[PATCH bpf-next v2 2/5] bpf: Preserve unique-field state across nested structs

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

btf_find_struct_field() initializes a fresh seen mask for every recursive
descent. Unique special fields in different levels of the same aggregate
therefore do not see one another. The duplicate fields can reach
btf_parse_fields(), where they trigger an invariant WARN_ON_ONCE(). A
crafted user BTF can consequently trigger the warning before map creation
checks capabilities.

Initialize the seen mask once in btf_find_field() and pass the same pointer
through struct, datasec, and nested-struct walks. This gives the entire field
traversal one shared uniqueness state.

Fixes: 64e8ee814819 ("bpf: look into the types of the fields of a struct type recursively.")
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 kernel/bpf/btf.c | 26 ++++++++++++++------------
 1 file changed, 14 insertions(+), 12 deletions(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index e7d4e9ba24e2..c577f00e9d88 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -3751,7 +3751,7 @@ static int btf_repeat_fields(struct btf_field_info *info, int info_cnt,
 static int btf_find_struct_field(const struct btf *btf,
 				 const struct btf_type *t, u32 field_mask,
 				 struct btf_field_info *info, int info_cnt,
-				 u32 level);
+				 u32 level, u32 *seen_mask);
 
 /* Find special fields in the struct type of a field.
  *
@@ -3762,7 +3762,7 @@ static int btf_find_struct_field(const struct btf *btf,
 static int btf_find_nested_struct(const struct btf *btf, const struct btf_type *t,
 				  u32 off, u32 nelems,
 				  u32 field_mask, struct btf_field_info *info,
-				  int info_cnt, u32 level)
+				  int info_cnt, u32 level, u32 *seen_mask)
 {
 	int ret, err, i;
 
@@ -3770,7 +3770,7 @@ static int btf_find_nested_struct(const struct btf *btf, const struct btf_type *
 	if (level >= MAX_RESOLVE_DEPTH)
 		return -E2BIG;
 
-	ret = btf_find_struct_field(btf, t, field_mask, info, info_cnt, level);
+	ret = btf_find_struct_field(btf, t, field_mask, info, info_cnt, level, seen_mask);
 
 	if (ret <= 0)
 		return ret;
@@ -3827,7 +3827,7 @@ static int btf_find_field_one(const struct btf *btf,
 		if (expected_size && expected_size != sz * nelems)
 			return 0;
 		ret = btf_find_nested_struct(btf, var_type, off, nelems, field_mask,
-					     &info[0], info_cnt, level);
+					     &info[0], info_cnt, level, seen_mask);
 		return ret;
 	}
 
@@ -3892,11 +3892,11 @@ static int btf_find_field_one(const struct btf *btf,
 static int btf_find_struct_field(const struct btf *btf,
 				 const struct btf_type *t, u32 field_mask,
 				 struct btf_field_info *info, int info_cnt,
-				 u32 level)
+				 u32 level, u32 *seen_mask)
 {
 	int ret, idx = 0;
 	const struct btf_member *member;
-	u32 i, off, seen_mask = 0;
+	u32 i, off;
 
 	for_each_member(i, t, member) {
 		const struct btf_type *member_type = btf_type_by_id(btf,
@@ -3910,7 +3910,7 @@ static int btf_find_struct_field(const struct btf *btf,
 
 		ret = btf_find_field_one(btf, t, member_type, i,
 					 off, 0,
-					 field_mask, &seen_mask,
+					 field_mask, seen_mask,
 					 &info[idx], info_cnt - idx, level);
 		if (ret < 0)
 			return ret;
@@ -3921,11 +3921,11 @@ static int btf_find_struct_field(const struct btf *btf,
 
 static int btf_find_datasec_var(const struct btf *btf, const struct btf_type *t,
 				u32 field_mask, struct btf_field_info *info,
-				int info_cnt, u32 level)
+				int info_cnt, u32 level, u32 *seen_mask)
 {
 	int ret, idx = 0;
 	const struct btf_var_secinfo *vsi;
-	u32 i, off, seen_mask = 0;
+	u32 i, off;
 
 	for_each_vsi(i, t, vsi) {
 		const struct btf_type *var = btf_type_by_id(btf, vsi->type);
@@ -3933,7 +3933,7 @@ static int btf_find_datasec_var(const struct btf *btf, const struct btf_type *t,
 
 		off = vsi->offset;
 		ret = btf_find_field_one(btf, var, var_type, -1, off, vsi->size,
-					 field_mask, &seen_mask,
+					 field_mask, seen_mask,
 					 &info[idx], info_cnt - idx,
 					 level);
 		if (ret < 0)
@@ -3947,10 +3947,12 @@ static int btf_find_field(const struct btf *btf, const struct btf_type *t,
 			  u32 field_mask, struct btf_field_info *info,
 			  int info_cnt)
 {
+	u32 seen_mask = 0;
+
 	if (__btf_type_is_struct(t))
-		return btf_find_struct_field(btf, t, field_mask, info, info_cnt, 0);
+		return btf_find_struct_field(btf, t, field_mask, info, info_cnt, 0, &seen_mask);
 	else if (btf_type_is_datasec(t))
-		return btf_find_datasec_var(btf, t, field_mask, info, info_cnt, 0);
+		return btf_find_datasec_var(btf, t, field_mask, info, info_cnt, 0, &seen_mask);
 	return -EINVAL;
 }
 
-- 
2.53.0