[PATCH bpf-next v2 12/15] libbpf: rewrite btf datasec names starting from '?'

[Eduard Zingerman <eddyz87@gmail.com>]

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 removing '?' prefix and adding ".optional" suffix.
For example:

    DATASEC ?.struct_ops -> DATASEC .struct_ops.optional

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
---
 tools/lib/bpf/features.c        | 22 ++++++++++++++++++++++
 tools/lib/bpf/libbpf.c          | 30 +++++++++++++++++++++++++++++-
 tools/lib/bpf/libbpf_internal.h |  2 ++
 3 files changed, 53 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/features.c b/tools/lib/bpf/features.c
index 6b0738ad7063..4e783cc7fc4b 100644
--- a/tools/lib/bpf/features.c
+++ b/tools/lib/bpf/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)
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 157d28aea186..af0bfb987928 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -2828,6 +2828,11 @@ 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);
@@ -2837,9 +2842,10 @@ 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)
@@ -2851,6 +2857,7 @@ 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;
@@ -2876,6 +2883,8 @@ static int bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *btf)
 			char *name;
 
 			name = (char *)btf__name_by_offset(btf, t->name_off);
+			if (*name == '?')
+				*name++ = '_';
 			while (*name) {
 				if (*name == '.')
 					*name = '_';
@@ -2892,6 +2901,25 @@ 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))) {
+			/* remove '?' prefix and add '.optional' suffix for
+			 * DATASEC names staring from '?':
+			 *
+			 *   DATASEC ?.foo -> DATASEC .foo.optional
+			 */
+			const char *name;
+			char buf[256];
+			int str;
+
+			name = btf__name_by_offset(btf, t->name_off);
+			snprintf(buf, sizeof(buf), "%s.optional", &name[1] /* skip '?' */);
+			str = btf__add_str(btf, buf);
+			if (str < 0)
+				return str;
+
+			t = (struct btf_type *)btf__type_by_id(btf, i);
+			t->name_off = str;
 		} else if (!has_func && btf_is_func_proto(t)) {
 			/* replace FUNC_PROTO with ENUM */
 			vlen = btf_vlen(t);
diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
index ad936ac5e639..864b36177424 100644
--- a/tools/lib/bpf/libbpf_internal.h
+++ b/tools/lib/bpf/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,
 };
 
-- 
2.43.0