Re: [PATCH v2] [gdb] Fix "value is not available" with debug frame

[Luis Machado <luis.machado@arm.com>]

On 2/18/24 13:30, Tom de Vries wrote:
> On 2/18/24 05:09, Kevin Buettner wrote:
>> Hi Tom,
>>
>> On Tue, 13 Feb 2024 18:17:18 +0100
>> Tom de Vries <tdevries@suse.de> wrote:
>>
>>> On arm-linux, with a started hello world, running "info frame" works fine, but
>>> when I set debug frame to on, I run into:
>>> ...
>>> (gdb) info frame
>>>    ...
>>> [frame] frame_unwind_register_value: exit
>>> value is not available
>>> (gdb)
>>> ...
>>>
>>> The problem is here in frame_unwind_register_value:
>>> ...
>>>            if (value->lazy ())
>>>              gdb_printf (&debug_file, " lazy");
>>>            else
>>>              {
>>>                int i;
>>>                gdb::array_view<const gdb_byte> buf = value->contents ();
>>> ...
>>> where we call value->contents () while !value->entirely_available ().
>>>
>>> Fix this by checking value->entirely_available () and printing:
>>> ...
>>>      [frame] frame_unwind_register_value:   -> register=91 unavailable
>>> ...
>>>
>>> Tested on arm-linux.
>>
>> The fix looks good to me.
>>
>> I'm wondering about the test case though.  Is it the case that the
>> failure depends on debug info from a particular compiler (or compiler
>> version)?  If so, would it be possible to devise a test using the
>> DWARF assembler which would not be dependent on compiler output that
>> may change over time?
>>
> 
> Hi Kevin,
> 
> thanks for the review.
> 
> This doesn't depend on debug info.  The test-case depends on a register being unavailable.  I don't know if or how we can reliably enforce this condition in a test-case.
> 
> To give some more detail, a minimal way to show the register as unavailable is:
> ...
> $ gcc hello.c
> $ gdb -q -batch a.out -ex start -ex 'p $tpidruro'
> Temporary breakpoint 1 at 0x512
> 
> Temporary breakpoint 1, 0xaaaaa512 in main ()
> $1 = <unavailable>
> ...
> 
> I'm not sure why it's unavailable.
> 
> It seems the register was added as part of some tls support for fbsd arm tdep.  It could be that support for the register is missing in linux arm tdep.
> 
> It also could be a kernel problem (used kernel is 4.4.190).
> 
> Or, something to do with the fact that I'm running a pinebook with 64-bit kernel, 32-bit userland setup.
> 
> Luis, any comments on this?  Is this a bug, or a missing feature, or expected behaviour?
> 
> Thanks,
> - Tom
> 
>> Kevin
>>
> 

Hi Tom,

I was out on time off. It looks like Thiago addressed this in a separate thread, is that right?