libbpf: Rewrite btf datasec names starting from '?'

Optional struct_ops maps are defined using question mark at the start
of the section name, e.g.:

    SEC("?.struct_ops")
    struct test_ops optional_map = { ... };

This commit teaches libbpf to detect if kernel allows '?' prefix
in datasec names, and if it doesn't then to rewrite such names
by replacing '?' with '_', e.g.:

    DATASEC ?.struct_ops -> DATASEC _.struct_ops

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240306104529.6453-13-eddyz87@gmail.com

src/features.c

@@ -147,6 +147,25 @@ static int probe_kern_btf_datasec(int token_fd)
 					     strs, sizeof(strs), token_fd));
 }
 
+static int probe_kern_btf_qmark_datasec(int token_fd)
+{
+	static const char strs[] = "\0x\0?.data";
+	/* static int a; */
+	__u32 types[] = {
+		/* int */
+		BTF_TYPE_INT_ENC(0, BTF_INT_SIGNED, 0, 32, 4),  /* [1] */
+		/* VAR x */                                     /* [2] */
+		BTF_TYPE_ENC(1, BTF_INFO_ENC(BTF_KIND_VAR, 0, 0), 1),
+		BTF_VAR_STATIC,
+		/* DATASEC ?.data */                            /* [3] */
+		BTF_TYPE_ENC(3, BTF_INFO_ENC(BTF_KIND_DATASEC, 0, 1), 4),
+		BTF_VAR_SECINFO_ENC(2, 0, 4),
+	};
+
+	return probe_fd(libbpf__load_raw_btf((char *)types, sizeof(types),
+					     strs, sizeof(strs), token_fd));
+}
+
 static int probe_kern_btf_float(int token_fd)
 {
 	static const char strs[] = "\0float";
@@ -534,6 +553,9 @@ static struct kern_feature_desc {
 	[FEAT_ARG_CTX_TAG] = {
 		"kernel-side __arg_ctx tag", probe_kern_arg_ctx_tag,
 	},
+	[FEAT_BTF_QMARK_DATASEC] = {
+		"BTF DATASEC names starting from '?'", probe_kern_btf_qmark_datasec,
+	},
 };
 
 bool feat_supported(struct kern_feature_cache *cache, enum kern_feature_id feat_id)

src/libbpf.c

@@ -2869,6 +2869,11 @@ static bool section_have_execinstr(struct bpf_object *obj, int idx)
 	return sh->sh_flags & SHF_EXECINSTR;
 }
 
+static bool starts_with_qmark(const char *s)
+{
+	return s && s[0] == '?';
+}
+
 static bool btf_needs_sanitization(struct bpf_object *obj)
 {
 	bool has_func_global = kernel_supports(obj, FEAT_BTF_GLOBAL_FUNC);
@@ -2878,9 +2883,10 @@ static bool btf_needs_sanitization(struct bpf_object *obj)
 	bool has_decl_tag = kernel_supports(obj, FEAT_BTF_DECL_TAG);
 	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);
 
 	return !has_func || !has_datasec || !has_func_global || !has_float ||
-	       !has_decl_tag || !has_type_tag || !has_enum64;
+	       !has_decl_tag || !has_type_tag || !has_enum64 || !has_qmark_datasec;
 }
 
 static int bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *btf)
@@ -2892,6 +2898,7 @@ static int bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *btf)
 	bool has_decl_tag = kernel_supports(obj, FEAT_BTF_DECL_TAG);
 	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);
 	int enum64_placeholder_id = 0;
 	struct btf_type *t;
 	int i, j, vlen;
@@ -2918,7 +2925,7 @@ static int bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *btf)
 
 			name = (char *)btf__name_by_offset(btf, t->name_off);
 			while (*name) {
-				if (*name == '.')
+				if (*name == '.' || *name == '?')
 					*name = '_';
 				name++;
 			}
@@ -2933,6 +2940,14 @@ static int bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *btf)
 				vt = (void *)btf__type_by_id(btf, v->type);
 				m->name_off = vt->name_off;
 			}
+		} else if (!has_qmark_datasec && btf_is_datasec(t) &&
+			   starts_with_qmark(btf__name_by_offset(btf, t->name_off))) {
+			/* replace '?' prefix with '_' for DATASEC names */
+			char *name;
+
+			name = (char *)btf__name_by_offset(btf, t->name_off);
+			if (name[0] == '?')
+				name[0] = '_';
 		} else if (!has_func && btf_is_func_proto(t)) {
 			/* replace FUNC_PROTO with ENUM */
 			vlen = btf_vlen(t);

src/libbpf_internal.h

@@ -374,6 +374,8 @@ enum kern_feature_id {
 	FEAT_UPROBE_MULTI_LINK,
 	/* Kernel supports arg:ctx tag (__arg_ctx) for global subprogs natively */
 	FEAT_ARG_CTX_TAG,
+	/* Kernel supports '?' at the front of datasec names */
+	FEAT_BTF_QMARK_DATASEC,
 	__FEAT_CNT,
 };