Re: [PATCH][gdb/symtab] Fix assert in write_one_signatured_type

[Simon Marchi via Gdb-patches <gdb-patches@sourceware.org>]

On 2021-02-01 9:43 a.m., Tom de Vries wrote:> Hi,
> 
> When running test-case gdb.dwarf2/fission-reread.exp with target board
> cc-with-gdb-index, we run into an abort during the generation of the gdb-index
> by cc-with-tweaks.sh:
> ...
> build/gdb/testsuite/cache/gdb.sh: line 1: 27275 Aborted  (core dumped)
> ...
> 
> This can be reproduced on the command line like this:
> ...
> $ gdb -batch ./outputs/gdb.dwarf2/fission-reread/fission-reread \
>   -ex 'save gdb-index  ./outputs/gdb.dwarf2/fission-reread'
> warning: Could not find DWO TU fission-reread.dwo(0x9022f1ceac7e8b19) \
>   referenced by TU at offset 0x0 [in module fission-reread]
> warning: Could not find DWO CU fission-reread.dwo(0x807060504030201) \
>   referenced by CU at offset 0x561 [in module fission-reread]
> Aborted (core dumped)
> ...
> 
> The abort is a segfault due to a using a nullptr psymtab in
> write_one_signatured_type.
> 
> The problem is that we're trying to write index entries for the type unit
> with signature:
> ...
> (gdb) p /x entry->signature
> $2 = 0x9022f1ceac7e8b19
> ...
> which is a skeleton type unit:
> ...
> Contents of the .debug_types section:
> 
>   Compilation Unit @ offset 0x0:
>    Length:        0x4a (32-bit)
>    Version:       4
>    Abbrev Offset: 0x165
>    Pointer Size:  4
>    Signature:     0x9022f1ceac7e8b19
>    Type Offset:   0x0
>  <0><17>: Abbrev Number: 2 (DW_TAG_type_unit)
>     <18>   DW_AT_comp_dir    : /tmp/src/gdb/testsuite
>     <2f>   DW_AT_GNU_dwo_name: fission-reread.dwo
>     <42>   DW_AT_GNU_pubnames: 0x0
>     <46>   DW_AT_GNU_pubtypes: 0x0
>     <4a>   DW_AT_GNU_addr_base: 0x0
> ...
> referring to a .dwo file, but as the warnings show, the .dwo file is not
> found.
> 
> Fix this by skipping the type unit in write_one_signatured_type if
> psymtab == nullptr.
> 
> Tested on x86_64-linux.
> 
> Any comments?
> 
> Thanks,
> - Tom
> 
> [gdb/symtab] Fix assert in write_one_signatured_type
> 
> gdb/ChangeLog:
> 
> 2021-02-01  Tom de Vries  <tdevries@suse.de>
> 
> 	PR symtab/24620
> 	* dwarf2/index-write.c (write_one_signatured_type): Skip if
> 	psymtab == nullptr.
> 
> ---
>  gdb/dwarf2/index-write.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/gdb/dwarf2/index-write.c b/gdb/dwarf2/index-write.c
> index 66781feaf46..a7b9ae66cae 100644
> --- a/gdb/dwarf2/index-write.c
> +++ b/gdb/dwarf2/index-write.c
> @@ -616,6 +616,14 @@ write_one_signatured_type (void **slot, void *d)
>    struct signatured_type *entry = (struct signatured_type *) *slot;
>    partial_symtab *psymtab = entry->per_cu.v.psymtab;
>  
> +  if (psymtab == nullptr)
> +    {
> +      /* We can end up here when processing a skeleton CU referring to a
> +	 .dwo file that hasn't been found.  There's not much we can do in
> +	 such a case, so skip this CU.  */
> +      return 1;
> +    }
> +
>    write_psymbols (info->symtab, info->psyms_seen,
>  		  psymtab->global_psymbols, info->cu_index,
>  		  0);
> 
LGTM, some questions:

1. Do we need to emit a warning here? Probably not because another one
   should have been emitted earlier, when parsing symbols.  But I thought
   I'd ask just in case.

2. Would it be somewhat easy to write a test for this?  Have a type unit
   with a dwo, delete the dwo, try to generate an index.

3. Is the same situation correctly handled for standard compile units?

Simon