Merge branch 'btf-arbitrary-__attribute__-encoding'

Ihor Solodrai says:

====================
BTF: arbitrary __attribute__ encoding

This patch series extends BPF Type Format (BTF) to support arbitrary
__attribute__ encoding.

Setting the kind_flag to 1 in BTF type tags and decl tags now changes
the meaning for the encoded tag, in particular with respect to
btf_dump in libbpf.

If the kflag is set, then the string encoded by the tag represents the
full attribute-list of an attribute specifier [1].

This feature will allow extending tools such as pahole and bpftool to
capture and use more granular type information, and make it easier to
manage compatibility between clang and gcc BPF compilers.

[1] https://gcc.gnu.org/onlinedocs/gcc-13.2.0/gcc/Attribute-Syntax.html

v2->v3: nit fixes suggested by Andrii
v1->v2:
  - When checking for specific BTF tags in the verifier, make sure the
    tag's kflag is 0
  - Split docs and libbpf changes into separate patches
  - Various renames, as suggested by Andrii and Eduard

v2: https://lore.kernel.org/bpf/20250127233955.2275804-1-ihor.solodrai@linux.dev/
v1: https://lore.kernel.org/bpf/20250122025308.2717553-1-ihor.solodrai@pm.me
====================

Link: https://patch.msgid.link/20250130201239.1429648-1-ihor.solodrai@linux.dev
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

1 files changed, 63 insertions, 23 deletions

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 560b519f820e2..eea99c766a207 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -2090,7 +2090,7 @@ static int validate_type_id(int id)
 }
 
 /* generic append function for PTR, TYPEDEF, CONST/VOLATILE/RESTRICT */
-static int btf_add_ref_kind(struct btf *btf, int kind, const char *name, int ref_type_id)
+static int btf_add_ref_kind(struct btf *btf, int kind, const char *name, int ref_type_id, int kflag)
 {
 	struct btf_type *t;
 	int sz, name_off = 0;
@@ -2113,7 +2113,7 @@ static int btf_add_ref_kind(struct btf *btf, int kind, const char *name, int ref
 	}
 
 	t->name_off = name_off;
-	t->info = btf_type_info(kind, 0, 0);
+	t->info = btf_type_info(kind, 0, kflag);
 	t->type = ref_type_id;
 
 	return btf_commit_type(btf, sz);
@@ -2128,7 +2128,7 @@ static int btf_add_ref_kind(struct btf *btf, int kind, const char *name, int ref
  */
 int btf__add_ptr(struct btf *btf, int ref_type_id)
 {
-	return btf_add_ref_kind(btf, BTF_KIND_PTR, NULL, ref_type_id);
+	return btf_add_ref_kind(btf, BTF_KIND_PTR, NULL, ref_type_id, 0);
 }
 
 /*
@@ -2506,7 +2506,7 @@ int btf__add_fwd(struct btf *btf, const char *name, enum btf_fwd_kind fwd_kind)
 		struct btf_type *t;
 		int id;
 
-		id = btf_add_ref_kind(btf, BTF_KIND_FWD, name, 0);
+		id = btf_add_ref_kind(btf, BTF_KIND_FWD, name, 0, 0);
 		if (id <= 0)
 			return id;
 		t = btf_type_by_id(btf, id);
@@ -2536,7 +2536,7 @@ int btf__add_typedef(struct btf *btf, const char *name, int ref_type_id)
 	if (!name || !name[0])
 		return libbpf_err(-EINVAL);
 
-	return btf_add_ref_kind(btf, BTF_KIND_TYPEDEF, name, ref_type_id);
+	return btf_add_ref_kind(btf, BTF_KIND_TYPEDEF, name, ref_type_id, 0);
 }
 
 /*
@@ -2548,7 +2548,7 @@ int btf__add_typedef(struct btf *btf, const char *name, int ref_type_id)
  */
 int btf__add_volatile(struct btf *btf, int ref_type_id)
 {
-	return btf_add_ref_kind(btf, BTF_KIND_VOLATILE, NULL, ref_type_id);
+	return btf_add_ref_kind(btf, BTF_KIND_VOLATILE, NULL, ref_type_id, 0);
 }
 
 /*
@@ -2560,7 +2560,7 @@ int btf__add_volatile(struct btf *btf, int ref_type_id)
  */
 int btf__add_const(struct btf *btf, int ref_type_id)
 {
-	return btf_add_ref_kind(btf, BTF_KIND_CONST, NULL, ref_type_id);
+	return btf_add_ref_kind(btf, BTF_KIND_CONST, NULL, ref_type_id, 0);
 }
 
 /*
@@ -2572,7 +2572,7 @@ int btf__add_const(struct btf *btf, int ref_type_id)
  */
 int btf__add_restrict(struct btf *btf, int ref_type_id)
 {
-	return btf_add_ref_kind(btf, BTF_KIND_RESTRICT, NULL, ref_type_id);
+	return btf_add_ref_kind(btf, BTF_KIND_RESTRICT, NULL, ref_type_id, 0);
 }
 
 /*
@@ -2588,7 +2588,24 @@ int btf__add_type_tag(struct btf *btf, const char *value, int ref_type_id)
 	if (!value || !value[0])
 		return libbpf_err(-EINVAL);
 
-	return btf_add_ref_kind(btf, BTF_KIND_TYPE_TAG, value, ref_type_id);
+	return btf_add_ref_kind(btf, BTF_KIND_TYPE_TAG, value, ref_type_id, 0);
+}
+
+/*
+ * Append new BTF_KIND_TYPE_TAG type with:
+ *   - *value*, non-empty/non-NULL tag value;
+ *   - *ref_type_id* - referenced type ID, it might not exist yet;
+ * Set info->kflag to 1, indicating this tag is an __attribute__
+ * Returns:
+ *   - >0, type ID of newly added BTF type;
+ *   - <0, on error.
+ */
+int btf__add_type_attr(struct btf *btf, const char *value, int ref_type_id)
+{
+	if (!value || !value[0])
+		return libbpf_err(-EINVAL);
+
+	return btf_add_ref_kind(btf, BTF_KIND_TYPE_TAG, value, ref_type_id, 1);
 }
 
 /*
@@ -2610,7 +2627,7 @@ int btf__add_func(struct btf *btf, const char *name,
 	    linkage != BTF_FUNC_EXTERN)
 		return libbpf_err(-EINVAL);
 
-	id = btf_add_ref_kind(btf, BTF_KIND_FUNC, name, proto_type_id);
+	id = btf_add_ref_kind(btf, BTF_KIND_FUNC, name, proto_type_id, 0);
 	if (id > 0) {
 		struct btf_type *t = btf_type_by_id(btf, id);
 
@@ -2845,18 +2862,8 @@ int btf__add_datasec_var_info(struct btf *btf, int var_type_id, __u32 offset, __
 	return 0;
 }
 
-/*
- * Append new BTF_KIND_DECL_TAG type with:
- *   - *value* - non-empty/non-NULL string;
- *   - *ref_type_id* - referenced type ID, it might not exist yet;
- *   - *component_idx* - -1 for tagging reference type, otherwise struct/union
- *     member or function argument index;
- * Returns:
- *   - >0, type ID of newly added BTF type;
- *   - <0, on error.
- */
-int btf__add_decl_tag(struct btf *btf, const char *value, int ref_type_id,
-		 int component_idx)
+static int btf_add_decl_tag(struct btf *btf, const char *value, int ref_type_id,
+			    int component_idx, int kflag)
 {
 	struct btf_type *t;
 	int sz, value_off;
@@ -2880,13 +2887,46 @@ int btf__add_decl_tag(struct btf *btf, const char *value, int ref_type_id,
 		return value_off;
 
 	t->name_off = value_off;
-	t->info = btf_type_info(BTF_KIND_DECL_TAG, 0, false);
+	t->info = btf_type_info(BTF_KIND_DECL_TAG, 0, kflag);
 	t->type = ref_type_id;
 	btf_decl_tag(t)->component_idx = component_idx;
 
 	return btf_commit_type(btf, sz);
 }
 
+/*
+ * Append new BTF_KIND_DECL_TAG type with:
+ *   - *value* - non-empty/non-NULL string;
+ *   - *ref_type_id* - referenced type ID, it might not exist yet;
+ *   - *component_idx* - -1 for tagging reference type, otherwise struct/union
+ *     member or function argument index;
+ * Returns:
+ *   - >0, type ID of newly added BTF type;
+ *   - <0, on error.
+ */
+int btf__add_decl_tag(struct btf *btf, const char *value, int ref_type_id,
+		      int component_idx)
+{
+	return btf_add_decl_tag(btf, value, ref_type_id, component_idx, 0);
+}
+
+/*
+ * Append new BTF_KIND_DECL_TAG type with:
+ *   - *value* - non-empty/non-NULL string;
+ *   - *ref_type_id* - referenced type ID, it might not exist yet;
+ *   - *component_idx* - -1 for tagging reference type, otherwise struct/union
+ *     member or function argument index;
+ * Set info->kflag to 1, indicating this tag is an __attribute__
+ * Returns:
+ *   - >0, type ID of newly added BTF type;
+ *   - <0, on error.
+ */
+int btf__add_decl_attr(struct btf *btf, const char *value, int ref_type_id,
+		       int component_idx)
+{
+	return btf_add_decl_tag(btf, value, ref_type_id, component_idx, 1);
+}
+
 struct btf_ext_sec_info_param {
 	__u32 off;
 	__u32 len;