Message-ID: <20181215221009.wryzhx3h4bmzmwtf@kafai-mbp>
Date:   Sat, 15 Dec 2018 22:10:12 +0000
From:   Martin Lau <kafai@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Yonghong Song <yhs@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next v2 2/8] bpf: btf: fix struct/union/fwd types with
 kind_flag

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@...com>
> > > Signed-off-by: Yonghong Song <yhs@...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; };"......

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