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Date: Mon, 11 Mar 2019 07:02:20 +0000
From: Yonghong Song <yhs@...com>
To: Andrii Nakryiko <andriin@...com>,
"andrii.nakryiko@...il.com" <andrii.nakryiko@...il.com>,
Kernel Team <Kernel-team@...com>,
"Alexei Starovoitov" <ast@...com>,
"acme@...nel.org" <acme@...nel.org>,
"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
"bpf@...r.kernel.org" <bpf@...r.kernel.org>,
"daniel@...earbox.net" <daniel@...earbox.net>
Subject: Re: [PATCH bpf-next 1/2] btf: resolve enum fwds in btf_dedup
On 3/10/19 5:44 PM, Andrii Nakryiko wrote:
> GCC and clang support enum forward declarations as an extension. Such
> forward-declared enums will be represented as normal BTF_KIND_ENUM types with
> vlen=0. This patch adds ability to resolve such enums to their corresponding
Thanks Andrii for reporting and trying to fix forward-declared enum
issue. The current approach to represent forward enum with
BTF_KIND_ENUM with vlen = 0 does work. "enum A {};" is illegal for C,
so vlen=0 must be a forward declaration.
But this is probably not the best since we have BTF_KIND_FWD to make
forward declaration explicit.
Currently BTF_KIND_FWD uses (btf_type.info >> 31) to distinguish
struct/union. If btf_type.info >> 31 == 0, it is a struct, otherwise
it is a union.
We could extend this to cover forward enum as well with backward
compability. We can extend by using both bit 30 and bit 31:
btf_type.info >> 31 (btf_type.info >> 30) & 0x1
0 0 <== struct
1 0 <== union
0 1 <== enum
Maybe this may be reasonable?
This will require pahole and llvm change to support, besides kernel
support. I will try to prototype llvm support tomorrow.
> fully defined enums. This helps to avoid duplicated BTF type graphs which only
> differ by some types referencing forward-declared enum vs full enum.
>
> One such example in kernel is enum irqchip_irq_state, defined in
> include/linux/interrupt.h and forward-declared in include/linux/irq.h. This
> causes entire struct task_struct and all referenced types to be duplicated in
> btf_dedup output. This patch eliminates such duplication cases.
>
> Signed-off-by: Andrii Nakryiko <andriin@...com>
> ---
> tools/lib/bpf/btf.c | 51 +++++++++++++++++++++++++++++++++------------
> 1 file changed, 38 insertions(+), 13 deletions(-)
>
> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index 1b8d8cdd3575..87e3020ac1bc 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -1602,16 +1602,12 @@ static bool btf_equal_int(struct btf_type *t1, struct btf_type *t2)
> /* Calculate type signature hash of ENUM. */
> static __u32 btf_hash_enum(struct btf_type *t)
> {
> - struct btf_enum *member = (struct btf_enum *)(t + 1);
> - __u32 vlen = BTF_INFO_VLEN(t->info);
> - __u32 h = btf_hash_common(t);
> - int i;
> + __u32 h;
>
> - for (i = 0; i < vlen; i++) {
> - h = hash_combine(h, member->name_off);
> - h = hash_combine(h, member->val);
> - member++;
> - }
> + /* don't hash vlen and enum members to support enum fwd resolving */
> + h = hash_combine(0, t->name_off);
> + h = hash_combine(h, t->info & ~0xffff);
> + h = hash_combine(h, t->size);
> return h;
> }
>
> @@ -1637,6 +1633,22 @@ static bool btf_equal_enum(struct btf_type *t1, struct btf_type *t2)
> return true;
> }
>
> +static inline bool btf_is_enum_fwd(struct btf_type *t)
> +{
> + return BTF_INFO_KIND(t->info) == BTF_KIND_ENUM &&
> + BTF_INFO_VLEN(t->info) == 0;
> +}
> +
> +static bool btf_compat_enum(struct btf_type *t1, struct btf_type *t2)
> +{
> + if (!btf_is_enum_fwd(t1) && !btf_is_enum_fwd(t2))
> + return btf_equal_enum(t1, t2);
> + /* ignore vlen when comparing */
> + return t1->name_off == t2->name_off &&
> + (t1->info & ~0xffff) == (t2->info & ~0xffff) &&
> + t1->size == t2->size;
> +}
> +
> /*
> * Calculate type signature hash of STRUCT/UNION, ignoring referenced type IDs,
> * as referenced type IDs equivalence is established separately during type
> @@ -1860,6 +1872,17 @@ static int btf_dedup_prim_type(struct btf_dedup *d, __u32 type_id)
> new_id = cand_node->type_id;
> break;
> }
> + if (d->opts.dont_resolve_fwds)
> + continue;
> + if (btf_compat_enum(t, cand)) {
> + if (btf_is_enum_fwd(t)) {
> + /* resolve fwd to full enum */
> + new_id = cand_node->type_id;
> + break;
> + }
> + /* resolve canonical enum fwd to full enum */
> + d->map[cand_node->type_id] = type_id;
> + }
> }
> break;
>
> @@ -2084,15 +2107,15 @@ static int btf_dedup_is_equiv(struct btf_dedup *d, __u32 cand_id,
> return fwd_kind == real_kind;
> }
>
> - if (cand_type->info != canon_type->info)
> - return 0;
> -
> switch (cand_kind) {
> case BTF_KIND_INT:
> return btf_equal_int(cand_type, canon_type);
>
> case BTF_KIND_ENUM:
> - return btf_equal_enum(cand_type, canon_type);
> + if (d->opts.dont_resolve_fwds)
> + return btf_equal_enum(cand_type, canon_type);
> + else
> + return btf_compat_enum(cand_type, canon_type);
>
> case BTF_KIND_FWD:
> return btf_equal_common(cand_type, canon_type);
> @@ -2103,6 +2126,8 @@ static int btf_dedup_is_equiv(struct btf_dedup *d, __u32 cand_id,
> case BTF_KIND_PTR:
> case BTF_KIND_TYPEDEF:
> case BTF_KIND_FUNC:
> + if (cand_type->info != canon_type->info)
> + return 0;
> return btf_dedup_is_equiv(d, cand_type->type, canon_type->type);
>
> case BTF_KIND_ARRAY: {
>
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