Re: [PATCH 2/2] Catching errors on probes-based dynamic linker interface

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

On Tuesday, August 25 2015, I wrote:

> Thanks for the review, Gary.

Any more comments (from Gary or anyone else) before I go ahead and apply
this?  I will wait until the end of tomorrow (Tuesday), and then I'll go
ahead.

Thanks,

> On Tuesday, August 25 2015, Gary Benson wrote:
>
>> Sergio Durigan Junior wrote:
>>> On Monday, August 24 2015, Gary Benson wrote:
>>> > Maybe this would be clearer and more robust:
>>> >
>>> >   TRY
>>> >     {
>>> >       unsigned probe_argc;
>>> >
>>> >       probe_argc = get_probe_argument_count (pa->probe, frame);
>>> >    
>>> >       if (probe_argc == 2)
>>> >         action = FULL_RELOAD;
>>> >       else if (probe_argc < 2)
>>> > 	action = PROBES_INTERFACE_FAILED;
>>> >     }
>>> >   CATCH (ex, RETURN_MASK_ERROR)
>>> >     {
>>> >       exception_print (gdb_stderr, ex);
>>> >       action = PROBES_INTERFACE_FAILED;
>>> >     }
>>> >   END_CATCH
>>> 
>>> Maybe it's a matter of preference, but I don't like this (and I
>>> don't see why it is more robust).  I prefer to have as little code
>>> as possible running on the TRY block, and handle everything else
>>> outside of it.  I think it also makes things a bit more confuse
>>> because you have two places where action can be
>>> PROBES_INTERFACE_FAILED.
>>
>> Well, there are two different failures:
>>
>>  1) get_probe_argument_count failed
>>  2) get_probe_argument_count returned < 2
>
> Yes, and both are covered by the proposed patch.  It is not really
> important to distinguish between these failures today: what really
> matters is that GDB recognizes both as failures and acts accordingly.
>
>> I think it's more robust because, imagine a future where someone adds
>> a zero-argument probe to glibc.  They update the "if (probe_argc)..."
>> block to allow zero-argument probes through.  If get_probe_argument_count
>> with such a GDB then it will not be treated as a failure.
>
> I think we should cross this bridge when we come to it.  Plus, the
> version you proposed does not take that scenario into account as well:
> if probe_argc is zero, action will be PROBES_INTERFACE_FAILED;
> therefore, this code would have to be rewritten anyway (in the scenario
> you're proposing).
>
>> FWIW I also like to keep code in TRY blocks to a minimum.  Maybe you
>> could do it your original way, but set probe_argc to -1 in the CATCH
>> and have the below block like:
>>
>>   if (probe_argc < 0)
>>     /* get_probe_argument_count failed */
>>     action = PROBES_INTERFACE_FAILED
>>   else if (probe_argc == 2)
>>     action = FULL_RELOAD;
>>   else if (probe_argc < 2)
>>     /* we don't understand this probe with too few arguments  */
>>     action = PROBES_INTERFACE_FAILED;
>>
>> It looks kind of silly but the compiler will optimize it out.
>
> This has crossed my mind when I was writing this part, but probe_argc is
> unsigned int and therefore is never < 0.
>
> Moreover, as I said above, we are not really interested in
> differentiating between the errors here; what we really want to know is
> if there was an error.
>
>>> > As an aside it would clarify this code greatly if "old_chain"
>>> > were renamed "disable_probes_interface" or similar.  It took
>>> > me a while to figure out what the code was doing, and I wrote
>>> > it!
>>> 
>>> Yeah.  I'll leave this to another patch.
>>
>> I'll do it if you like (but I'll wait til you've got this through).
>
> Sure, no problem.
>
> Cheers,
>
> -- 
> Sergio
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> Please send encrypted e-mail if possible
> http://sergiodj.net/

-- 
Sergio
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