bpf: Rename ARG_PTR_TO_KPTR -> ARG_KPTR_XCHG_DEST

ARG_PTR_TO_KPTR is currently only used by the bpf_kptr_xchg helper.
Although it limits reg types for that helper's first arg to
PTR_TO_MAP_VALUE, any arbitrary mapval won't do: further custom
verification logic ensures that the mapval reg being xchgd-into is
pointing to a kptr field. If this is not the case, it's not safe to xchg
into that reg's pointee.

Let's rename the bpf_arg_type to more accurately describe the fairly
specific expectations that this arg type encodes.

This is a nonfunctional change.

Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Signed-off-by: Amery Hung <amery.hung@bytedance.com>
Link: https://lore.kernel.org/r/20240813212424.2871455-4-amery.hung@bytedance.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: davemarchevsky

include/linux/bpf.h

@@ -744,7 +744,7 @@ enum bpf_arg_type {
 	ARG_PTR_TO_STACK,	/* pointer to stack */
 	ARG_PTR_TO_CONST_STR,	/* pointer to a null terminated read-only string */
 	ARG_PTR_TO_TIMER,	/* pointer to bpf_timer */
-	ARG_PTR_TO_KPTR,	/* pointer to referenced kptr */
+	ARG_KPTR_XCHG_DEST,	/* pointer to destination that kptrs are bpf_kptr_xchg'd into */
 	ARG_PTR_TO_DYNPTR,      /* pointer to bpf_dynptr. See bpf_type_flag for dynptr type */
 	__BPF_ARG_TYPE_MAX,
 

kernel/bpf/helpers.c

@@ -1636,7 +1636,7 @@ static const struct bpf_func_proto bpf_kptr_xchg_proto = {
 	.gpl_only     = false,
 	.ret_type     = RET_PTR_TO_BTF_ID_OR_NULL,
 	.ret_btf_id   = BPF_PTR_POISON,
-	.arg1_type    = ARG_PTR_TO_KPTR,
+	.arg1_type    = ARG_KPTR_XCHG_DEST,
 	.arg2_type    = ARG_PTR_TO_BTF_ID_OR_NULL | OBJ_RELEASE,
 	.arg2_btf_id  = BPF_PTR_POISON,
 };

kernel/bpf/verifier.c

@@ -8412,7 +8412,7 @@ static const struct bpf_reg_types func_ptr_types = { .types = { PTR_TO_FUNC } };
 static const struct bpf_reg_types stack_ptr_types = { .types = { PTR_TO_STACK } };
 static const struct bpf_reg_types const_str_ptr_types = { .types = { PTR_TO_MAP_VALUE } };
 static const struct bpf_reg_types timer_types = { .types = { PTR_TO_MAP_VALUE } };
-static const struct bpf_reg_types kptr_types = { .types = { PTR_TO_MAP_VALUE } };
+static const struct bpf_reg_types kptr_xchg_dest_types = { .types = { PTR_TO_MAP_VALUE } };
 static const struct bpf_reg_types dynptr_types = {
 	.types = {
 		PTR_TO_STACK,
@@ -8444,7 +8444,7 @@ static const struct bpf_reg_types *compatible_reg_types[__BPF_ARG_TYPE_MAX] = {
 	[ARG_PTR_TO_STACK]		= &stack_ptr_types,
 	[ARG_PTR_TO_CONST_STR]		= &const_str_ptr_types,
 	[ARG_PTR_TO_TIMER]		= &timer_types,
-	[ARG_PTR_TO_KPTR]		= &kptr_types,
+	[ARG_KPTR_XCHG_DEST]		= &kptr_xchg_dest_types,
 	[ARG_PTR_TO_DYNPTR]		= &dynptr_types,
 };
 
@@ -9044,7 +9044,7 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 			return err;
 		break;
 	}
-	case ARG_PTR_TO_KPTR:
+	case ARG_KPTR_XCHG_DEST:
 		err = process_kptr_func(env, regno, meta);
 		if (err)
 			return err;