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

[Christophe Lyon <christophe.lyon@linaro.org>]

On Tue, 29 Sep 2020 at 00:02, Martin Sebor via Gcc-patches
<gcc-patches@gcc.gnu.org> 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.  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

Hi Martin,

One of t new tests fails on many arm configurations and on aarch64
FAIL: gcc.dg/Wstringop-overflow-47.c  (test for warnings, line 29)
FAIL: gcc.dg/Wstringop-overflow-47.c  (test for warnings, line 32)
FAIL: gcc.dg/Wstringop-overflow-47.c  (test for warnings, line 37)

It looks like it is also failing on x86, s390 and ia64 according to
gcc-testresults.

Can you check?

Thanks