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

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

Em Mon, Oct 03, 2022 at 10:56:36AM +0200, Martin Liška escreveu:
> I noticed one can't build 5.19 with latest binutils master.
> 
> One see the following error:
> [ 1413s]   BTF     .btf.vmlinux.bin.o
> [ 1413s] Unsupported DW_TAG_unspecified_type(0x3b)
> [ 1413s] Encountered error while encoding BTF.
> 
> It's caused by DWARF coming from .S files and the change is introduced since
> the following binutils revision:
> 
> commit 5578fbf672ee497ea19826edeb509f4cc3e825a8
> Author: Nick Clifton <nickc@redhat.com>
> Date:   Thu Aug 25 11:48:00 2022 +0100
> 
>     GAS: Add a return type tag to DWARF DIEs generated for function symbols.
> 
> for entry.S the output changes to:
> 
> $ as-new --gdwarf-5 --64 -o entry.o entry.S && readelf -wi entry.o
> Contents of the .debug_info section:
> 
>   Compilation Unit @ offset 0x0:
>    Length:        0x35 (32-bit)
>    Version:       5
>    Unit Type:     DW_UT_compile (1)
>    Abbrev Offset: 0x0
>    Pointer Size:  8
>  <0><c>: Abbrev Number: 3 (DW_TAG_unspecified_type) <--- the problematic TAG
>  <0><d>: Abbrev Number: 1 (DW_TAG_compile_unit)
>     <e>   DW_AT_stmt_list   : 0x0
>     <12>   DW_AT_low_pc      : 0x0
>     <1a>   DW_AT_high_pc     : 19
>     <1b>   DW_AT_name        : (indirect string, offset: 0x0): ../arch/x86/entry/entry.S
>     <1f>   DW_AT_comp_dir    : (indirect string, offset: 0x1a): /tmp
>     <23>   DW_AT_producer    : (indirect string, offset: 0x1f): GNU AS 2.39.50
>     <27>   DW_AT_language    : 32769	(MIPS assembler)
>  <1><29>: Abbrev Number: 2 (DW_TAG_subprogram)
>     <2a>   DW_AT_name        : (indirect string, offset: 0x2e): entry_ibpb
>     <2e>   DW_AT_external    : 1
>     <2e>   DW_AT_type        : <0xc>

Ok, it happens at the top level of a CU and there are users for it, now
to try to figure out how to best support this in the pretty printer,
DWARF loader and BTF encoder.

- Arnaldo

>     <2f>   DW_AT_low_pc      : 0x0
>     <37>   DW_AT_high_pc     : 19
>  <1><38>: Abbrev Number: 0
> 
> Is it something pahole should handle?
> 
> Thanks,
> Martin

-- 

- Arnaldo