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libbpf: Resolve unambigous forward declarations

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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@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221109142611.879983-3-eddyz87@gmail.com
This commit is contained in:
Eduard Zingerman 2022-11-09 16:26:10 +02:00 committed by Andrii Nakryiko
parent c302378bc1
commit 082108fd69
1 changed files with 139 additions and 4 deletions
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143
tools/lib/bpf/btf.c
View File

@ -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.
*