Re: [PATCH bpf-next] bpf: add a new bpf argument type ARG_CONST_MAP_PTR_OR_NULL

From: Daniel Borkmann <daniel@iogearbox.net>
To: Hangbin Liu <liuhangbin@gmail.com>, bpf@vger.kernel.org
Cc: netdev@vger.kernel.org,
	"Toke Høiland-Jørgensen" <toke@redhat.com>,
	"Jesper Dangaard Brouer" <brouer@redhat.com>,
	ast@kernel.org
Subject: Re: [PATCH bpf-next] bpf: add a new bpf argument type ARG_CONST_MAP_PTR_OR_NULL
Date: Thu, 16 Jul 2020 00:28:16 +0200	[thread overview]
Message-ID: <67a68a77-f287-1bb1-3221-24e8b3351958@iogearbox.net> (raw)
In-Reply-To: <20200715070001.2048207-1-liuhangbin@gmail.com>

On 7/15/20 9:00 AM, Hangbin Liu wrote:
> Add a new bpf argument type ARG_CONST_MAP_PTR_OR_NULL which could be
> used when we want to allow NULL pointer for map parameter. The bpf helper
> need to take care and check if the map is NULL when use this type.
> 
> Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>

Is this patch to be merged into the set in [0] for passing NULL ex_map as discussed?
Seems you sent out two incomplete sets?

   [0] https://lore.kernel.org/bpf/20200709013008.3900892-1-liuhangbin@gmail.com/T/#m99a8fa8ffe79d5f00d305c0800ad3abe619294f2

> ---
>   include/linux/bpf.h   |  1 +
>   kernel/bpf/verifier.c | 11 ++++++++---
>   2 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index c67c88ad35f8..9d4dbef3c943 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -253,6 +253,7 @@ enum bpf_arg_type {
>   	ARG_PTR_TO_ALLOC_MEM,	/* pointer to dynamically allocated memory */
>   	ARG_PTR_TO_ALLOC_MEM_OR_NULL,	/* pointer to dynamically allocated memory or NULL */
>   	ARG_CONST_ALLOC_SIZE_OR_ZERO,	/* number of allocated bytes requested */
> +	ARG_CONST_MAP_PTR_OR_NULL,	/* const argument used as pointer to bpf_map or NULL */
>   };
>   
>   /* type of values returned from helper functions */
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 3c1efc9d08fd..d3551a19853a 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -3849,9 +3849,13 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
>   		expected_type = SCALAR_VALUE;
>   		if (type != expected_type)
>   			goto err_type;
> -	} else if (arg_type == ARG_CONST_MAP_PTR) {
> +	} else if (arg_type == ARG_CONST_MAP_PTR ||
> +		   arg_type == ARG_CONST_MAP_PTR_OR_NULL) {
>   		expected_type = CONST_PTR_TO_MAP;
> -		if (type != expected_type)
> +		if (register_is_null(reg) &&
> +		    arg_type == ARG_CONST_MAP_PTR_OR_NULL)
> +			/* final test in check_stack_boundary() */;
> +		else if (type != expected_type)
>   			goto err_type;
>   	} else if (arg_type == ARG_PTR_TO_CTX ||
>   		   arg_type == ARG_PTR_TO_CTX_OR_NULL) {
> @@ -3957,7 +3961,8 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
>   		return -EFAULT;
>   	}
>   
> -	if (arg_type == ARG_CONST_MAP_PTR) {
> +	if (arg_type == ARG_CONST_MAP_PTR ||
> +	    (arg_type == ARG_CONST_MAP_PTR_OR_NULL && !register_is_null(reg))) {
>   		/* bpf_map_xxx(map_ptr) call: remember that map_ptr */
>   		meta->map_ptr = reg->map_ptr;

I would probably have the semantics a bit different in the sense that I would
update meta->map_ptr to the last ARG_CONST_MAP_PTR, meaning:

     meta->map_ptr = register_is_null(reg) ? NULL : reg->map_ptr;

>   	} else if (arg_type == ARG_PTR_TO_MAP_KEY) {
> 

In combination with the set, this also needs test_verifier selftests in order to
exercise BPF insn snippets for the good & [expected] bad case.

Thanks,
Daniel