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

[Tom de Vries <tdevries@suse.de>]

On 2/1/21 5:16 PM, Simon Marchi wrote:
> 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.
> 

Yeah, I think the existing warning is sufficient.

> 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.
> 

I've added a test-case to fission-reread.exp.

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

I've tried the following experiment: I've done a gdb testsuite run with
target board fission-dwp, with a hack added to cc-with-tweaks.sh to
remove the .dwp file.  Apart from a lot of FAILs, all I ran into was
PR27313 "Segfault in insert_catch_syscall" (
https://sourceware.org/bugzilla/show_bug.cgi?id=27313 ).

Committed with test-case added.

Thanks,
- Tom