[cfe-dev] [Analyzer] Equality of pointers

[Artem Dergachev via cfe-dev]

I believe in our current situation we should simply generate a sink 
whenever we discover symbolic pointer aliasing, because RegionStore 
always implicitly assumes that there's no aliasing. Eg., consider:

   void example1(int *x, int *y) {
     *x = 1; // (SymRegion{reg_$1<x>}, 0, direct): 1 S32b
     *y = 2; // (SymRegion{reg_$2<y>}, 0, direct): 2 S32b
     if (x == y) {
       // ???
     }
   }

The correct way to model the then-branch of the if-statement is to 
figure out which of the bindings is *newer* and keep it; in this case 
it'll mean dropping the binding of 'x' and keeping the binding of 'y'. 
This would mean that the value of the common pointee of 'x' and 'y' 
would be equal to 2. However, in its current shape RegionStore simply 
does not track history of bindings. We can make it track the history, 
but it'll be a ridiculously complex data structure - and even the 
current simple structure is so complicated and messy that nobody 
understands how it works.

All right, suppose we make a new Store implementation that'll handle 
aliasing perfectly and is otherwise perfect in every way. Suppose we 
also have a perfect constraint solver that knows that in the following code

   void example2(int *x, int *y) {
     if (*x > 5)
       if (*y < 10)
         if (x == y) {
           // ???
         }
   }

the valid constraint range for the common pointee of 'x' and 'y' would 
be [6, 9] (this won't be immediately solved by the Z3 solver because 
heap shape information still needs to be stuffed into it). Now, how 
about these examples?:

   void example3(int *x, int *y) {
     delete x;
     if (x == y) {
       // ???
     }
   }

   void example4(int *x, int *y) {
     delete x;
     delete y;
     if (x == y) {
       // ???
     }
   }

In example 3 the common pointee would be deleted. It'd cause a warning 
if 'y' is used later. In example 4 it's an outright retroactive 
double-free. We need to modify MallocChecker to handle both of these 
cases correctly.

If the same happens to RetainCountChecker, we'll need to *add* reference 
counts of the two pointers together. That's already a large variety of 
very non-trivial operations that we need to perform. How about aliasing 
file descriptors (which aren't even necessarily pointers)? How about 
aliasing two mutexes one of which is locked and the other is unlocked? 
How much completely non-trivial and non-reusable code do we need to 
write in dozens of checkers to solve this problem, given that we can 
barely solve it for RegionStore?

===========

What we *can* do, however, is to not only generate the sink, but 
*restart* the analysis from the beginning, assuming that the two 
pointers point to the same memory from the start. Say, force the 
SymbolManager that's owned by this analysis's ExprEngine to produce 
reg_$1<x> whenever it's asked for reg_$2<y>. This would make sure that 
we don't make wrong assumptions in the first place, so that we didn't 
have to undo them in the middle of the analysis. This wouldn't require 
much checker-side support, so it'll be much more in the spirit of the 
Analyzer. And it's also much easier to implement. As usual, we'll need 
some heuristics to make sure we don't exponentially explode on the 
number of different combinations of pointers that can potentially alias.


On 05.06.2019 05:23, Gábor Horváth via cfe-dev wrote:
> Hi!
>
> This is an excellent point and it is not easy to solve. One of the 
> reason why your original idea is hard to implement is the following. 
> Let's look at the code snippet below:
> void f(int *p, int *q) {
>   int *p2 = p + 2;
>   if (p == q) {
>     // Here we learned something new about p2 as well.
>     // So we might need to update all the symbolic expressions and 
> bindings, not just p.
>   }
> }
>
> I think your second idea should work well, but I have no idea about 
> the performance implications. As a starting point we could implement 
> your second solution and do some benchmarks both performance wise and 
> looking at the results from some open source projects to see what 
> happens to the false positive rate.
>
> Out of curiosity, do we have the same problems with invalidations?
>
> I am sure Artem will have some more to add but as far as I know he is 
> busy with WWDC this week.
>
> Regards,
> Gabor
>
> On Tue, 4 Jun 2019 at 14:53, Ádám Balogh via cfe-dev 
> <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>
>     Hello,
>
>     This is not the first time that we are facing the following problem.
>
>     Given a code that clears all non-sentinel elements from a circular
>     double-linked list with sentinel element:
>
>     typedef struct ListNode {
>
>       struct ListNode *next;
>
>       struct ListNode *prev;
>
>       void *data;
>
>     } ListNode;
>
>     void ListClear(ListNode *list) {
>
>       while (list->next != list) {
>
>         ListNode *node = list->next;
>
>         node->next->prev = node->prev;
>
>         node->prev->next = node->next;
>
>         node->next = NULL;
>
>         node->prev = NULL;
>
>         free(node);
>
>       }
>
>     }
>
>     unix.Malloc Use after free is reported in the second iteration of
>     the loop for `node`.
>
>     This is not necessarily a false positive if analyzed top level.
>     The function that clears the nodes of the list implicitly assumes
>     that it is a consistent circular double-linked list with a
>     sentinel node. However this invariant is not ensured anywhere
>     inside the function. It is thus not known that the `prev` pointer
>     of the first non-sentinel node points back to the sentinel node,
>     neither is it known that applying a `prev` and a `next` after each
>     other in any order leads back to the original node. Without these
>     invariants the analyzer cannot know that after unlinking and
>     freeing the first non-sentinel element and then taking again the
>     first non-sentinel element is not equal to the original sentinel
>     element. So it assumes that they are equal and used again after
>     freeing its memory space. Thus from the perspective of the
>     standalone function it can be a use-after-free situation.
>
>     So I fixed the function by inserting assertions to ensure the
>     invariant:
>
>     void ListClear(ListNode *list) {
>
>       while (list->next != list) {
>
>         ListNode *node = list->next;
>
>         assert(node->prev == list);
>
>         assert(node->next->prev == node);
>
>         assert(node->prev->next == node);
>
>         node->next->prev = node->prev;
>
>         node->prev->next = node->next;
>
>         node->next = NULL;
>
>         node->prev = NULL;
>
>         free(node);
>
>       }
>
>     }
>
>     However, it did not help because the analyzer cannot handle
>     equality of pointers correctly. If we have an assumption that two
>     pointers are equal the analyzer still creates two symbolic regions
>     and records a constraint that the symbols of the symbolic regions
>     are equal. Later this constraint can be used for deciding whether
>     they are equal or not but it does not handle them as the same
>     region. So if we change the pointer `node->prev->next` to
>     `node->next` then `node->prev->next` is changed but `list->next`
>     not even if we assumed that they are the same. That is the reason
>     the analyzer takes the freed region again in the next iteration of
>     the loop and reports an error.
>
>     A strange thing is that the constraint manager rearranges the
>     comparison of the two symbols (of the symbolic regions) to a
>     difference without `aggressive-binary-operation-simplification`
>     disabled. (It should not do it even if this option is enabled
>     since the ranges of symbolic values are not constrained to `MAX/4`
>     of the type.)
>
>     The question is how to fix this. I think that if two pointers are
>     the same, then they must point to exactly the same memory region
>     which means exactly the same symbol if it is a symbolic region.
>     Thus instead of assigning `$reg2` to `node->prev` and recording
>     whether the range of `$reg2-$reg0` is `[0..0]` the analyzer should
>     assign `$reg0` to `node->prev` which is the region of `list`.
>     Similarly, `node->next->prev` should not become `$reg7` where the
>     range of `$reg7-$reg1` is `[0..0]` and `node->prev->next ` should
>     not become `$reg11` where the range of `$reg11-$reg1` is `[0..0]`
>     but both should become `$reg1`. This way when we unlink `node`
>     `node->next->prev` becomes `reg0` (thus the region of `list`) and
>     `node->prev->next` e.g. `$reg2`. This way in the next iteration
>     `node->next` is `$reg2` instead of the freed `$reg1` so the false
>     warning goes away. Of course I know this is not easy to implement.
>     If both pointers already have regions assigned then which one to
>     chose and how to replace the other one?
>
>     Another solution could be iterating all the existing regions upon
>     bindings and do the same binding to the regions equal to the
>     affected region. (I hope I managed to express myself clearly
>     enough here.)
>
>     This is an old weakness of the Analyzer which should be solved
>     eventually because it causes false positives. A new constraint
>     manager does not help because the problem is not there.
>
>     Any ideas?
>
>     Regards,
>
>     Ádám
>
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