bpf: Allow union argument in trampoline based programs

Currently, functions with 'union' arguments cannot be traced with
fentry/fexit:

bpftrace -e 'fentry:release_pages { exit(); }' -v

The function release_pages arg0 type UNION is unsupported.

The type of the 'release_pages' arg0 is defined as:

typedef union {
	struct page **pages;
	struct folio **folios;
	struct encoded_page **encoded_pages;
} release_pages_arg __attribute__ ((__transparent_union__));

This patch relaxes the restriction by allowing function arguments of type
'union' to be traced in verifier.

Reviewed-by: Amery Hung <[email protected]>
Signed-off-by: Leon Hwang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: Asphaltt

include/linux/bpf.h

@@ -1128,7 +1128,7 @@ struct bpf_prog_offload {
  */
 #define MAX_BPF_FUNC_REG_ARGS 5
 
-/* The argument is a structure. */
+/* The argument is a structure or a union. */
 #define BTF_FMODEL_STRUCT_ARG		BIT(0)
 
 /* The argument is signed. */

kernel/bpf/btf.c

@@ -6751,7 +6751,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 	/* skip modifiers */
 	while (btf_type_is_modifier(t))
 		t = btf_type_by_id(btf, t->type);
-	if (btf_type_is_small_int(t) || btf_is_any_enum(t) || __btf_type_is_struct(t))
+	if (btf_type_is_small_int(t) || btf_is_any_enum(t) || btf_type_is_struct(t))
 		/* accessing a scalar */
 		return true;
 	if (!btf_type_is_ptr(t)) {
@@ -7323,7 +7323,7 @@ static int __get_type_size(struct btf *btf, u32 btf_id,
 	if (btf_type_is_ptr(t))
 		/* kernel size of pointer. Not BPF's size of pointer*/
 		return sizeof(void *);
-	if (btf_type_is_int(t) || btf_is_any_enum(t) || __btf_type_is_struct(t))
+	if (btf_type_is_int(t) || btf_is_any_enum(t) || btf_type_is_struct(t))
 		return t->size;
 	return -EINVAL;
 }
@@ -7332,7 +7332,7 @@ static u8 __get_type_fmodel_flags(const struct btf_type *t)
 {
 	u8 flags = 0;
 
-	if (__btf_type_is_struct(t))
+	if (btf_type_is_struct(t))
 		flags |= BTF_FMODEL_STRUCT_ARG;
 	if (btf_type_is_signed_int(t))
 		flags |= BTF_FMODEL_SIGNED_ARG;
@@ -7373,7 +7373,7 @@ int btf_distill_func_proto(struct bpf_verifier_log *log,
 		return -EINVAL;
 	}
 	ret = __get_type_size(btf, func->type, &t);
-	if (ret < 0 || __btf_type_is_struct(t)) {
+	if (ret < 0 || btf_type_is_struct(t)) {
 		bpf_log(log,
 			"The function %s return type %s is unsupported.\n",
 			tname, btf_type_str(t));