[PATCH] include size and offset in -Wstringop-overflow

[Jeff Law]

On 11/6/19 3:34 PM, Martin Sebor wrote:
> On 11/6/19 2:06 PM, Martin Sebor wrote:
>> On 11/6/19 1:39 PM, Jeff Law wrote:
>>> On 11/6/19 1:27 PM, Martin Sebor wrote:
>>>> On 11/6/19 11:55 AM, Jeff Law wrote:
>>>>> On 11/6/19 11:00 AM, Martin Sebor wrote:
>>>>>> The -Wstringop-overflow warnings for single-byte and multi-byte
>>>>>> stores mention the amount of data being stored and the amount of
>>>>>> space remaining in the destination, such as:
>>>>>>
>>>>>> warning: writing 4 bytes into a region of size 0 [-Wstringop-overflow=]
>>>>>>
>>>>>>     123 |   *p = 0;
>>>>>>         |   ~~~^~~
>>>>>> note: destination object declared here
>>>>>>      45 |   char b[N];
>>>>>>         |        ^
>>>>>>
>>>>>> A warning like this can take some time to analyze.  First, the size
>>>>>> of the destination isn't mentioned and may not be easy to tell from
>>>>>> the sources.  In the note above, when N's value is the result of
>>>>>> some non-trivial computation, chasing it down may be a small project
>>>>>> in and of itself.  Second, it's also not clear why the region size
>>>>>> is zero.  It could be because the offset is exactly N, or because
>>>>>> it's negative, or because it's in some range greater than N.
>>>>>>
>>>>>> Mentioning both the size of the destination object and the offset
>>>>>> makes the existing messages clearer, are will become essential when
>>>>>> GCC starts diagnosing overflow into allocated buffers (as my
>>>>>> follow-on patch does).
>>>>>>
>>>>>> The attached patch enhances -Wstringop-overflow to do this by
>>>>>> letting compute_objsize return the offset to its caller, doing
>>>>>> something similar in get_stridx, and adding a new function to
>>>>>> the strlen pass to issue this enhanced warning (eventually, I'd
>>>>>> like the function to replace the -Wstringop-overflow handler in
>>>>>> builtins.c).  With the change, the note above might read something
>>>>>> like:
>>>>>>
>>>>>> note: at offset 11 to object ‘b’ with size 8 declared here
>>>>>>      45 |   char b[N];
>>>>>>         |        ^
>>>>>>
>>>>>> Tested on x86_64-linux.
>>>>>>
>>>>>> Martin
>>>>>>
>>>>>> gcc-store-offset.diff
>>>>>>
>>>>>> gcc/ChangeLog:
>>>>>>
>>>>>>      * builtins.c (compute_objsize): Add an argument and set it to
>>>>>> offset
>>>>>>      into destination.
>>>>>>      * builtins.h (compute_objsize): Add an argument.
>>>>>>      * tree-object-size.c (addr_object_size): Add an argument and
>>>>>> set it
>>>>>>      to offset into destination.
>>>>>>      (compute_builtin_object_size): Same.
>>>>>>      * tree-object-size.h (compute_builtin_object_size): Add an
>>>>>> argument.
>>>>>>      * tree-ssa-strlen.c (get_addr_stridx): Add an argument and
>>>>>> set it
>>>>>>      to offset into destination.
>>>>>>      (maybe_warn_overflow): New function.
>>>>>>      (handle_store): Call maybe_warn_overflow to issue warnings.
>>>>>>
>>>>>> gcc/testsuite/ChangeLog:
>>>>>>
>>>>>>      * c-c++-common/Wstringop-overflow-2.c: Adjust text of expected
>>>>>> messages.
>>>>>>      * g++.dg/warn/Wstringop-overflow-3.C: Same.
>>>>>>      * gcc.dg/Wstringop-overflow-17.c: Same.
>>>>>>
>>>>>
>>>>>> Index: gcc/tree-ssa-strlen.c
>>>>>> ===================================================================
>>>>>> --- gcc/tree-ssa-strlen.c    (revision 277886)
>>>>>> +++ gcc/tree-ssa-strlen.c    (working copy)
>>>>>> @@ -189,6 +189,52 @@ struct laststmt_struct
>>>>>>    static int get_stridx_plus_constant (strinfo *, unsigned
>>>>>> HOST_WIDE_INT, tree);
>>>>>>    static void handle_builtin_stxncpy (built_in_function,
>>>>>> gimple_stmt_iterator *);
>>>>>>    +/* Sets MINMAX to either the constant value or the range VAL
>>>>>> is in
>>>>>> +   and returns true on success.  */
>>>>>> +
>>>>>> +static bool
>>>>>> +get_range (tree val, wide_int minmax[2], const vr_values *rvals =
>>>>>> NULL)
>>>>>> +{
>>>>>> +  if (tree_fits_uhwi_p (val))
>>>>>> +    {
>>>>>> +      minmax[0] = minmax[1] = wi::to_wide (val);
>>>>>> +      return true;
>>>>>> +    }
>>>>>> +
>>>>>> +  if (TREE_CODE (val) != SSA_NAME)
>>>>>> +    return false;
>>>>>> +
>>>>>> +  if (rvals)
>>>>>> +    {
>>>>>> +      gimple *def = SSA_NAME_DEF_STMT (val);
>>>>>> +      if (gimple_assign_single_p (def)
>>>>>> +      && gimple_assign_rhs_code (def) == INTEGER_CST)
>>>>>> +    {
>>>>>> +      /* get_value_range returns [0, N] for constant
>>>>>> assignments.  */
>>>>>> +      val = gimple_assign_rhs1 (def);
>>>>>> +      minmax[0] = minmax[1] = wi::to_wide (val);
>>>>>> +      return true;
>>>>>> +    }
>>>>> Umm, something seems really off with this hunk.  If the SSA_NAME is
>>>>> set
>>>>> via a simple constant assignment, then the range ought to be a
>>>>> singleton
>>>>> ie [CONST,CONST].   Is there are particular test were this is not
>>>>> true?
>>>>>
>>>>> The only way offhand I could see this happening is if originally
>>>>> the RHS
>>>>> wasn't a constant, but due to optimizations it either simplified
>>>>> into a
>>>>> constant or a constant was propagated into an SSA_NAME appearing on
>>>>> the
>>>>> RHS.  This would have to happen between the last range analysis and
>>>>> the
>>>>> point where you're making this query.
>>>>
>>>> Yes, I think that's right.  Here's an example where it happens:
>>>>
>>>>    void f (void)
>>>>    {
>>>>      char s[] = "1234";
>>>>      unsigned n = strlen (s);
>>>>      char vla[n];   // or malloc (n)
>>>>      vla[n] = 0;    // n = [4, 4]
>>>>      ...
>>>>    }
>>>>
>>>> The strlen call is folded to 4 but that's not propagated to
>>>> the access until sometime after the strlen pass is done.
>>> Hmm.  Are we calling set_range_info in that case?  That goes behind the
>>> back of pass instance of vr_values.  If so, that might argue we want to
>>> be setting it in vr_values too.
>>
>> No, set_range_info is only called for ranges.  In this case,
>> handle_builtin_strlen replaces the strlen() call with 4:
>>
>>    s = "1234";
>>    _1 = __builtin_strlen (&s);
>>    n_2 = (unsigned int) _1;
>>    a.1_8 = __builtin_alloca_with_align (_1, 8);
>>    (*a.1_8)[n_2] = 0;
>>
>> When the access is made, the __builtin_alloca_with_align call
>> is found as the destination and the _1 SSA_NAME is used to
>> get its size.  We get back the range [4, 4].
> 
> By the way, I glossed over one detail.  The above doesn't work
> exactly as is because the allocation size is the SSA_NAME _1
> (with the range [4, 4]) but the index is the SSA_NAME n_2 (with
> the range [0, 4]; the range is [0, 4] because it was set based
> on the size of the argument to the strlen() call well before
> the strlen pass even ran).
Which would tend to argue that we should forward propagate the constant
to the uses of _1.  That should expose that the RHS of the assignment to
n_2 is a constant as well.


> 
> To make it work across assignments we need to propagate the strlen
> results down the CFG somehow.  I'm hoping the on-demand VRP will
> do this automagically.
It would, but it's probably more heavyweight than we need.  We just need
to forward propagate the discovered constant to the use points and pick
up any secondary opportunities that arise.

jeff