Re: PING [PATCH] warn for strlen of arrays with missing nul (PR 86552, 86711, 86714) )

[Jeff Law <law@redhat.com>]

On 08/25/2018 12:32 AM, Bernd Edlinger wrote:
> On 08/25/18 01:54, Jeff Law wrote:
>> On 08/24/2018 11:26 AM, Bernd Edlinger wrote:
>>> On 08/24/18 18:51, Jeff Law wrote:
>>>>> Well, this is broken for wide character strings.
>>>>> but I hope we can get rid of STRING_CST which are
>>>>> not explicitly null terminated.
>>>
>>> I am afraid that is not going to happen.
>>> Maybe we can get STRING_CST that are never longer
>>> than the TYPE_UNIT_SIZE, but c_strlen and c_getstr
>>> need to take care that the string is zero-terminated.
>>>
>>> string_constant, should not promise the string is zero terminated.
>>> But instead it can promise that:
>>> 1) the STRING_CST is valid up to TREE_STRING_LENGTH
>>> 2) mem_size is >= TREE_STRING_LENGTH
>>> 3) memory between TREE_STRING_LENGTH and mem_size is ZERO.
>>>
>>> It will not guarantee anything about zero termination any more.
>> Interesting because those conditions would be sufficient to deal with a
>> regression I stumbled over after fixing Martin's patch to not assume
>> that all STRING_CSTs are NUL terminated.
>>
>> But I need to think about this a bit more.  Essentially the question
>> we'd need to ask is whether or not these are sufficient in general or
>> just in specific cases.
>>
>> I tend to think they're not sufficient in general. If a string returned
>> by string_constant that didn't have a terminating NUL, but which did
>> pass the tests above were ultimately passed to the runtime's str*
>> routines, then the call may run off the end of the string.  We'd like to
>> be able to warn for that.
>>
>> So ISTM those rules are only valid in contexts where we know the result
>> isn't going to be passed to str* and friends within the C library.
>>
>> I do think they're sufficient to avoid problems with the
>> tree-ssa-forwprop code we've looked at.  So what may make the most sense
>> is to have that routine indicate it's willing to accept unterminated
>> strings, then check the conditions above before optimizing the code.
>>
> 
> There are not too many callers of string_constant.
> Not all need zero termination.
Right.  And in retrospect we probably should have avoided default
parameter overloads and just fixed the callers.  But that can be a
follow-up.

> 
> But I think if the are interested in zero-termination
> they should simply call c_strlen or c_getstr.
Perhaps.


> 
> 
>>>
>>> In the end, the best approach might be to either merge my patch
>>> with Martins, or step-wise, first fixing wrong code, and then
>>> implementing warnings without fixing wrong code.
>> Unsure at this time.  I've been working with both.  I suspect that if we
>> went with yours that we'd then turn around and layer Martin's on top of
>> it because of the desire to signal to callers that we have an
>> unterminated string and have the callers take appropriate action.  Which
>> begs the question of whether or not we just go with Martin's -- ie, is
>> there really any value in using both.  I haven't seen indications there
>> is value in that approach, but I'm still poking at things.
>>
> 
> Well, ya call it "layer one patch over the other"
> I call it "incremental improvements".
It is (of course) a case by case basis.  The way I try to look at these
things is to ask whether or not the first patch under consideration
would have any value/purpose after the second patch was installed.  If
so, then it may make sense to include both.  If not, then we really just
want one patch.

Jeff