Re: ERROR: INT DW_ATE_unsigned_1 Error emitting BTF type

[Sedat Dilek <sedat.dilek@gmail.com>]

On Sat, Feb 6, 2021 at 8:17 PM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 2/6/21 10:10 AM, Sedat Dilek wrote:
> > On Sat, Feb 6, 2021 at 6:53 PM Yonghong Song <yhs@fb.com> wrote:
> >>
> >>
> >>
> >> On 2/6/21 8:24 AM, Mark Wieelard wrote:
> >>> Hi,
> >>>
> >>> On Sat, Feb 06, 2021 at 12:26:44AM -0800, Yonghong Song wrote:
> >>>> With the above vmlinux, the issue appears to be handling
> >>>> DW_ATE_signed_1, DW_ATE_unsigned_{1,24,40}.
> >>>>
> >>>> The following patch should fix the issue:
> >>>
> >>> That doesn't really make sense to me. Why is the compiler emitting a
> >>> DW_TAG_base_type that needs to be interpreted according to the
> >>> DW_AT_name attribute?
> >>>
> >>> If the issue is that the size of the base type cannot be expressed in
> >>> bytes then the DWARF spec provides the following option:
> >>>
> >>>       If the value of an object of the given type does not fully occupy
> >>>       the storage described by a byte size attribute, the base type
> >>>       entry may also have a DW_AT_bit_size and a DW_AT_data_bit_offset
> >>>       attribute, both of whose values are integer constant values (see
> >>>       Section 2.19 on page 55). The bit size attribute describes the
> >>>       actual size in bits used to represent values of the given
> >>>       type. The data bit offset attribute is the offset in bits from the
> >>>       beginning of the containing storage to the beginning of the
> >>>       value. Bits that are part of the offset are padding.  If this
> >>>       attribute is omitted a default data bit offset of zero is assumed.
> >>>
> >>> Would it be possible to use that encoding of those special types?  If
> >>
> >> I agree with you. I do not like comparing me as well. Unfortunately,
> >> there is no enough information in dwarf to find out actual information.
> >> The following is the dwarf dump with vmlinux (Sedat provided) for
> >> DW_ATE_unsigned_1.
> >>
> >> 0x000e97e9:   DW_TAG_base_type
> >>                   DW_AT_name      ("DW_ATE_unsigned_1")
> >>                   DW_AT_encoding  (DW_ATE_unsigned)
> >>                   DW_AT_byte_size (0x00)
> >>
> >> There is no DW_AT_bit_size and DW_AT_bit_offset for base type.
> >> AFAIK, these two attributes typically appear in struct/union members
> >> together with DW_AT_byte_size.
> >>
> >> Maybe compilers (clang in this case) can emit DW_AT_bit_size = 1
> >> and DW_AT_bit_offset = 0/7 (depending on big/little endian) and
> >> this case, we just test and get DW_AT_bit_size and it should work.
> >>
> >> But I think BTF does not need this (DW_ATE_unsigned_1) for now.
> >> I checked dwarf dump and it is mostly used for some arith operation
> >> encoded in dump (in this case, e.g., shift by 1 bit)
> >>
> >> 0x000015cf:   DW_TAG_base_type
> >>                   DW_AT_name      ("DW_ATE_unsigned_1")
> >>                   DW_AT_encoding  (DW_ATE_unsigned)
> >>                   DW_AT_byte_size (0x00)
> >>
> >> 0x00010ed9:         DW_TAG_formal_parameter
> >>                         DW_AT_location    (DW_OP_lit0, DW_OP_not,
> >> DW_OP_convert (0x000015cf) "DW_ATE_unsigned_1", DW_OP_convert
> >> (0x000015d4) "DW_ATE_unsigned_8", DW_OP_stack_value)
> >>                         DW_AT_abstract_origin     (0x00013984 "branch")
> >>
> >> Look at clang frontend, only the following types are encoded with
> >> unsigned dwarf type.
> >>
> >>     case BuiltinType::UShort:
> >>     case BuiltinType::UInt:
> >>     case BuiltinType::UInt128:
> >>     case BuiltinType::ULong:
> >>     case BuiltinType::WChar_U:
> >>     case BuiltinType::ULongLong:
> >>       Encoding = llvm::dwarf::DW_ATE_unsigned;
> >>       break;
> >>
> >>
> >>> not, can we try to come up with some extension that doesn't require
> >>> consumers to match magic names?
> >>>
> >
> > You want me to upload mlx5_core.ko?
>
> I just sent out a patch. You are cc'ed. I also attached in this email.
> Yes, it would be great if you can upload mlx5_core.ko so I can
> double check with this DW_ATE_unsigned_160 which is really usual.
>

Yupp, just built a new pahole :-).
Re-building linux-kernel...

Will upload mlx5_core.ko - need zstd-ed it before.

- Sedat -

> >
> > When looking with llvm-dwarf for DW_ATE_unsigned_160:
> >
> > 0x00d65616:   DW_TAG_base_type
> >                 DW_AT_name      ("DW_ATE_unsigned_160")
> >                 DW_AT_encoding  (DW_ATE_unsigned)
> >                 DW_AT_byte_size (0x14)
> >
> > If you need further information, please let me know.
> >
> > Thanks.
> >
> > - Sedat -
> >