[PATCH 2/2] libbpf: add check if kernel supports kind flag and fix the bitfield members in union and structs if not

Timur Chernykh
2025-03-31T20:50:00Z

Signed-off-by: Timur Chernykh <tim.cherry.co@gmail.com>
---
tools/lib/bpf/features.c | 30 ++++++++++++++++
tools/lib/bpf/libbpf.c | 62 ++++++++++++++++++++++++++++++++-
tools/lib/bpf/libbpf_internal.h | 2 ++
3 files changed, 93 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/features.c b/tools/lib/bpf/features.c
index 760657f5224c..dfab65f30f0c 100644
--- a/tools/lib/bpf/features.c
+++ b/tools/lib/bpf/features.c
@@ -507,6 +507,33 @@ static int probe_kern_arg_ctx_tag(int token_fd)
return probe_fd(prog_fd);
}

+static int probe_kern_btf_type_kind_flag(int token_fd)
+{
+ const char strs[] = "\0bpf_spin_lock\0val\0cnt\0l";
+ /* struct bpf_spin_lock {
+ * int val;
+ * };
+ * struct val {
+ * int cnt;
+ * struct bpf_spin_lock l;
+ * };
+ */
+ __u32 types[] = {
+ /* int */
+ BTF_TYPE_INT_ENC(0, BTF_INT_SIGNED, 0, 32, 4), /* [1] */
+ /* struct bpf_spin_lock */ /* [2] */
+ BTF_TYPE_ENC(1, BTF_INFO_ENC(BTF_KIND_STRUCT, 1 /* kind bit */, 1), 4),
+ BTF_MEMBER_ENC(15, 1, 0), /* int val; */
+ /* struct val */ /* [3] */
+ BTF_TYPE_ENC(15, BTF_INFO_ENC(BTF_KIND_STRUCT, 1 /* kind bit */, 2), 8),
+ BTF_MEMBER_ENC(19, 1, 0), /* int cnt; */
+ BTF_MEMBER_ENC(23, 2, 32),/* struct bpf_spin_lock l; */
+ };
+
+ return probe_fd(libbpf__load_raw_btf((char *)types, sizeof(types),
+ strs, sizeof(strs), token_fd));
+}
+
typedef int (*feature_probe_fn)(int /* token_fd */);

static struct kern_feature_cache feature_cache;
@@ -582,6 +609,9 @@ static struct kern_feature_desc {
[FEAT_BTF_QMARK_DATASEC] = {
"BTF DATASEC names starting from '?'", probe_kern_btf_qmark_datasec,
},
+ [FEAT_BTF_TYPE_KIND_FLAG] = {
+ "BTF btf_type can have the kind flags set", probe_kern_btf_type_kind_flag,
+ },
};

bool feat_supported(struct kern_feature_cache *cache, enum kern_feature_id feat_id)
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 8e1edba443dd..13ed8f03cc65 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -3113,9 +3113,10 @@ static bool btf_needs_sanitization(struct bpf_object *obj)
bool has_type_tag = kernel_supports(obj, FEAT_BTF_TYPE_TAG);
bool has_enum64 = kernel_supports(obj, FEAT_BTF_ENUM64);
bool has_qmark_datasec = kernel_supports(obj, FEAT_BTF_QMARK_DATASEC);
+ bool has_kind_bit_support = kernel_supports(obj, FEAT_BTF_TYPE_KIND_FLAG);

return !has_func || !has_datasec || !has_func_global || !has_float ||
- !has_decl_tag || !has_type_tag || !has_enum64 || !has_qmark_datasec;
+ !has_decl_tag || !has_type_tag || !has_enum64 || !has_qmark_datasec || !has_kind_bit_support;
}

static int bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *btf)
@@ -3128,6 +3129,7 @@ static int bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *btf)
bool has_type_tag = kernel_supports(obj, FEAT_BTF_TYPE_TAG);
bool has_enum64 = kernel_supports(obj, FEAT_BTF_ENUM64);
bool has_qmark_datasec = kernel_supports(obj, FEAT_BTF_QMARK_DATASEC);
+ bool has_kind_bit_support = kernel_supports(obj, FEAT_BTF_TYPE_KIND_FLAG);

char name_gen_buff[32] = {0};
int enum64_placeholder_id = 0;
@@ -3263,6 +3265,64 @@ static int bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *btf)
m->type = enum64_placeholder_id;
m->offset = 0;
}
+ } else if (!has_kind_bit_support && (btf_is_struct(t) || btf_is_union(t) || btf_is_fwd(t) || btf_is_enum(t) || btf_is_enum64(t))) {
+ const uint16_t members_cnt = btf_vlen(t);
+
+ /* type encoded with a kind flag */
+ if (t->info != BTF_INFO_ENC(btf_kind(t), 1, members_cnt)) {
+ continue;
+ }
+
+ /* unset kind flag anyway */
+ t->info = BTF_INFO_ENC(btf_kind(t), 0, btf_vlen(t));
+
+ /* structs an unions has a different bitfield processing behaviour is kind flag is set */
+ if (btf_is_struct(t) || btf_is_union(t)) {
+ struct btf_member* members = btf_members(t);
+ struct btf_type* new_int_type = NULL;
+ int new_int_type_id;
+ __u32* new_int_type_data;
+ int encoding = 0;
+ int nmember;
+
+ for (nmember = 0; nmember < members_cnt; nmember++) {
+ struct btf_member* member = &members[nmember];
+ const struct btf_type* member_type = btf_type_by_id(btf, member->type);
+
+ while (btf_is_typedef(member_type)) { /* unwrap typedefs */
+ member_type = btf_type_by_id(btf, member_type->type);
+ }
+
+ /* bitfields can be only int or enum values */
+ if (!(btf_is_int(member_type) || btf_is_enum(member_type))) {
+ continue;
+ }
+
+ encoding = btf_int_encoding(member_type);
+ if (btf_is_enum(member_type) && member_type->info & 0x80000000 /* kind flag */) {
+ /* enum value encodes integer signed/unsigned info in the kind flag */
+ encoding = BTF_INT_SIGNED;
+ }
+
+ /* create new integral type with the same info */
+ snprintf(name_gen_buff, sizeof(name_gen_buff), "__int_%d_%d", i, nmember);
+ new_int_type_id = btf__add_int(btf, name_gen_buff, member_type->size, encoding);
+
+ if (new_int_type_id < 0) {
+ pr_warn("Error adding integer type for a bitfield %d of [%d]", nmember, i);
+ return new_int_type_id;
+ }
+
+ new_int_type = btf_type_by_id(btf, new_int_type_id);
+
+ /* encode int in legacy way, keep offset 0 and specify bit size as set in the member */
+ new_int_type_data = (__u32*)(new_int_type + 1);
+ *new_int_type_data = BTF_INT_ENC(encoding, 0, BTF_MEMBER_BITFIELD_SIZE(member->offset));
+
+ member->type = new_int_type_id;
+ member->offset = BTF_MEMBER_BIT_OFFSET(member->offset) /* old kernels looks only on offset */;
+ }
+ }
}
}

diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
index 76669c73dcd1..6369c5520fce 100644
--- a/tools/lib/bpf/libbpf_internal.h
+++ b/tools/lib/bpf/libbpf_internal.h
@@ -380,6 +380,8 @@ enum kern_feature_id {
FEAT_ARG_CTX_TAG,
/* Kernel supports '?' at the front of datasec names */
FEAT_BTF_QMARK_DATASEC,
+ /* Kernel supports kind flag */
+ FEAT_BTF_TYPE_KIND_FLAG,
__FEAT_CNT,
};

--
2.49.0