Re: [PATCH bpf-next v2 2/8] bpf: btf: fix struct/union/fwd types with kind_flag

[Yonghong Song <yhs@fb.com>]

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 <kafai@fb.com>
>>>> Signed-off-by: Yonghong Song <yhs@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?

>