[RFC PATCH bpf-next seccomp 11/12] bpf/verifier: support NULL-able ptr to BTF ID as helper argument

[YiFei Zhu <zhuyifei1999@gmail.com>]

From: YiFei Zhu <yifeifz2@illinois.edu>

This is to allow progs with no access to ptr to BPF ID still be
able to call some helpers, with these arguments set as NULL, so
the helper implementation may set a fallback when NULL is passed in.

Signed-off-by: YiFei Zhu <yifeifz2@illinois.edu>
---
 include/linux/bpf.h   |  1 +
 kernel/bpf/verifier.c | 12 +++++++++---
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 2019c0893250..efa6444b88d3 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -303,6 +303,7 @@ enum bpf_arg_type {
 	ARG_PTR_TO_SOCKET,	/* pointer to bpf_sock (fullsock) */
 	ARG_PTR_TO_SOCKET_OR_NULL,	/* pointer to bpf_sock (fullsock) or NULL */
 	ARG_PTR_TO_BTF_ID,	/* pointer to in-kernel struct */
+	ARG_PTR_TO_BTF_ID_OR_NULL,	/* pointer to in-kernel struct or NULL */
 	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 */
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 8eec1796caaa..8a08a27e0abc 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -484,7 +484,8 @@ static bool arg_type_may_be_null(enum bpf_arg_type type)
 	       type == ARG_PTR_TO_CTX_OR_NULL ||
 	       type == ARG_PTR_TO_SOCKET_OR_NULL ||
 	       type == ARG_PTR_TO_ALLOC_MEM_OR_NULL ||
-	       type == ARG_PTR_TO_STACK_OR_NULL;
+	       type == ARG_PTR_TO_STACK_OR_NULL ||
+	       type == ARG_PTR_TO_BTF_ID_OR_NULL;
 }
 
 /* Determine whether the function releases some resources allocated by another
@@ -4808,6 +4809,7 @@ static const struct bpf_reg_types *compatible_reg_types[__BPF_ARG_TYPE_MAX] = {
 	[ARG_PTR_TO_SOCKET]		= &fullsock_types,
 	[ARG_PTR_TO_SOCKET_OR_NULL]	= &fullsock_types,
 	[ARG_PTR_TO_BTF_ID]		= &btf_ptr_types,
+	[ARG_PTR_TO_BTF_ID_OR_NULL]	= &btf_ptr_types,
 	[ARG_PTR_TO_SPIN_LOCK]		= &spin_lock_types,
 	[ARG_PTR_TO_MEM]		= &mem_types,
 	[ARG_PTR_TO_MEM_OR_NULL]	= &mem_types,
@@ -5436,10 +5438,14 @@ static bool check_btf_id_ok(const struct bpf_func_proto *fn)
 	int i;
 
 	for (i = 0; i < ARRAY_SIZE(fn->arg_type); i++) {
-		if (fn->arg_type[i] == ARG_PTR_TO_BTF_ID && !fn->arg_btf_id[i])
+		if ((fn->arg_type[i] == ARG_PTR_TO_BTF_ID ||
+		     fn->arg_type[i] == ARG_PTR_TO_BTF_ID_OR_NULL) &&
+		    !fn->arg_btf_id[i])
 			return false;
 
-		if (fn->arg_type[i] != ARG_PTR_TO_BTF_ID && fn->arg_btf_id[i])
+		if ((fn->arg_type[i] != ARG_PTR_TO_BTF_ID &&
+		     fn->arg_type[i] != ARG_PTR_TO_BTF_ID_OR_NULL) &&
+		    fn->arg_btf_id[i])
 			return false;
 	}
 
-- 
2.31.1