bpf: Add bpf_sys_close() helper.

Add bpf_sys_close() helper to be used by the syscall/loader program to close
intermediate FDs and other cleanup.
Note this helper must never be allowed inside fdget/fdput bracketing.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210514003623.28033-11-alexei.starovoitov@gmail.com

include/uapi/linux/bpf.h

@@ -4754,6 +4754,12 @@ union bpf_attr {
  * 		Find BTF type with given name and kind in vmlinux BTF or in module's BTFs.
  * 	Return
  * 		Returns btf_id and btf_obj_fd in lower and upper 32 bits.
+ *
+ * long bpf_sys_close(u32 fd)
+ * 	Description
+ * 		Execute close syscall for given FD.
+ * 	Return
+ * 		A syscall result.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -4924,6 +4930,7 @@ union bpf_attr {
 	FN(snprintf),			\
 	FN(sys_bpf),			\
 	FN(btf_find_by_name_kind),	\
+	FN(sys_close),			\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper

kernel/bpf/syscall.c

@@ -4578,6 +4578,23 @@ tracing_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 	return bpf_base_func_proto(func_id);
 }
 
+BPF_CALL_1(bpf_sys_close, u32, fd)
+{
+	/* When bpf program calls this helper there should not be
+	 * an fdget() without matching completed fdput().
+	 * This helper is allowed in the following callchain only:
+	 * sys_bpf->prog_test_run->bpf_prog->bpf_sys_close
+	 */
+	return close_fd(fd);
+}
+
+const struct bpf_func_proto bpf_sys_close_proto = {
+	.func		= bpf_sys_close,
+	.gpl_only	= false,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_ANYTHING,
+};
+
 static const struct bpf_func_proto *
 syscall_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 {
@@ -4586,6 +4603,8 @@ syscall_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_sys_bpf_proto;
 	case BPF_FUNC_btf_find_by_name_kind:
 		return &bpf_btf_find_by_name_kind_proto;
+	case BPF_FUNC_sys_close:
+		return &bpf_sys_close_proto;
 	default:
 		return tracing_prog_func_proto(func_id, prog);
 	}

tools/include/uapi/linux/bpf.h

@@ -4754,6 +4754,12 @@ union bpf_attr {
  * 		Find BTF type with given name and kind in vmlinux BTF or in module's BTFs.
  * 	Return
  * 		Returns btf_id and btf_obj_fd in lower and upper 32 bits.
+ *
+ * long bpf_sys_close(u32 fd)
+ * 	Description
+ * 		Execute close syscall for given FD.
+ * 	Return
+ * 		A syscall result.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -4924,6 +4930,7 @@ union bpf_attr {
 	FN(snprintf),			\
 	FN(sys_bpf),			\
 	FN(btf_find_by_name_kind),	\
+	FN(sys_close),			\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper