Resolve forward declarations that don't take part in type graphs
comparisons if declaration name is unambiguous. Example:
CU #1:
struct foo; // standalone forward declaration
struct foo *some_global;
CU #2:
struct foo { int x; };
struct foo *another_global;
The `struct foo` from CU #1 is not a part of any definition that is
compared against another definition while `btf_dedup_struct_types`
processes structural types. The the BTF after `btf_dedup_struct_types`
the BTF looks as follows:
[1] STRUCT 'foo' size=4 vlen=1 ...
[2] INT 'int' size=4 ...
[3] PTR '(anon)' type_id=1
[4] FWD 'foo' fwd_kind=struct
[5] PTR '(anon)' type_id=4
This commit adds a new pass `btf_dedup_resolve_fwds`, that maps such
forward declarations to structs or unions with identical name in case
if the name is not ambiguous.
The pass is positioned before `btf_dedup_ref_types` so that types
[3] and [5] could be merged as a same type after [1] and [4] are merged.
The final result for the example above looks as follows:
[1] STRUCT 'foo' size=4 vlen=1
'x' type_id=2 bits_offset=0
[2] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[3] PTR '(anon)' type_id=1
For defconfig kernel with BTF enabled this removes 63 forward
declarations. Examples of removed declarations: `pt_regs`, `in6_addr`.
The running time of `btf__dedup` function is increased by about 3%.
Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
Reviewed-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
---
tools/lib/bpf/btf.c | 143 ++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 139 insertions(+), 4 deletions(-)
diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 442d4d0f98b8..3bd812bf88ff 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -2881,6 +2881,7 @@ static int btf_dedup_strings(struct btf_dedup *d);
static int btf_dedup_prim_types(struct btf_dedup *d);
static int btf_dedup_struct_types(struct btf_dedup *d);
static int btf_dedup_ref_types(struct btf_dedup *d);
+static int btf_dedup_resolve_fwds(struct btf_dedup *d);
static int btf_dedup_compact_types(struct btf_dedup *d);
static int btf_dedup_remap_types(struct btf_dedup *d);
@@ -2988,15 +2989,16 @@ static int btf_dedup_remap_types(struct btf_dedup *d);
* Algorithm summary
* =================
*
- * Algorithm completes its work in 6 separate passes:
+ * Algorithm completes its work in 7 separate passes:
*
* 1. Strings deduplication.
* 2. Primitive types deduplication (int, enum, fwd).
* 3. Struct/union types deduplication.
- * 4. Reference types deduplication (pointers, typedefs, arrays, funcs, func
+ * 4. Resolve unambiguous forward declarations.
+ * 5. Reference types deduplication (pointers, typedefs, arrays, funcs, func
* protos, and const/volatile/restrict modifiers).
- * 5. Types compaction.
- * 6. Types remapping.
+ * 6. Types compaction.
+ * 7. Types remapping.
*
* Algorithm determines canonical type descriptor, which is a single
* representative type for each truly unique type. This canonical type is the
@@ -3060,6 +3062,11 @@ int btf__dedup(struct btf *btf, const struct btf_dedup_opts *opts)
pr_debug("btf_dedup_struct_types failed:%d\n", err);
goto done;
}
+ err = btf_dedup_resolve_fwds(d);
+ if (err < 0) {
+ pr_debug("btf_dedup_resolve_fwds failed:%d\n", err);
+ goto done;
+ }
err = btf_dedup_ref_types(d);
if (err < 0) {
pr_debug("btf_dedup_ref_types failed:%d\n", err);
@@ -4501,6 +4508,134 @@ static int btf_dedup_ref_types(struct btf_dedup *d)
return 0;
}
+/*
+ * Collect a map from type names to type ids for all canonical structs
+ * and unions. If the same name is shared by several canonical types
+ * use a special value 0 to indicate this fact.
+ */
+static int btf_dedup_fill_unique_names_map(struct btf_dedup *d, struct hashmap *names_map)
+{
+ __u32 nr_types = btf__type_cnt(d->btf);
+ struct btf_type *t;
+ __u32 type_id;
+ __u16 kind;
+ int err;
+
+ /*
+ * Iterate over base and split module ids in order to get all
+ * available structs in the map.
+ */
+ for (type_id = 1; type_id < nr_types; ++type_id) {
+ t = btf_type_by_id(d->btf, type_id);
+ kind = btf_kind(t);
+
+ if (kind != BTF_KIND_STRUCT && kind != BTF_KIND_UNION)
+ continue;
+
+ /* Skip non-canonical types */
+ if (type_id != d->map[type_id])
+ continue;
+
+ err = hashmap__add(names_map, t->name_off, type_id);
+ if (err == -EEXIST)
+ err = hashmap__set(names_map, t->name_off, 0, NULL, NULL);
+
+ if (err)
+ return err;
+ }
+
+ return 0;
+}
+
+static int btf_dedup_resolve_fwd(struct btf_dedup *d, struct hashmap *names_map, __u32 type_id)
+{
+ struct btf_type *t = btf_type_by_id(d->btf, type_id);
+ enum btf_fwd_kind fwd_kind = btf_kflag(t);
+ __u16 cand_kind, kind = btf_kind(t);
+ struct btf_type *cand_t;
+ uintptr_t cand_id;
+
+ if (kind != BTF_KIND_FWD)
+ return 0;
+
+ /* Skip if this FWD already has a mapping */
+ if (type_id != d->map[type_id])
+ return 0;
+
+ if (!hashmap__find(names_map, t->name_off, &cand_id))
+ return 0;
+
+ /* Zero is a special value indicating that name is not unique */
+ if (!cand_id)
+ return 0;
+
+ cand_t = btf_type_by_id(d->btf, cand_id);
+ cand_kind = btf_kind(cand_t);
+ if ((cand_kind == BTF_KIND_STRUCT && fwd_kind != BTF_FWD_STRUCT) ||
+ (cand_kind == BTF_KIND_UNION && fwd_kind != BTF_FWD_UNION))
+ return 0;
+
+ d->map[type_id] = cand_id;
+
+ return 0;
+}
+
+/*
+ * Resolve unambiguous forward declarations.
+ *
+ * The lion's share of all FWD declarations is resolved during
+ * `btf_dedup_struct_types` phase when different type graphs are
+ * compared against each other. However, if in some compilation unit a
+ * FWD declaration is not a part of a type graph compared against
+ * another type graph that declaration's canonical type would not be
+ * changed. Example:
+ *
+ * CU #1:
+ *
+ * struct foo;
+ * struct foo *some_global;
+ *
+ * CU #2:
+ *
+ * struct foo { int u; };
+ * struct foo *another_global;
+ *
+ * After `btf_dedup_struct_types` the BTF looks as follows:
+ *
+ * [1] STRUCT 'foo' size=4 vlen=1 ...
+ * [2] INT 'int' size=4 ...
+ * [3] PTR '(anon)' type_id=1
+ * [4] FWD 'foo' fwd_kind=struct
+ * [5] PTR '(anon)' type_id=4
+ *
+ * This pass assumes that such FWD declarations should be mapped to
+ * structs or unions with identical name in case if the name is not
+ * ambiguous.
+ */
+static int btf_dedup_resolve_fwds(struct btf_dedup *d)
+{
+ int i, err;
+ struct hashmap *names_map;
+
+ names_map = hashmap__new(btf_dedup_identity_hash_fn, btf_dedup_equal_fn, NULL);
+ if (IS_ERR(names_map))
+ return PTR_ERR(names_map);
+
+ err = btf_dedup_fill_unique_names_map(d, names_map);
+ if (err < 0)
+ goto exit;
+
+ for (i = 0; i < d->btf->nr_types; i++) {
+ err = btf_dedup_resolve_fwd(d, names_map, d->btf->start_id + i);
+ if (err < 0)
+ break;
+ }
+
+exit:
+ hashmap__free(names_map);
+ return err;
+}
+
/*
* Compact types.
*
--
2.34.1
