Re: [PING][PATCH] correct handling of indices into arrays with elements larger than 1 (PR c++/96511)

[Jason Merrill <jason@redhat.com>]

On 10/7/20 11:19 AM, Martin Sebor wrote:
> On 10/7/20 9:07 AM, Jason Merrill wrote:
>> On 10/7/20 10:42 AM, Martin Sebor wrote:
>>> On 10/7/20 8:26 AM, Jason Merrill wrote:
>>>> On 9/28/20 6:01 PM, Martin Sebor wrote:
>>>>> On 9/25/20 11:17 PM, Jason Merrill wrote:
>>>>>> On 9/22/20 4:05 PM, Martin Sebor wrote:
>>>>>>> The rebased and retested patches are attached.
>>>>>>>
>>>>>>> On 9/21/20 3:17 PM, Martin Sebor wrote:
>>>>>>>> Ping: 
>>>>>>>> https://gcc.gnu.org/pipermail/gcc-patches/2020-September/553906.html 
>>>>>>>>
>>>>>>>>
>>>>>>>> (I'm working on rebasing the patch on top of the latest trunk which
>>>>>>>> has changed some of the same code but it'd be helpful to get a go-
>>>>>>>> ahead on substance the changes.  I don't expect the rebase to
>>>>>>>> require any substantive modifications.)
>>>>>>>>
>>>>>>>> Martin
>>>>>>>>
>>>>>>>> On 9/14/20 4:01 PM, Martin Sebor wrote:
>>>>>>>>> On 9/4/20 11:14 AM, Jason Merrill wrote:
>>>>>>>>>> On 9/3/20 2:44 PM, Martin Sebor wrote:
>>>>>>>>>>> On 9/1/20 1:22 PM, Jason Merrill wrote:
>>>>>>>>>>>> On 8/11/20 12:19 PM, Martin Sebor via Gcc-patches wrote:
>>>>>>>>>>>>> -Wplacement-new handles array indices and pointer offsets 
>>>>>>>>>>>>> the same:
>>>>>>>>>>>>> by adjusting them by the size of the element.  That's 
>>>>>>>>>>>>> correct for
>>>>>>>>>>>>> the latter but wrong for the former, causing false 
>>>>>>>>>>>>> positives when
>>>>>>>>>>>>> the element size is greater than one.
>>>>>>>>>>>>>
>>>>>>>>>>>>> In addition, the warning doesn't even attempt to handle 
>>>>>>>>>>>>> arrays of
>>>>>>>>>>>>> arrays.  I'm not sure if I forgot or if I simply didn't 
>>>>>>>>>>>>> think of
>>>>>>>>>>>>> it.
>>>>>>>>>>>>>
>>>>>>>>>>>>> The attached patch corrects these oversights by replacing most
>>>>>>>>>>>>> of the -Wplacement-new code with a call to compute_objsize 
>>>>>>>>>>>>> which
>>>>>>>>>>>>> handles all this correctly (plus more), and is also better 
>>>>>>>>>>>>> tested.
>>>>>>>>>>>>> But even compute_objsize has bugs: it trips up while 
>>>>>>>>>>>>> converting
>>>>>>>>>>>>> wide_int to offset_int for some pointer offset ranges.  Since
>>>>>>>>>>>>> handling the C++ IL required changes in this area the patch 
>>>>>>>>>>>>> also
>>>>>>>>>>>>> fixes that.
>>>>>>>>>>>>>
>>>>>>>>>>>>> For review purposes, the patch affects just the middle end.
>>>>>>>>>>>>> The C++ diff pretty much just removes code from the front end.
>>>>>>>>>>>>
>>>>>>>>>>>> The C++ changes are OK.
>>>>>>>>>>>
>>>>>>>>>>> Thank you for looking at the rest as well.
>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>> -compute_objsize (tree ptr, int ostype, access_ref *pref,
>>>>>>>>>>>>> -                bitmap *visited, const vr_values *rvals /* 
>>>>>>>>>>>>> = NULL */)
>>>>>>>>>>>>> +compute_objsize (tree ptr, int ostype, access_ref *pref, 
>>>>>>>>>>>>> bitmap *visited,
>>>>>>>>>>>>> +                const vr_values *rvals)
>>>>>>>>>>>>
>>>>>>>>>>>> This reformatting seems unnecessary, and I prefer to keep 
>>>>>>>>>>>> the comment about the default argument.
>>>>>>>>>>>
>>>>>>>>>>> This overload doesn't take a default argument.  (There was a 
>>>>>>>>>>> stray
>>>>>>>>>>> declaration of a similar function at the top of the file that 
>>>>>>>>>>> had
>>>>>>>>>>> one.  I've removed it.)
>>>>>>>>>>
>>>>>>>>>> Ah, true.
>>>>>>>>>>
>>>>>>>>>>>>> -      if (!size || TREE_CODE (size) != INTEGER_CST)
>>>>>>>>>>>>> -       return false;
>>>>>>>>>>>>  >...
>>>>>>>>>>>>
>>>>>>>>>>>> You change some failure cases in compute_objsize to return 
>>>>>>>>>>>> success with a maximum range, while others continue to 
>>>>>>>>>>>> return failure. This needs commentary about the design 
>>>>>>>>>>>> rationale.
>>>>>>>>>>>
>>>>>>>>>>> This is too much for a comment in the code but the background is
>>>>>>>>>>> this: compute_objsize initially returned the object size as a 
>>>>>>>>>>> constant.
>>>>>>>>>>> Recently, I have enhanced it to return a range to improve 
>>>>>>>>>>> warnings for
>>>>>>>>>>> allocated objects.  With that, a failure can be turned into 
>>>>>>>>>>> success by
>>>>>>>>>>> having the function set the range to that of the largest 
>>>>>>>>>>> object. That
>>>>>>>>>>> should simplify the function's callers and could even improve
>>>>>>>>>>> the detection of some invalid accesses.  Once this change is 
>>>>>>>>>>> made
>>>>>>>>>>> it might even be possible to change its return type to void.
>>>>>>>>>>>
>>>>>>>>>>> The change that caught your eye is necessary to make the 
>>>>>>>>>>> function
>>>>>>>>>>> a drop-in replacement for the C++ front end code which makes 
>>>>>>>>>>> this
>>>>>>>>>>> same assumption.  Without it, a number of test cases that 
>>>>>>>>>>> exercise
>>>>>>>>>>> VLAs fail in g++.dg/warn/Wplacement-new-size-5.C.  For example:
>>>>>>>>>>>
>>>>>>>>>>>    void f (int n)
>>>>>>>>>>>    {
>>>>>>>>>>>      char a[n];
>>>>>>>>>>>      new (a - 1) int ();
>>>>>>>>>>>    }
>>>>>>>>>>>
>>>>>>>>>>> Changing any of the other places isn't necessary for existing 
>>>>>>>>>>> tests
>>>>>>>>>>> to pass (and I didn't want to introduce too much churn).  But 
>>>>>>>>>>> I do
>>>>>>>>>>> want to change the rest of the function along the same lines 
>>>>>>>>>>> at some
>>>>>>>>>>> point.
>>>>>>>>>>
>>>>>>>>>> Please do change the other places to be consistent; better to 
>>>>>>>>>> have more churn than to leave the function half-updated.  That 
>>>>>>>>>> can be a separate patch if you prefer, but let's do it now 
>>>>>>>>>> rather than later.
>>>>>>>>>
>>>>>>>>> I've made most of these changes in the other patch (also 
>>>>>>>>> attached).
>>>>>>>>> I'm quite happy with the result but it turned out to be a lot more
>>>>>>>>> work than either of us expected, mostly due to the amount of 
>>>>>>>>> testing.
>>>>>>>>>
>>>>>>>>> I've left a couple of failing cases in place mainly as reminders
>>>>>>>>> to handle them better (which means I also didn't change the caller
>>>>>>>>> to avoid testing for failures).  I've also added TODO notes with
>>>>>>>>> reminders to handle some of the new codes more completely.
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>>>> +  special_array_member sam{ };
>>>>>>>>>>>>
>>>>>>>>>>>> sam is always set by component_ref_size, so I don't think 
>>>>>>>>>>>> it's necessary to initialize it at the declaration.
>>>>>>>>>>>
>>>>>>>>>>> I find initializing pass-by-pointer local variables helpful but
>>>>>>>>>>> I don't insist on it.
>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>> @@ -187,7 +187,7 @@ decl_init_size (tree decl, bool min)
>>>>>>>>>>>>>    tree last_type = TREE_TYPE (last);
>>>>>>>>>>>>>    if (TREE_CODE (last_type) != ARRAY_TYPE
>>>>>>>>>>>>>        || TYPE_SIZE (last_type))
>>>>>>>>>>>>> -    return size;
>>>>>>>>>>>>> +    return size ? size : TYPE_SIZE_UNIT (type);
>>>>>>>>>>>>
>>>>>>>>>>>> This change seems to violate the comment for the function.
>>>>>>>>>>>
>>>>>>>>>>> By my reading (and writing) the change is covered by the first
>>>>>>>>>>> sentence:
>>>>>>>>>>>
>>>>>>>>>>>     Returns the size of the object designated by DECL 
>>>>>>>>>>> considering
>>>>>>>>>>>     its initializer if it either has one or if it would not 
>>>>>>>>>>> affect
>>>>>>>>>>>     its size, ...
>>>>>>>>>>
>>>>>>>>>> OK, I see it now.
>>>>>>>>>>
>>>>>>>>>>> It handles a number of cases in Wplacement-new-size.C fail that
>>>>>>>>>>> construct a larger object in an extern declaration of a 
>>>>>>>>>>> template,
>>>>>>>>>>> like this:
>>>>>>>>>>>
>>>>>>>>>>>    template <class> struct S { char c; };
>>>>>>>>>>>    extern S<int> s;
>>>>>>>>>>>
>>>>>>>>>>>    void f ()
>>>>>>>>>>>    {
>>>>>>>>>>>      new (&s) int ();
>>>>>>>>>>>    }
>>>>>>>>>>>
>>>>>>>>>>> I don't know why DECL_SIZE isn't set here (I don't think it can
>>>>>>>>>>> be anything but equal to TYPE_SIZE, can it?) and other than 
>>>>>>>>>>> struct
>>>>>>>>>>> objects with a flexible array member where this identity doesn't
>>>>>>>>>>> hold I can't think of others.  Am I missing something?
>>>>>>>>>>
>>>>>>>>>> Good question.  The attached patch should fix that, so you 
>>>>>>>>>> shouldn't need the change to decl_init_size:
>>>>>>>>>
>>>>>>>>> I've integrated it into the bug fix.
>>>>>>>>>
>>>>>>>>> Besides the usual x86_64-linux bootstrap/regtest I tested both
>>>>>>>>> patches by building a few packages, including Binutils/GDB, Glibc,
>>>>>>>>> and  verifying no new warnings show up.
>>>>>>>>>
>>>>>>>>> Martin
>>>>>>
>>>>>>> +offset_int
>>>>>>> +access_ref::size_remaining (offset_int *pmin /* = NULL */) const
>>>>>>
>>>>>> For the various member functions, please include the comments with 
>>>>>> the definition as well as the in-class declaration.
>>>>>
>>>>> Only one access_ref member function is defined out-of-line: 
>>>>> offset_bounded().  I've adjusted the comment and copied it above
>>>>> the function definition.
>>>>>
>>>>>>
>>>>>>> +      if (offrng[1] < offrng[0])
>>>>>>
>>>>>> What does it mean for the max offset to be less than the min 
>>>>>> offset? I wouldn't expect that to ever happen with wide integers.
>>>>>
>>>>> The offset is represented in sizetype with negative values represented
>>>>> as large positive values, but has to be converted to ptrdiff_t.
>>>>
>>>> It looks to me like the offset is offset_int, which is both signed 
>>>> and big enough to hold all values of sizetype without turning large 
>>>> positive values into negative values.  Where are these 
>>>> sign-switching conversions happening?
>>>
>>> In get_offset_range in builtins.c.
>>
>> Since we're converting to offset_int there, why not give the 
>> offset_int the real value rather than a bogus negative value?
> 
> I don't understand the question: the real offset (in the program)
> is negative when its sizetype representation is greater than
> PTRDIFF_MAX.  It's worked this way for years.

OK, then we're back to my original question: why is the max offset less 
than the min offset?  If the range includes negative values, why isn't 
the more-negative one the minimum?

>>>>> These
>>>>> cases come up when the unsigned offset is an ordinary range that
>>>>> corresponds to an anti-range, such as here:
>>>>>
>>>>>    extern char a[2];
>>>>>
>>>>>    void f (unsigned long i)
>>>>>    {
>>>>>      if (i == 0)
>>>>>        return;
>>>>>      a[i] = 0;   // i's range is [1, -1] (i.e., [1, SIZE_MAX]
>>>>>    }
>>>>>
>>>>>>
>>>>>>> +  /* Return true if OFFRNG is bounded to a subrange of possible 
>>>>>>> offset
>>>>>>> +     values.  */
>>>>>>> +  bool offset_bounded () const;
>>>>>>
>>>>>> I don't understand how you're using this.  The implementation 
>>>>>> checks for the possible offset values falling outside those 
>>>>>> representable by ptrdiff_t, unless the range is only a single 
>>>>>> value.  And then the only use is
>>>>>>
>>>>>>> +  if (ref.offset_zero () || !ref.offset_bounded ())
>>>>>>> +    inform (DECL_SOURCE_LOCATION (ref.ref),
>>>>>>> +        "%qD declared here", ref.ref);
>>>>>>> +  else if (ref.offrng[0] == ref.offrng[1])
>>>>>>> +    inform (DECL_SOURCE_LOCATION (ref.ref),
>>>>>>> +        "at offset %wi from %qD declared here",
>>>>>>> +        ref.offrng[0].to_shwi (), ref.ref);
>>>>>>> +  else
>>>>>>> +    inform (DECL_SOURCE_LOCATION (ref.ref),
>>>>>>> +        "at offset [%wi, %wi] from %qD declared here",
>>>>>>> +        ref.offrng[0].to_shwi (), ref.offrng[1].to_shwi (), 
>>>>>>> ref.ref);
>>>>>>
>>>>>> So if the possible offsets are all representable by ptrdiff_t, we 
>>>>>> don't print the range?  The middle case also looks unreachable, 
>>>>>> since offset_bounded will return false in that case.
>>>>>
>>>>> The function was originally named "offset_unbounded."  I changed
>>>>> it to "offset_bounded" but looks like I didn't finish the job or
>>>>> add any tests for it.
>>>>>
>>>>> The goal of conditionals is to avoid overwhelming the user with
>>>>> excessive numbers that may not be meaningful or even relevant
>>>>> to the warning.  I've corrected the function body, tweaked and
>>>>> renamed the get_range function to get_offset_range to do a better
>>>>> job of extracting ranges from the types of some nonconstant
>>>>> expressions the front end passes it, and added a new test for
>>>>> all this.  Attached is the new revision.
>>>>>
>>>>> Martin
>>>>
>>>
>>
>