Re: [PATCH bpf-next v2 4/8] bpf: Implement get_current_task_btf and RET_PTR_TO_BTF_ID

[Song Liu <songliubraving@fb.com>]

> On Nov 3, 2020, at 7:31 AM, KP Singh <kpsingh@chromium.org> wrote:
> 
> From: KP Singh <kpsingh@google.com>
> 
> The currently available bpf_get_current_task returns an unsigned integer
> which can be used along with BPF_CORE_READ to read data from
> the task_struct but still cannot be used as an input argument to a
> helper that accepts an ARG_PTR_TO_BTF_ID of type task_struct.
> 
> In order to implement this helper a new return type, RET_PTR_TO_BTF_ID,
> is added. This is similar to RET_PTR_TO_BTF_ID_OR_NULL but does not
> require checking the nullness of returned pointer.
> 
> Signed-off-by: KP Singh <kpsingh@google.com>

Acked-by: Song Liu <songliubraving@fb.com>

> ---
> include/linux/bpf.h            |  1 +
> include/uapi/linux/bpf.h       |  9 +++++++++
> kernel/bpf/verifier.c          |  7 +++++--
> kernel/trace/bpf_trace.c       | 16 ++++++++++++++++
> tools/include/uapi/linux/bpf.h |  9 +++++++++
> 5 files changed, 40 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 2fffd30e13ac..73d5381a5d5c 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -310,6 +310,7 @@ enum bpf_return_type {
> 	RET_PTR_TO_BTF_ID_OR_NULL,	/* returns a pointer to a btf_id or NULL */
> 	RET_PTR_TO_MEM_OR_BTF_ID_OR_NULL, /* returns a pointer to a valid memory or a btf_id or NULL */
> 	RET_PTR_TO_MEM_OR_BTF_ID,	/* returns a pointer to a valid memory or a btf_id */
> +	RET_PTR_TO_BTF_ID,		/* returns a pointer to a btf_id */
> };
> 
> /* eBPF function prototype used by verifier to allow BPF_CALLs from eBPF programs
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index f4037b2161a6..9879d6793e90 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -3779,6 +3779,14 @@ union bpf_attr {
>  *		0 on success.
>  *
>  *		**-ENOENT** if the bpf_local_storage cannot be found.
> + *
> + * struct task_struct *bpf_get_current_task_btf(void)
> + *	Description
> + *		Return a BTF pointer to the "current" task.
> + *		This pointer can also be used in helpers that accept an
> + *		*ARG_PTR_TO_BTF_ID* of type *task_struct*.
> + *	Return
> + *		Pointer to the current task.
>  */
> #define __BPF_FUNC_MAPPER(FN)		\
> 	FN(unspec),			\
> @@ -3939,6 +3947,7 @@ union bpf_attr {
> 	FN(redirect_peer),		\
> 	FN(task_storage_get),		\
> 	FN(task_storage_delete),	\
> +	FN(get_current_task_btf),	\
> 	/* */
> 
> /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index b0790876694f..314018e8fc12 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -5186,11 +5186,14 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
> 				PTR_TO_BTF_ID : PTR_TO_BTF_ID_OR_NULL;
> 			regs[BPF_REG_0].btf_id = meta.ret_btf_id;
> 		}
> -	} else if (fn->ret_type == RET_PTR_TO_BTF_ID_OR_NULL) {
> +	} else if (fn->ret_type == RET_PTR_TO_BTF_ID_OR_NULL ||
> +		   fn->ret_type == RET_PTR_TO_BTF_ID) {
> 		int ret_btf_id;
> 
> 		mark_reg_known_zero(env, regs, BPF_REG_0);
> -		regs[BPF_REG_0].type = PTR_TO_BTF_ID_OR_NULL;
> +		regs[BPF_REG_0].type = fn->ret_type == RET_PTR_TO_BTF_ID ?
> +						     PTR_TO_BTF_ID :
> +						     PTR_TO_BTF_ID_OR_NULL;
> 		ret_btf_id = *fn->ret_btf_id;
> 		if (ret_btf_id == 0) {
> 			verbose(env, "invalid return type %d of func %s#%d\n",
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 4517c8b66518..e4515b0f62a8 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1022,6 +1022,20 @@ const struct bpf_func_proto bpf_get_current_task_proto = {
> 	.ret_type	= RET_INTEGER,
> };
> 
> +BPF_CALL_0(bpf_get_current_task_btf)
> +{
> +	return (unsigned long) current;
> +}
> +
> +BTF_ID_LIST_SINGLE(bpf_get_current_btf_ids, struct, task_struct)
> +
> +static const struct bpf_func_proto bpf_get_current_task_btf_proto = {
> +	.func		= bpf_get_current_task_btf,
> +	.gpl_only	= true,
> +	.ret_type	= RET_PTR_TO_BTF_ID,
> +	.ret_btf_id	= &bpf_get_current_btf_ids[0],
> +};
> +
> BPF_CALL_2(bpf_current_task_under_cgroup, struct bpf_map *, map, u32, idx)
> {
> 	struct bpf_array *array = container_of(map, struct bpf_array, map);
> @@ -1265,6 +1279,8 @@ bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> 		return &bpf_get_current_pid_tgid_proto;
> 	case BPF_FUNC_get_current_task:
> 		return &bpf_get_current_task_proto;
> +	case BPF_FUNC_get_current_task_btf:
> +		return &bpf_get_current_task_btf_proto;
> 	case BPF_FUNC_get_current_uid_gid:
> 		return &bpf_get_current_uid_gid_proto;
> 	case BPF_FUNC_get_current_comm:
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index f4037b2161a6..9879d6793e90 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -3779,6 +3779,14 @@ union bpf_attr {
>  *		0 on success.
>  *
>  *		**-ENOENT** if the bpf_local_storage cannot be found.
> + *
> + * struct task_struct *bpf_get_current_task_btf(void)
> + *	Description
> + *		Return a BTF pointer to the "current" task.
> + *		This pointer can also be used in helpers that accept an
> + *		*ARG_PTR_TO_BTF_ID* of type *task_struct*.
> + *	Return
> + *		Pointer to the current task.
>  */
> #define __BPF_FUNC_MAPPER(FN)		\
> 	FN(unspec),			\
> @@ -3939,6 +3947,7 @@ union bpf_attr {
> 	FN(redirect_peer),		\
> 	FN(task_storage_get),		\
> 	FN(task_storage_delete),	\
> +	FN(get_current_task_btf),	\
> 	/* */
> 
> /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> -- 
> 2.29.1.341.ge80a0c044ae-goog
>