[RFC v2 2/3] bpftool: add attribute preserve_static_offset for context types

[Eduard Zingerman <eddyz87@gmail.com>]

When printing vmlinux.h emit attribute preserve_static_offset [0] for
types that are used as context parameters for BPF programs. To avoid
hacking libbpf dump logic emit forward declarations annotated with
attribute. Such forward declarations have to come before structure
definitions.

Only emit such forward declarations when context types are present in
target BTF (identified by name).

C language standard wording in section "6.7.2.1 Structure and union
specifiers" [1] is vague, but example in 6.7.2.1.21 explicitly allows
such notation, and it matches clang behavior.

Here is how 'bpftool btf gen ... format c' looks after this change:

    #ifndef __VMLINUX_H__
    #define __VMLINUX_H__

    #if !defined(BPF_NO_PRESERVE_STATIC_OFFSET) && \
        __has_attribute(preserve_static_offset)
    #pragma clang attribute push \
              (__attribute__((preserve_static_offset)), apply_to = record)

    struct bpf_cgroup_dev_ctx;
    ...

    #pragma clang attribute pop
    #endif

    ... rest of the output unchanged ...

This is a follow up for discussion in thread [2].

[0] https://clang.llvm.org/docs/AttributeReference.html#preserve-static-offset
[1] https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3088.pdf
[2] https://lore.kernel.org/bpf/20231208000531.19179-1-eddyz87@gmail.com/

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
---
 tools/bpf/bpftool/btf.c | 131 +++++++++++++++++++++++++++++++++++-----
 1 file changed, 116 insertions(+), 15 deletions(-)

diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
index 91fcb75babe3..2abe71194afb 100644
--- a/tools/bpf/bpftool/btf.c
+++ b/tools/bpf/bpftool/btf.c
@@ -460,11 +460,118 @@ static void __printf(2, 0) btf_dump_printf(void *ctx,
 	vfprintf(stdout, fmt, args);
 }
 
+static const char * const context_types[] = {
+	"bpf_cgroup_dev_ctx",
+	"bpf_nf_ctx",
+	"bpf_perf_event_data",
+	"bpf_raw_tracepoint_args",
+	"bpf_sk_lookup",
+	"bpf_sock",
+	"bpf_sock_addr",
+	"bpf_sock_ops",
+	"bpf_sockopt",
+	"bpf_sysctl",
+	"__sk_buff",
+	"sk_msg_md",
+	"sk_reuseport_md",
+	"xdp_md",
+	"pt_regs",
+};
+
+static bool is_context_type_name(const struct btf *btf, const char *name)
+{
+	__u32 i;
+
+	for (i = 0; i < ARRAY_SIZE(context_types); ++i)
+		if (strcmp(name, context_types[i]) == 0)
+			return true;
+
+	return false;
+}
+
+/* When root_type_ids == NULL represents an iterator
+ * over all type ids in BTF: [1 .. btf__type_cnt(btf)].
+ *
+ * When root_type_ids != NULL represents an iterator
+ * over all type ids in root_type_ids array.
+ */
+struct root_type_iter {
+	__u32 *root_type_ids;
+	__u32 cnt;
+	__u32 pos;
+};
+
+static struct root_type_iter make_root_type_iter(const struct btf *btf,
+						 __u32 *root_type_ids, int root_type_cnt)
+{
+	if (root_type_cnt)
+		return (struct root_type_iter) { root_type_ids, root_type_cnt, 0 };
+
+	return (struct root_type_iter) { NULL, btf__type_cnt(btf), 1 };
+}
+
+static __u32 root_type_iter_next(struct root_type_iter *iter)
+{
+	if (iter->pos >= iter->cnt)
+		return 0;
+
+	if (iter->root_type_ids)
+		return iter->root_type_ids[iter->pos++];
+
+	return iter->pos++;
+}
+
+/* Iterate all types in 'btf', if there are types with name matching
+ * name of a BPF program context parameter type - emit a forward
+ * declaration for this type annotated with preserve_static_offset
+ * attribute [0].
+ *
+ * [0] https://clang.llvm.org/docs/AttributeReference.html#preserve-static-offset
+ */
+static void emit_static_offset_protos(const struct btf *btf, struct root_type_iter iter)
+{
+	bool first = true;
+	__u32 id;
+
+	while ((id = root_type_iter_next(&iter))) {
+		const struct btf_type *t;
+		const char *name;
+
+		t = btf__type_by_id(btf, id);
+		if (!t)
+			continue;
+
+		name = btf__name_by_offset(btf, t->name_off);
+		if (!name)
+			continue;
+
+		if (!btf_is_struct(t) || !is_context_type_name(btf, name))
+			continue;
+
+		if (first) {
+			first = false;
+			printf("#if !defined(BPF_NO_PRESERVE_STATIC_OFFSET) && __has_attribute(preserve_static_offset)\n");
+			printf("#pragma clang attribute push (__attribute__((preserve_static_offset)), apply_to = record)\n");
+			printf("\n");
+		}
+
+		printf("struct %s;\n", name);
+	}
+
+	if (!first) {
+		printf("\n");
+		printf("#pragma clang attribute pop\n");
+		printf("#endif /* BPF_NO_PRESERVE_STATIC_OFFSET */\n\n");
+	}
+}
+
 static int dump_btf_c(const struct btf *btf,
 		      __u32 *root_type_ids, int root_type_cnt)
 {
+	struct root_type_iter iter;
 	struct btf_dump *d;
-	int err = 0, i;
+	int err = 0;
+	__u32 id;
 
 	d = btf_dump__new(btf, btf_dump_printf, NULL, NULL);
 	if (!d)
@@ -473,24 +580,18 @@ static int dump_btf_c(const struct btf *btf,
 	printf("#ifndef __VMLINUX_H__\n");
 	printf("#define __VMLINUX_H__\n");
 	printf("\n");
+
+	emit_static_offset_protos(btf, make_root_type_iter(btf, root_type_ids, root_type_cnt));
+
 	printf("#ifndef BPF_NO_PRESERVE_ACCESS_INDEX\n");
 	printf("#pragma clang attribute push (__attribute__((preserve_access_index)), apply_to = record)\n");
 	printf("#endif\n\n");
 
-	if (root_type_cnt) {
-		for (i = 0; i < root_type_cnt; i++) {
-			err = btf_dump__dump_type(d, root_type_ids[i]);
-			if (err)
-				goto done;
-		}
-	} else {
-		int cnt = btf__type_cnt(btf);
-
-		for (i = 1; i < cnt; i++) {
-			err = btf_dump__dump_type(d, i);
-			if (err)
-				goto done;
-		}
+	iter = make_root_type_iter(btf, root_type_ids, root_type_cnt);
+	while ((id = root_type_iter_next(&iter))) {
+		err = btf_dump__dump_type(d, id);
+		if (err)
+			goto done;
 	}
 
 	printf("#ifndef BPF_NO_PRESERVE_ACCESS_INDEX\n");
-- 
2.42.1