On 12/15/18 2:10 PM, Martin Lau wrote:
> On Sat, Dec 15, 2018 at 09:44:44AM -0800, Alexei Starovoitov wrote:
>> On Sat, Dec 15, 2018 at 04:37:06PM +0000, Martin Lau wrote:
>>> On Fri, Dec 14, 2018 at 03:34:27PM -0800, Yonghong Song wrote:
>>>> This patch fixed two issues with BTF. One is related to
>>>> struct/union bitfield encoding and the other is related to
>>>> forward type.
>>>>
>>>> Issue #1 and solution:
>>>> ======================
>>>>
>>>> Current btf encoding of bitfield follows what pahole generates.
>>>> For each bitfield, pahole will duplicate the type chain and
>>>> put the bitfield size at the final int or enum type.
>>>> Since the BTF enum type cannot encode bit size,
>>>> pahole workarounds the issue by generating
>>>> an int type whenever the enum bit size is not 32.
>>>>
>>>> For example,
>>>> -bash-4.4$ cat t.c
>>>> typedef int ___int;
>>>> enum A { A1, A2, A3 };
>>>> struct t {
>>>> int a[5];
>>>> ___int b:4;
>>>> volatile enum A c:4;
>>>> } g;
>>>> -bash-4.4$ gcc -c -O2 -g t.c
>>>> The current kernel supports the following BTF encoding:
>>>> $ pahole -JV t.o
>>>> [1] TYPEDEF ___int type_id=2
>>>> [2] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
>>>> [3] ENUM A size=4 vlen=3
>>>> A1 val=0
>>>> A2 val=1
>>>> A3 val=2
>>>> [4] STRUCT t size=24 vlen=3
>>>> a type_id=5 bits_offset=0
>>>> b type_id=9 bits_offset=160
>>>> c type_id=11 bits_offset=164
>>>> [5] ARRAY (anon) type_id=2 index_type_id=2 nr_elems=5
>>>> [6] INT sizetype size=8 bit_offset=0 nr_bits=64 encoding=(none)
>>>> [7] VOLATILE (anon) type_id=3
>>>> [8] INT int size=1 bit_offset=0 nr_bits=4 encoding=(none)
>>>> [9] TYPEDEF ___int type_id=8
>>>> [10] INT (anon) size=1 bit_offset=0 nr_bits=4 encoding=SIGNED
>>>> [11] VOLATILE (anon) type_id=10
>>>>
>>>> Two issues are in the above:
>>>> . by changing enum type to int, we lost the original
>>>> type information and this will not be ideal later
>>>> when we try to convert BTF to a header file.
>>>> . the type duplication for bitfields will cause
>>>> BTF bloat. Duplicated types cannot be deduplicated
>>>> later if the bitfield size is different.
>>>>
>>>> To fix this issue, this patch implemented a compatible
>>>> change for BTF struct type encoding:
>>>> . the bit 31 of struct_type->info, previously reserved,
>>>> now is used to indicate whether bitfield_size is
>>>> encoded in btf_member or not.
>>>> . if bit 31 of struct_type->info is set,
>>>> btf_member->offset will encode like:
>>>> bit 0 - 23: bit offset
>>>> bit 24 - 31: bitfield size
>>>> if bit 31 is not set, the old behavior is preserved:
>>>> bit 0 - 31: bit offset
>>>>
>>>> So if the struct contains a bit field, the maximum bit offset
>>>> will be reduced to (2^24 - 1) instead of MAX_UINT. The maximum
>>>> bitfield size will be 256 which is enough for today as maximum
>>>> bitfield in compiler can be 128 where int128 type is supported.
>>>>
>>>> This kernel patch intends to support the new BTF encoding:
>>>> $ pahole -JV t.o
>>>> [1] TYPEDEF ___int type_id=2
>>>> [2] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
>>>> [3] ENUM A size=4 vlen=3
>>>> A1 val=0
>>>> A2 val=1
>>>> A3 val=2
>>>> [4] STRUCT t kind_flag=1 size=24 vlen=3
>>>> a type_id=5 bitfield_size=0 bits_offset=0
>>>> b type_id=1 bitfield_size=4 bits_offset=160
>>>> c type_id=7 bitfield_size=4 bits_offset=164
>>>> [5] ARRAY (anon) type_id=2 index_type_id=2 nr_elems=5
>>>> [6] INT sizetype size=8 bit_offset=0 nr_bits=64 encoding=(none)
>>>> [7] VOLATILE (anon) type_id=3
>>>>
>>>> Issue #2 and solution:
>>>> ======================
>>>>
>>>> Current forward type in BTF does not specify whether the original
>>>> type is struct or union. This will not work for type pretty print
>>>> and BTF-to-header-file conversion as struct/union must be specified.
>>>> $ cat tt.c
>>>> struct t;
>>>> union u;
>>>> int foo(struct t *t, union u *u) { return 0; }
>>>> $ gcc -c -g -O2 tt.c
>>>> $ pahole -JV tt.o
>>>> [1] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
>>>> [2] FWD t type_id=0
>>>> [3] PTR (anon) type_id=2
>>>> [4] FWD u type_id=0
>>>> [5] PTR (anon) type_id=4
>>>>
>>>> To fix this issue, similar to issue #1, type->info bit 31
>>>> is used. If the bit is set, it is union type. Otherwise, it is
>>>> a struct type.
>>>>
>>>> $ pahole -JV tt.o
>>>> [1] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
>>>> [2] FWD t kind_flag=0 type_id=0
>>>> [3] PTR (anon) kind_flag=0 type_id=2
>>>> [4] FWD u kind_flag=1 type_id=0
>>>> [5] PTR (anon) kind_flag=0 type_id=4
>>>>
>>>> Pahole/LLVM change:
>>>> ===================
>>>>
>>>> The new kind_flag functionality has been implemented in pahole
>>>> and llvm:
>>>> https://github.com/yonghong-song/pahole/tree/bitfield
>>>> https://github.com/yonghong-song/llvm/tree/bitfield
>>>>
>>>> Note that pahole hasn't implemented func/func_proto kind
>>>> and .BTF.ext. So to print function signature with bpftool,
>>>> the llvm compiler should be used.
>>>>
>>>> Fixes: 69b693f0aefa ("bpf: btf: Introduce BPF Type Format (BTF)")
>>>> Signed-off-by: Martin KaFai Lau <ka...@fb.com>
>>>> Signed-off-by: Yonghong Song <y...@fb.com>
>>>> ---
>>>> include/uapi/linux/btf.h | 15 ++-
>>>> kernel/bpf/btf.c | 274 ++++++++++++++++++++++++++++++++++++---
>>>> 2 files changed, 267 insertions(+), 22 deletions(-)
>>>>
>>>> diff --git a/include/uapi/linux/btf.h b/include/uapi/linux/btf.h
>>>> index 14f66948fc95..34aba40ed926 100644
>>>> --- a/include/uapi/linux/btf.h
>>>> +++ b/include/uapi/linux/btf.h
>>>> @@ -34,7 +34,9 @@ struct btf_type {
>>>> * bits 0-15: vlen (e.g. # of struct's members)
>>>> * bits 16-23: unused
>>>> * bits 24-27: kind (e.g. int, ptr, array...etc)
>>>> - * bits 28-31: unused
>>>> + * bits 28-30: unused
>>>> + * bit 31: kind_flag, currently used by
>>>> + * struct, union and fwd
>>>> */
>>>> __u32 info;
>>>> /* "size" is used by INT, ENUM, STRUCT and UNION.
>>>> @@ -52,6 +54,7 @@ struct btf_type {
>>>>
>>>> #define BTF_INFO_KIND(info) (((info) >> 24) & 0x0f)
>>>> #define BTF_INFO_VLEN(info) ((info) & 0xffff)
>>>> +#define BTF_INFO_KFLAG(info) ((info) >> 31)
>>>>
>>>> #define BTF_KIND_UNKN 0 /* Unknown */
>>>> #define BTF_KIND_INT 1 /* Integer */
>>>> @@ -110,9 +113,17 @@ struct btf_array {
>>>> struct btf_member {
>>>> __u32 name_off;
>>>> __u32 type;
>>>> - __u32 offset; /* offset in bits */
>>>> + __u32 offset; /* [bitfield_size and] offset in bits */
>>>> };
>>>>
>>>> +/* If the type info kind_flag set, the btf_member.offset
>>>> + * contains both member bit offset and bitfield size, and
>>>> + * bitfield size will set for struct/union bitfield members.
>>>> + * Otherwise, it contains only bit offset.
>>>> + */
>>> nit. It may be better to move this comment to the btf_member.offset
>>> above.
>>>
>>>> +#define BTF_MEMBER_BITFIELD_SIZE(val) ((val) >> 24)
>>>> +#define BTF_MEMBER_BIT_OFFSET(val) ((val) & 0xffffff)
>>> After re-thinking this setup again, I still think
>>> having these macros in btf.h to also do the kflag checking
>>> would be nice.
>>>
>>> Unlike BTF_INFO_KIND() and BTF_INT_ENCODING() which don't
>>> depend on other facts, the btf.h raw user must check kflag
>>> anyway before calling BTF_MEMBER_BIT*().
>>> Forcing a kflag check before the user can access these convenient
>>> 0xfffff and >>24 conversions may enforce this kflag check to
>>> some extend.
>>>
>>> Since it is in uapi, it will not be easy to change later.
>>> The above concern could be overkill ;), just want to ensure
>>> it has been thought through a bit more here.
>>>
>>> It could be as easy as moving the new btf_member_bit*() from
>>> btf.c to here and remove these two macros (or move them back to btf.c).
>>
>> I think moving:
>> +static u32 btf_member_bitfield_size(const struct btf_type *struct_type,
>> + const struct btf_member *member)
>> +{
>> + return btf_type_kflag(struct_type) ?
>> BTF_MEMBER_BITFIELD_SIZE(member->offset)
>> + : 0;
>> +}
>>
>> into uapi/btf.h may or may not be useful for btf uapi users.
>> What are the chances that these static inline helpers will be
>> reused by BTF logic in libbpf or other libs?
>> At this point we don't know.
>
>> So I would keep btf.h minimal.
> ok. Make sense
>
>> I agree that BTF_MEMBER_BIT_OFFSET() shouldn't be reused blindly.
>> The users have to do BTF_INFO_KFLAG() check first.
>> But this is the case for pretty much all of BTF data structures.
> Other similar situation in btf.h (i.e. a single u32 field can be
> interpreted differently) has at least an union as an indication
> (e.g. size and type in btf_type)
>
> Here we cannot add the union (bitfield_offset:24 and bitfield_size:8)
> and we cannot change the name "offset" also. I am worry about
> m->offset will directly be used without checking the BTF_INFO_KFLAG().
>
> may be a "union { __u32 offset; __u32 bitsize_offset; };"......
The union with two __u32 is great idea. Maybe the
bitsize_offset becomes "bitfield_size_offset" to reflect
its real intention?
>
