Re: gdb.Value returning a string of length 1 (linux lx-version bug)

[Kieran Bingham <kieran dot bingham at linaro dot org>]

On 22/03/16 17:56, Jan Kiszka wrote:
> Hi Kieran,
> 
> On 2016-03-22 18:54, Kieran Bingham wrote:
>> Hi Jan,
>>
>> Me and Pete have just been looking into the gdb.Value object bug where
>> the lx-version command  returns a string of length 1.
>>
>> As I recall at FOSDEM, you were also hitting the bug.
>> Out of interest, what compiler version are you using?
> 
> gcc (SUSE Linux) 4.8.3 20140627 [gcc-4_8-branch revision 212064]

Thanks, well that means it's unlikely to be any optimiser changes, as
your compiler is earlier than Petes.

>>
>> We've made some head way, but we are suspicious that compiler versions
>> may affect the issue ! :(
>>
>> The crux of the issue, is that the valpy_string() calls c_get_string()
>> which in turn calls get_discrete_bounds().
>>
>> On my broken version, get_discrete_bounds, is setting both lowp, and
>> highp to 0x0; where as on Peters working version, his highp gets set
>> correctly. (there is a +1 added later which results in the string
>> producing a strlen of 1)
>>
>> I'm running gcc 5.2.1-22ubuntu2, whereas Pete is running gcc 4.8.4.
>>
>> Pete's version of his compiled binutils-gdb always seems to function
>> correctly, where as I hit the bug - *occasionally*
>>
>> Of course, since I have compiled with -g3 -O0 -fsanitize=undefined I
>> can't reproduce the issue, and now even removing the optimise levels I
>> haven't been able to reproduce.
>>
>> We have a Heisenbug :(
> 
> Maybe wait for someone seeing this more reproducibly? :-/

I think I may have found a way to reproduce reliably!

By running some commands, and ensured the issue occurred, and then
reducing the command set down until only one remained, I've narrowed
down the cause a little.

I have discovered that the issue only appears if you have run a
'backtrace (bt)' *before* calling lx-version.

(gdb) bt
  <bt output omitted>
(gdb) lx-version
L(gdb) ## Note the single char output on the left


Now - having determined how to reliably reproduce, I have re-enabled
-fsanitize=undefined :

(gdb) bt
/home/linuxembedded/linaro/lkd/openst-lkd/sources/binutils-gdb/gdb/dwarf2-frame.c:254:3:
runtime error: null pointer passed as argument 2, which is declared to
never be null
#0  0xc0222c68 in cpu_v7_do_idle ()
    at
/home/linuxembedded/linaro/lkd/openst/sources/linux/arch/arm/mm/proc-v7.S:74
#1  0xc0210728 in arch_cpu_idle ()
    at
/home/linuxembedded/linaro/lkd/openst/sources/linux/arch/arm/kernel/process.c:72
#2  0xc027de78 in cpu_startup_entry ()
    at
/home/linuxembedded/linaro/lkd/openst/sources/linux/kernel/sched/idle.c:150
#3  0xc027de78 in cpu_startup_entry ()
    at
/home/linuxembedded/linaro/lkd/openst/sources/linux/kernel/sched/idle.c:246
#4  0xc027de78 in cpu_startup_entry (state=<optimized out>)
    at
/home/linuxembedded/linaro/lkd/openst/sources/linux/kernel/sched/idle.c:294
#5  0xc09e0160 in rest_init () at
/home/linuxembedded/linaro/lkd/openst/sources/linux/init/main.c:412
#6  0xc0da2ca8 in start_kernel ()
    at /home/linuxembedded/linaro/lkd/openst/sources/linux/init/main.c:683
#7  0x8020807c in  ()
(gdb) lx-version
L(gdb)


UBSAN to the rescue!

I'll see if I can determine the cause / effect of the Null pointer, and
that is likely worth submitting as a dedicated GDB bug.
--
Kieran

> Jan
> 
>>
>> Has anyone else on the GDB Mailinglist experienced any intermittent
>> errors with gdb.Value strings from python?
>> --
>> Kieran
>>
>