[PATCH bpf-next v2 4/8] bpftool: Skip __bpf_relocatable on verifier-rewritten context types

[Ihor Solodrai <ihor.solodrai@linux.dev>]

The verifier translates accesses to UAPI context and socket types,
such as __sk_buff and bpf_sock, into accesses to kernel structures.
It picks the kernel field by the instruction's offset and keeps the
base register. For example, in bpf_convert_ctx_access() [1]:

	switch (si->off) {
	...
	case offsetof(struct __sk_buff, mark):
		...
		*insn++ = BPF_LDX_MEM(BPF_W, si->dst_reg, si->src_reg,
				      bpf_target_off(struct sk_buff, mark,
						     4, target_size));

So these accesses require a load or store with an immediate offset
from an unmodified pointer. Their UAPI field offsets are fixed, so
CO-RE relocation is unnecessary. With preserve_access_index, the
compiler can instead emit pointer arithmetic, for example
for ((u8 *)&skb->mark)[1].  The verifier then rejects an otherwise
valid program.

Omit the attribute from the 13 such types when dumping C. Also omit it
from their nested anonymous records: keeping it there can root
relocations at an unnamed type, which libbpf cannot resolve.

The last addition to this set was bpf_sk_lookup, introduced in 2020 by
commit e9ddbb7707ff ("bpf: Introduce SK_LOOKUP program type with a
dedicated attach point"). Keep the set hardcoded in bpftool so the
exclusion also works with older kernels.

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/core/filter.c?h=v7.2#n9939

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
 tools/bpf/bpftool/btf.c | 84 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 82 insertions(+), 2 deletions(-)

diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
index 5a2c5341b736..5c88f665e6b9 100644
--- a/tools/bpf/bpftool/btf.c
+++ b/tools/bpf/bpftool/btf.c
@@ -6,6 +6,7 @@
 #endif
 #include <errno.h>
 #include <fcntl.h>
+#include <linux/bitmap.h>
 #include <linux/err.h>
 #include <stdbool.h>
 #include <stdio.h>
@@ -1092,22 +1093,101 @@ static struct sort_datum *sort_btf_c(const struct btf *btf)
 	return datums;
 }
 
+/*
+ * The verifier translates field accesses of some types into accesses
+ * to kernel structures, see bpf_convert_ctx_accesses().
+ * The translation keys on the immediate offset of each load or store,
+ * so offsets into these types must stay immediates.
+ */
+static const char * const btf_c_static_offset_types[] = {
+	"__sk_buff",
+	"bpf_perf_event_data",
+	"bpf_sk_lookup",
+	"bpf_sock",
+	"bpf_sock_addr",
+	"bpf_sock_ops",
+	"bpf_sockopt",
+	"bpf_sysctl",
+	"bpf_tcp_sock",
+	"bpf_xdp_sock",
+	"sk_msg_md",
+	"sk_reuseport_md",
+	"xdp_md",
+};
+
+static bool btf_c_needs_static_offset(const struct btf *btf, const struct btf_type *t)
+{
+	const char *name;
+	size_t i;
+
+	if (!btf_is_struct(t))
+		return false;
+
+	name = btf__name_by_offset(btf, t->name_off);
+	for (i = 0; i < ARRAY_SIZE(btf_c_static_offset_types); i++) {
+		if (!strcmp(name, btf_c_static_offset_types[i]))
+			return true;
+	}
+	return false;
+}
+
+/*
+ * Anonymous records within types that need static offsets must not start
+ * CO-RE access chains. Named records, typedefs and pointer targets are
+ * separate types.
+ */
+static void btf_c_mark_no_pai(const struct btf *btf, __u32 id, unsigned long *no_pai)
+{
+	const struct btf_type *t = btf__type_by_id(btf, id);
+	const struct btf_member *m = btf_members(t);
+	__u32 i;
+
+	__set_bit(id, no_pai);
+	for (i = 0; i < btf_vlen(t); i++, m++) {
+		const struct btf_type *mt;
+		__u32 mid = m->type;
+
+		mt = btf__type_by_id(btf, mid);
+		while (btf_is_array(mt) || btf_is_mod(mt)) {
+			mid = btf_is_array(mt) ? btf_array(mt)->type : mt->type;
+			mt = btf__type_by_id(btf, mid);
+		}
+		if (btf_is_composite(mt) && !mt->name_off && !test_bit(mid, no_pai))
+			btf_c_mark_no_pai(btf, mid, no_pai);
+	}
+}
+
 static int btf_c_annotate_records(struct btf *btf)
 {
 	__u32 i, cnt = btf__type_cnt(btf);
+	unsigned long *no_pai;
 	int err;
 
+	no_pai = bitmap_zalloc(cnt);
+	if (!no_pai)
+		return -ENOMEM;
+
+	for (i = 1; i < cnt; i++) {
+		if (btf_c_needs_static_offset(btf, btf__type_by_id(btf, i)))
+			btf_c_mark_no_pai(btf, i, no_pai);
+	}
+
 	for (i = 1; i < cnt; i++) {
 		const struct btf_type *t = btf__type_by_id(btf, i);
 
 		if (!btf_is_composite(t))
 			continue;
+		if (test_bit(i, no_pai))
+			continue;
 		err = btf__add_decl_attr(btf, "__bpf_relocatable", i, -1);
 		if (err < 0)
-			return err;
+			goto out;
 	}
 
-	return 0;
+	err = 0;
+out:
+	bitmap_free(no_pai);
+	return err;
 }
 
 static int dump_btf_c(struct btf *btf,
-- 
2.56.0