Re: [PATCH] gdb: nat/x86-dregs.c: add and use x86_dr_low_can_get_status

[Simon Marchi <simon.marchi@polymtl.ca>]

On 2021-07-18 6:39 p.m., John Baldwin wrote:
> On 7/16/21 1:38 PM, John Baldwin wrote:
>> On 7/15/21 12:53 PM, Simon Marchi via Gdb-patches wrote:
>>> I built and tried gdb on OpenBSD, and it immediately segfaults when
>>> running a program.  I tracked down the problem to x86_dr_low.get_status
>>> being nullptr at this point:
>>
>> I've looked, and another way to possibly fix this is to make the x86_bsd_nat_target
>> class not inherit from x86_nat_target<> when x86 debug registers aren't supported.
>> Something like this in x86-bsd-nat.h:
>>
>> /* A prototype *BSD/x86 target.  */
>>
>> #ifdef HAVE_PT_GETDBREGS
>> template<typename BaseTarget>
>> class x86bsd_nat_target : public x86_nat_target<BaseTarget>
>> {
>>     using base_class = x86_nat_target<BaseTarget>
>> public:
>>     void mourn_inferior () override
>>     {
>>       x86_cleanup_dregs ();
>>       base_class::mourn_inferior ();
>>     }
>> };
>> #else /* !HAVE_PT_GETDBREGS */
>> template<typename BaseTarget>
>> class x86bsd_nat_target : public BaseTarget
>> {
>> };
>> #endif /* HAVE_PT_GETDBREGS */
> 
> I think this change is ok (but have only tested it on FreeBSD so far).
> 
>> I would be further tempted to then remove x86-bsd-nat.o from OpenBSD's configure.nat
>> lines entirely sine it shouldn't be needed at that point, and maybe adjust x86-bsd-nat.c
>> to start with something like:
>>
>> #ifndef HAVE_PT_GETDBREGS
>> #error "x86-bsd-nat.c requires PT_GETDBREGS"
>> #endif
> 
> These changes are probably premature (though might be realistic after my
> other series lands that pulls x86bsd_xsave_len into the FreeBSD-specific
> targets such that x86-bsd-nat.c is purely about debug registers).  It
> should still be possible to remove x86-nat.c from OpenBSD in configure.nat
> with the x86-bsd-nat.h change.  I can try to test it once I get an
> OpenBSD VM spun up.

All you said sounds good to me.  If you want to do it while you do the
changes to FreeBSD, please go ahead, because this is pretty far down on
my todo list (I just happened to give a try on OpenBSD and wanted to
minimally unbreak it).

Simon