btf: Verify BTF sorting

This patch checks whether the BTF is sorted by name in ascending order.
If sorted, binary search will be used when looking up types.

Specifically, vmlinux and kernel module BTFs are always sorted during
the build phase with anonymous types placed before named types, so we
only need to identify the starting ID of named types.

Signed-off-by: Donglin Peng <pengdonglin@xiaomi.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260109130003.3313716-8-dolinux.peng@gmail.com

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index be8aa31e9e94225d37666db4063fe0d403abe206..d5b3a37c8560dea5393052065efc00cc3ce7de64 100644 (file)
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -550,6 +550,47 @@ u32 btf_nr_types(const struct btf *btf)
        return total;
 }
 
+/*
+ * Note that vmlinux and kernel module BTFs are always sorted
+ * during the building phase.
+ */
+static void btf_check_sorted(struct btf *btf)
+{
+       u32 i, n, named_start_id = 0;
+
+       n = btf_nr_types(btf);
+       if (btf_is_vmlinux(btf)) {
+               for (i = btf_start_id(btf); i < n; i++) {
+                       const struct btf_type *t = btf_type_by_id(btf, i);
+                       const char *n = btf_name_by_offset(btf, t->name_off);
+
+                       if (n[0] != '\0') {
+                               btf->named_start_id = i;
+                               return;
+                       }
+               }
+               return;
+       }
+
+       for (i = btf_start_id(btf) + 1; i < n; i++) {
+               const struct btf_type *ta = btf_type_by_id(btf, i - 1);
+               const struct btf_type *tb = btf_type_by_id(btf, i);
+               const char *na = btf_name_by_offset(btf, ta->name_off);
+               const char *nb = btf_name_by_offset(btf, tb->name_off);
+
+               if (strcmp(na, nb) > 0)
+                       return;
+
+               if (named_start_id == 0 && na[0] != '\0')
+                       named_start_id = i - 1;
+               if (named_start_id == 0 && nb[0] != '\0')
+                       named_start_id = i;
+       }
+
+       if (named_start_id)
+               btf->named_start_id = named_start_id;
+}
+
 /*
  * btf_named_start_id - Get the named starting ID for the BTF
  * @btf: Pointer to the target BTF object
@@ -6301,6 +6342,7 @@ static struct btf *btf_parse_base(struct btf_verifier_env *env, const char *name
        if (err)
                goto errout;
 
+       btf_check_sorted(btf);
        refcount_set(&btf->refcnt, 1);
 
        return btf;
@@ -6435,6 +6477,7 @@ static struct btf *btf_parse_module(const char *module_name, const void *data,
        }
 
        btf_verifier_env_free(env);
+       btf_check_sorted(btf);
        refcount_set(&btf->refcnt, 1);
        return btf;