bpf: Replace offsetof() with struct_size()

Compared to offsetof(), struct_size() provides additional compile-time
checks for structs with flexible arrays (e.g., __must_be_array()).

No functional changes intended.

Signed-off-by: Thorsten Blum <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: toblux

kernel/bpf/syscall.c

@@ -36,6 +36,7 @@
 #include <linux/memcontrol.h>
 #include <linux/trace_events.h>
 #include <linux/tracepoint.h>
+#include <linux/overflow.h>
 
 #include <net/netfilter/nf_bpf_link.h>
 #include <net/netkit.h>
@@ -693,7 +694,7 @@ struct btf_record *btf_record_dup(const struct btf_record *rec)
 
 	if (IS_ERR_OR_NULL(rec))
 		return NULL;
-	size = offsetof(struct btf_record, fields[rec->cnt]);
+	size = struct_size(rec, fields, rec->cnt);
 	new_rec = kmemdup(rec, size, GFP_KERNEL | __GFP_NOWARN);
 	if (!new_rec)
 		return ERR_PTR(-ENOMEM);
@@ -748,7 +749,7 @@ bool btf_record_equal(const struct btf_record *rec_a, const struct btf_record *r
 		return false;
 	if (rec_a->cnt != rec_b->cnt)
 		return false;
-	size = offsetof(struct btf_record, fields[rec_a->cnt]);
+	size = struct_size(rec_a, fields, rec_a->cnt);
 	/* btf_parse_fields uses kzalloc to allocate a btf_record, so unused
 	 * members are zeroed out. So memcmp is safe to do without worrying
 	 * about padding/unused fields.