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Reply-To: sedat.dilek@gmail.com
From:   Sedat Dilek <sedat.dilek@gmail.com>
Date:   Sat, 6 Feb 2021 19:10:37 +0100
Message-ID: <CA+icZUVfTH=yONintyJ+T8kvTrR4Q0gumJYNUCs6Ybraff5Kpg@mail.gmail.com>
Subject: Re: ERROR: INT DW_ATE_unsigned_1 Error emitting BTF type
To:     Yonghong Song <yhs@fb.com>
Cc:     Mark Wieelard <mark@klomp.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
        dwarves@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        bpf@vger.kernel.org, Jiri Olsa <jolsa@kernel.org>,
        Jan Engelhardt <jengelh@inai.de>,
        Domenico Andreoli <cavok@debian.org>,
        Matthias Schwarzott <zzam@gentoo.org>,
        Andrii Nakryiko <andriin@fb.com>,
        Paul Moore <paul@paul-moore.com>,
        Ondrej Mosnacek <omosnace@redhat.com>,
        =?UTF-8?Q?Daniel_P=2E_Berrang=C3=A9?= <berrange@redhat.com>,
        Tom Stellard <tstellar@redhat.com>
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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?

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 -