netdev - Re: [PATCH 3/5] bpf: Add strict alignment flag for BPF_PROG

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Message-ID: <59145EE6.3030409@iogearbox.net>
Date:   Thu, 11 May 2017 14:53:58 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     David Miller <davem@...emloft.net>, ast@...com
CC:     alexei.starovoitov@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH 3/5] bpf: Add strict alignment flag for BPF_PROG_LOAD.

On 05/10/2017 09:09 PM, David Miller wrote:
>
> Add a new field, "prog_flags", and an initial flag value
> BPF_F_STRCIT_ALIGNMENT.
>
> When set, the verifier will enforce strict pointer alignment
> regardless of the setting of CONFIG_EFFICIENT_UNALIGNED_ACCESS.
>
> The verifier, in this mode, will also use a fixed value of "2" in
> place of NET_IP_ALIGN.
>
> This facilitates test cases that will exercise and validate this part
> of the verifier even when run on architectures where alignment doesn't
> matter.
>
> Signed-off-by: David S. Miller <davem@...emloft.net>

[...]
> @@ -833,10 +838,12 @@ static int check_val_ptr_alignment(const struct bpf_reg_state *reg,
>   	return 0;
>   }
>
> +static bool strict_alignment;
> +
>   static int check_ptr_alignment(const struct bpf_reg_state *reg,
>   			       int off, int size)
>   {
> -	bool strict = false;
> +	bool strict = strict_alignment;
>
>   	if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS))
>   		strict = true;
> @@ -3574,6 +3581,10 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr)
>   	} else {
>   		log_level = 0;
>   	}
> +	if (attr->prog_flags & BPF_F_STRICT_ALIGNMENT)
> +		strict_alignment = true;
> +	else
> +		strict_alignment = false;

Just minor nit: Can we move this into struct bpf_verifier_env
here instead of global var? The only change it would need is
in check_ptr_alignment() to pass the env from check_mem_access().
check_ptr_alignment() can then infer this from env.

>   	ret = replace_map_fd_with_map_ptr(env);
>   	if (ret < 0)
> @@ -3679,6 +3690,7 @@ int bpf_analyzer(struct bpf_prog *prog, const struct bpf_ext_analyzer_ops *ops,
>   	mutex_lock(&bpf_verifier_lock);
>
>   	log_level = 0;
> +	strict_alignment = false;
>
>   	env->explored_states = kcalloc(env->prog->len,
>   				       sizeof(struct bpf_verifier_state_list *),

Rest looks good:

Acked-by: Daniel Borkmann <daniel@...earbox.net>