, but feels like overkill.