Re: Encountered error while encoding BTF due to Unsupported DW_TAG_unspecified_type(0x3b)

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Thu, Oct 06, 2022 at 02:23:16PM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Thu, Oct 06, 2022 at 09:04:58AM -0700, Andrii Nakryiko escreveu:
> > On Thu, Oct 6, 2022 at 7:00 AM Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com> wrote:
> > > Em Thu, Oct 06, 2022 at 10:43:22AM -0300, Arnaldo Carvalho de Melo escreveu:
> > > > 15e20ce2324a:~/git/linux # readelf -wi ./arch/x86/entry/entry.o
> > > > Contents of the .debug_info section:
> 
> > > >   Compilation Unit @ offset 0:
> > > >    Length:        0x35 (32-bit)
> > > >    Version:       5
> > > >    Unit Type:     DW_UT_compile (1)
> > > >    Abbrev Offset: 0
> > > >    Pointer Size:  8
> > > >  <0><c>: Abbrev Number: 1 (DW_TAG_compile_unit)
> > > >     <d>   DW_AT_stmt_list   : 0
> > > >     <11>   DW_AT_low_pc      : 0
> > > >     <19>   DW_AT_high_pc     : 19
> > > >     <1a>   DW_AT_name        : (indirect string, offset: 0): arch/x86/entry/entry.S
> > > >     <1e>   DW_AT_comp_dir    : (indirect string, offset: 0x17): /root/git/linux
> > > >     <22>   DW_AT_producer    : (indirect string, offset: 0x27): GNU AS 2.39.50
> > > >     <26>   DW_AT_language    : 32769  (MIPS assembler)
> > > >  <1><28>: Abbrev Number: 2 (DW_TAG_subprogram)
> > > >     <29>   DW_AT_name        : (indirect string, offset: 0x36): entry_ibpb
> > > >     <2d>   DW_AT_external    : 1
> > > >     <2d>   DW_AT_type        : <0x37>
> > > >     <2e>   DW_AT_low_pc      : 0
> > > >     <36>   DW_AT_high_pc     : 19
> > > >  <1><37>: Abbrev Number: 3 (DW_TAG_unspecified_type)
> > > >  <1><38>: Abbrev Number: 0
> 
> > > > 15e20ce2324a:~/git/linux #
> 
> > > > Which pahole -J barfs on:
> 
> > > > 15e20ce2324a:~/git/linux # pahole -J ./arch/x86/entry/entry.o
> > > > [1] UNKNOWN (anon) Unexpected kind for reference
> > > > 15e20ce2324a:~/git/linux #
> 
> > > > But if we ask it to exclude asm CUs (<26>   DW_AT_language    : 32769
> > > > (MIPS assembler)) then it ignores it, so this is a workaround.
> 
> > > > 15e20ce2324a:~/git/linux # pahole --lang_exclude asm -V -J ./arch/x86/entry/entry.o
> > > > 15e20ce2324a:~/git/linux #
> 
> > > > Now I'm looking at how to get the BTF encoder grokking this.
> 
> > > This is what I came up with, Andrii, Yonghong, wdyt?
>  
> > Does `const void` make sense? Why not just keeping it as "void"?
> > "const void" might confuse tooling and BTF verifier in kernel, but I
> > haven't checked. Just trying to understand why we need extra "const".
> 
> I was trying to have an entry for that DW_TAG_unspecified_type type
> instead of plain changing the type for entry_ibpb (a DW_TAG_subprogram)
> straight to 0 (void).

I think this is the best way forward at the moment, i.e. to just change
the type for such DW_TAG_subprogram to zero, aka void, losing this
information by default.

I'll have a --btf_encode_unspecified_type as this const void trick as an
opt-in, who knows if this ends up being useful :-)

- Arnaldo
 
> I.e. we wouldn't lose that info that the return type for that "function"
> (it was encoded as a DW_TAG_subprogram) isn't defined. Using straight
> void would state that it doesn't return anything.
> 
> I couldn't find a way to use KCFLAGS to ask Kbuild to pass that -B flag
> to gcc so that I could rebuild the whole kernel with it to check if the
> build goes thru and then try booting with such BTF info.
> 
> - Arnaldo
>  
> > > As an additional info clang 14 (haven't tested with other versions)
> > > encodes such ASM Labels as DW_TAG_label and this thus isn't an issue
> > > there.
> > >
> > > - Arnaldo
> > >
> > > commit 15ec614672da043008df31aa6ee85ebc5105d4fd
> > > Author: Arnaldo Carvalho de Melo <acme@redhat.com>
> > > Date:   Tue Oct 4 18:22:53 2022 -0300
> > >
> > >     btf_encoder: Encode DW_TAG_unspecified_type as BTF_KIND_CONST
> > >
> > >     This first appeared for assembler files in the Linux kernel with recent
> > >     GNU compilers, we don't have anything in BTF, AFAIK, to properly
> > >     represent that, so, for now, lets go with BTF_KIND_CONST.
> > >
> > >     Testing it:
> > >
> > >     Built binutils from git://sourceware.org/git/binutils-gdb.git, then used
> > >     gcc's -B option to point to the directory with the new as, that is built
> > >     as as-new, so make a symlink, ending up with:
> > >
> > >       15e20ce2324a:~/git/linux # readelf -wi ./arch/x86/entry/entry.o
> > >       Contents of the .debug_info section:
> > >
> > >         Compilation Unit @ offset 0:
> > >          Length:        0x35 (32-bit)
> > >          Version:       5
> > >          Unit Type:     DW_UT_compile (1)
> > >          Abbrev Offset: 0
> > >          Pointer Size:  8
> > >        <0><c>: Abbrev Number: 1 (DW_TAG_compile_unit)
> > >           <d>   DW_AT_stmt_list   : 0
> > >           <11>   DW_AT_low_pc      : 0
> > >           <19>   DW_AT_high_pc     : 19
> > >           <1a>   DW_AT_name        : (indirect string, offset: 0): arch/x86/entry/entry.S
> > >           <1e>   DW_AT_comp_dir    : (indirect string, offset: 0x17): /root/git/linux
> > >           <22>   DW_AT_producer    : (indirect string, offset: 0x27): GNU AS 2.39.50
> > >           <26>   DW_AT_language    : 32769  (MIPS assembler)
> > >        <1><28>: Abbrev Number: 2 (DW_TAG_subprogram)
> > >           <29>   DW_AT_name        : (indirect string, offset: 0x36): entry_ibpb
> > >           <2d>   DW_AT_external    : 1
> > >           <2d>   DW_AT_type        : <0x37>
> > >           <2e>   DW_AT_low_pc      : 0
> > >           <36>   DW_AT_high_pc     : 19
> > >        <1><37>: Abbrev Number: 3 (DW_TAG_unspecified_type)
> > >        <1><38>: Abbrev Number: 0
> > >
> > >     So we have that asm label encoded by GNU AS 2.39.50 as a
> > >     DW_TAG_subprogram that has as its DW_AT_type the DW_TAG_unspecified_type
> > >     0x37 that we encode as a BTF_KIND_CONST pointing to 0 (void):
> > >
> > >       15e20ce2324a:~/git/linux # pahole -J ./arch/x86/entry/entry.o
> > >       15e20ce2324a:~/git/linux # pahole -JV ./arch/x86/entry/entry.o
> > >       btf_encoder__new: './arch/x86/entry/entry.o' doesn't have '.data..percpu' section
> > >       Found 0 per-CPU variables!
> > >       Found 1 functions!
> > >       File ./arch/x86/entry/entry.o:
> > >       [1] CONST (anon) type_id=0
> > >       [2] FUNC_PROTO (anon) return=1 args=(void)
> > >       [3] FUNC entry_ibpb type_id=2
> > >       15e20ce2324a:~/git/linux # pfunct -F btf ./arch/x86/entry/entry.o
> > >       entry_ibpb
> > >       15e20ce2324a:~/git/linux # pfunct --proto -F btf ./arch/x86/entry/entry.o
> > >       const void  entry_ibpb(void);
> > >       15e20ce2324a:~/git/linux #
> > >
> > >       15e20ce2324a:~/git/linux # tools/bpf/bpftool/bpftool btf dump file ./arch/x86/entry/entry.o format raw
> > >       [1] CONST '(anon)' type_id=0
> > >       [2] FUNC_PROTO '(anon)' ret_type_id=1 vlen=0
> > >       [3] FUNC 'entry_ibpb' type_id=2 linkage=static
> > >       15e20ce2324a:~/git/linux #
> > >
> > >     I think this is what can be done to avoid having to skip ASM DWARF when
> > >     gets widely used, i.e. binutils gets updated.
> > >
> > >     Reported-by: Martin Liška <mliska@suse.cz>
> > >     Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
> > >     Cc: Yonghong Song <yhs@fb.com>
> > >     Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
> > >
> > > diff --git a/btf_encoder.c b/btf_encoder.c
> > > index 7ad3f29ef153d8d6..7e50ba711ccc14d6 100644
> > > --- a/btf_encoder.c
> > > +++ b/btf_encoder.c
> > > @@ -962,6 +962,8 @@ static int btf_encoder__encode_tag(struct btf_encoder *encoder, struct tag *tag,
> > >                 return btf_encoder__add_enum_type(encoder, tag, conf_load);
> > >         case DW_TAG_subroutine_type:
> > >                 return btf_encoder__add_func_proto(encoder, tag__ftype(tag), type_id_off);
> > > +       case DW_TAG_unspecified_type:
> > > +               return btf_encoder__add_ref_type(encoder, BTF_KIND_CONST, 0, NULL, false);
> > >         default:
> > >                 fprintf(stderr, "Unsupported DW_TAG_%s(0x%x): type: 0x%x\n",
> > >                         dwarf_tag_name(tag->tag), tag->tag, ref_type_id);
> 
> -- 
> 
> - Arnaldo

-- 

- Arnaldo