Re: [RFC/PATCH] New convenience variable $_exitsignal

[Sergio Durigan Junior <sergiodj at redhat dot com>]

On Monday, September 16 2013, Pedro Alves wrote:

> On 06/19/2013 05:59 AM, Sergio Durigan Junior wrote:
>> On Monday, June 17 2013, I wrote:
>> 
>>> On Monday, June 17 2013, Pierre Muller wrote:
>>>
>>>>   Hi Sergio,
>>>>
>>>>   Is there a reason why you don't handle
>>>> corelow.c anymore in your new patch?
>>>
>>> Hi Pierre,
>>>
>>> Yes, corelow.c is not important to this patch because (as Pedro
>>> explained on
>>> <http://sourceware.org/ml/gdb-patches/2013-06/msg00337.html>)
>>> $_exitsignal should not be set for corefiles, because the inferior has
>>> not exited.
>>>
>>> corelow.c will be touched in my next patch, which will add $_signo (but
>>> with the modifications proposed by Pedro).
>> 
>> I've been thinking about this answer I gave to Pierre.  After
>> investigating how corefiles handle the signal, I guess the right choice
>> would indeed be to set $_exitsignal in corelow.c as well.  This is my
>> rationale.
>
> Looks like I completely missed this email.  Sorry about that.

No problem, I forgot about it as well, and I wrote it mostly to keep my
opinions recorded.  Glad you replied to it, though!

>> 1) Single-threaded program + generate-core-file
>> 
>> In this case, NT_SIGINFO will not be filled by GDB's generate-core-file
>> (bug) because PRSTATUS generation does not contemplate that yet (which
>> reminds me of the PRPSINFO work I did few months ago, and the PRSTATUS
>> work I still need to do, which will fix this bug).  So, in this case,
>> "print $_siginfo.si_signo" will not display the correct signal, and we
>> can only rely on "bfd_core_file_failing_signal" (called inside
>> corelow.c).  Thus, setting $_signo to "bfd_core_file_failing_signal" is
>> the logical choice (of course, if we want to avoid having to use
>> NT_SIGINFO, that is the *only* choice).
>> 
>> 2) Single-threaded program + SIGSEGV (or another "Core" signal)
>> 
>> In this case, the Linux kernel correctly generates the NT_SIGINFO, which
>> can be displayed by $_siginfo.  However, we don't want to use
>> NT_SIGINFO, so "bfd_core_file_failing_signal" is the only choice again.
>> 
>> 3) Multi-threaded program + generate-core-file
>> 
>> Again, NT_SIGINFO is not generated by GDB.  Again,
>> "bfd_core_file_failing_signal" is the only choice.  (Back to this case
>> later)
>> 
>> 4) Multi-threaded program + SIGSEGV (or another "Core" signal)
>> 
>> Linux kernel generated NT_SIGINFO, but we don't want to use it.
>> However, the kernel put in NT_SIGINFO the same signal number (which
>> killed the process) for all threads.
>
> Really?  That's ......, to say the least.  ;-)
>
> Actually, in my 3.9.10-100.fc17.x86_64 kernel (Fedora 17),
> what I see is that the kernel only generates the NT_SIGINFO
> note for the thread that actually crashed.
>
> Hmm, actually, for a program with 3 threads, that has one thread
> call abort, I get:
>
> $ readelf -n ~/gdb/tests/threads-crash.core.32195
>
> Displaying notes found at file offset 0x000005f0 with length 0x00001998:
>   Owner                 Data size       Description
>   CORE                 0x00000150       NT_PRSTATUS (prstatus structure)
>   CORE                 0x00000088       NT_PRPSINFO (prpsinfo structure)
>   CORE                 0x00000080       NT_SIGINFO (siginfo_t data)
>   CORE                 0x00000130       NT_AUXV (auxiliary vector)
>   CORE                 0x000002aa       NT_FILE (mapped files)
> ...
>   CORE                 0x00000200       NT_FPREGSET (floating point registers)
>   LINUX                0x00000340       NT_X86_XSTATE (x86 XSAVE extended state)
>   CORE                 0x00000150       NT_PRSTATUS (prstatus structure)
>   CORE                 0x00000200       NT_FPREGSET (floating point registers)
>   LINUX                0x00000340       NT_X86_XSTATE (x86 XSAVE extended state)
>   CORE                 0x00000150       NT_PRSTATUS (prstatus structure)
>   CORE                 0x00000200       NT_FPREGSET (floating point registers)
>   LINUX                0x00000340       NT_X86_XSTATE (x86 XSAVE extended state)
>
> Which kind of makes sense, given the other threads didn't actually get
> any signal.

Yes, it does make sense.  And it was what I was expecting to see.
However, I cannot really remember how I came to the conclusion I wrote
above, since I did the same things that you did.

>> Thus, using
>> "bfd_core_file_failing_signal" is OK since there is no concept of "this
>> signal number killed only this thread".
>> 
>> 
>> Case (3) is the most difficult IMO.  I don't know how we are going to
>> handle it when I/we implement NT_SIGINFO generation on PRSTATUS.  My
>> first reaction is to do it using the same logic as the Linux kernel,
>> i.e., putting the same signal number in every thread's siginfo.  But I
>> don't think we should bikeshed too much now, so I'm stopping my e-mail
>> here.
>> 
>> I'd like to hear opinions.
>
> I can't say I really understand how any of that argues against my
> original rationale for not setting $_exitsignal on corefiles (because
> the inferior has not really exited at the point the core has been
> generated), rather than point at implementation choices.

Interesting.  I thought setting it made sense because it seems to me
that the inferior has exited when the corefile has been generated.  I am
clearly missing some knowledge here, then...

> Now, if one were to instead argue that _user interface_ -wise, it'd
> make sense to set $_exitsignal, because we also print
> "Program terminated with signal", (emphasis on "terminated"), then
> I'd agree:
>
>   siggy = bfd_core_file_failing_signal (core_bfd);
>   if (siggy > 0)
>     {
> ...
>       printf_filtered (_("Program terminated with signal %s, %s.\n"),
> 		       gdb_signal_to_name (sig), gdb_signal_to_string (sig));
>     }

...or not.  Apparently, you are differentiating between "exited" and
"terminated", right?  Could you expand a little more on this?

And BTW, I guess my reasoning for setting $_exitsignal here is indeed
because we already assume that the inferior has been terminated (or
exited?) indeed.

Thanks,

-- 
Sergio