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Re: [PATCH 0/6] improve handling of char arrays with missing nul (PR 86552, 86711, 86714)


On 08/15/2018 08:47 AM, Martin Sebor wrote:
> On 08/15/2018 12:02 AM, Jeff Law wrote:
>> On 08/13/2018 03:23 PM, Martin Sebor wrote:
>>> To make reviewing the changes easier I've split up the patch
>>> into a series:
>> [ ... ]
>> I'm about done for the night and thus won't get into the series (and as
>> you know Bernd has a competing patch in this space).  But I did want to
>> chime in on two things...
>>
>>>
>>> There are many more string functions where unterminated (constant
>>> or otherwise) should be diagnosed.  I plan to continue to work on
>>> those (with the constant ones first)  but I want to post this
>>> updated patch for review now, mainly so that the wrong code bug
>>> (PR 86711) can be resolved and the basic detection infrastructure
>>> agreed on.
>> Yes, I think we definitely want to focus on the wrong code bug first.
>>
>>>
>>> An open question in my mind is what should GCC do with such calls
>>> after issuing a warning: replace them with traps?  Fold them into
>>> constants?  Or continue to pass them through to the corresponding
>>> library functions?
>> My personal preference is to turn them into traps.  I don't think we
>> have to preserve the call itself in this case.   I think the sequencing
>> is to insert the trap before the call point, split the block after the
>> trap, remove the outgoing edges, let DCE clean up the rest.  At least I
>> think that's the sequencing.
> 
> That sounds fine to me.  It would be close in its effects to
> what _FORTIFY_SOURCE does.
The bad guys are exceedingly resourceful in how they exploit undefined
behavior.  By trapping immediately they don't have any window to do
anything nefarious.

> 
> It would be helpful to get a broader consensus on this and start
> adopting the same consistent solution in all contexts.  The question
> has come up a few times, most recently also in PR 86519 (folding
> memcmp(a, "a", 3)) where GCC ends up calling the library function.
Yup.  We've got a mish-mash of strategies here.

> 
> FWIW, if there are other preferences it might be worthwhile to
> consider providing an option to control the behavior in these
> cases.  There may also be interactions with or implications for
> the sanitizers to consider.
There's some (Marc Glisse IIRC) that would prefer to see the control
path to the undefined behavior zapped entirely.  We didn't initially do
that because the path my have other observable side effects.  However,
there may be cases where it makes sense.

> 
> Once there is agreement on what the solution should be I can look
> into implementing it at some point in the future.
ACK.  Certainly lower priority than the stuff in flight right now.

jeff


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