Re: ERROR: INT DW_ATE_unsigned_1 Error emitting BTF type

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

On Sat, Feb 6, 2021 at 8:33 PM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 2/6/21 11:28 AM, Sedat Dilek wrote:
> > On Sat, Feb 6, 2021 at 8:22 PM Sedat Dilek <sedat.dilek@gmail.com> wrote:
> >>
> >> 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.
> >>
> >
> > Hmm, I guess you want a mlx5_core.ko with your patch applied-to-pahole-1.20 :-)?
>
> this should work too. I want to check dwarf data. My patch won't impact
> dwarf generation.
>

Usual Dropbox-Link:

https://www.dropbox.com/sh/kvyh8ps7na0r1h5/AABfyNfDZ2bESse_bo4h05fFa?dl=0

See "for-yhs" directory:

1. mlx5-module_yhs-v1 ("[PATCH dwarves] btf_encoder: sanitize
non-regular int base type")
2. mlx5-module_yhs-dileks-v4 (with the last diff-v4 I tried successfully)

- Sedat -

> >
> >> - 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 -
> >>>>