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

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

Thanks for the review, Gary.

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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