Re: [PATCH] extend -Wstringop-overflow to allocated objects (PR 91582)

[Martin Sebor <msebor@gmail.com>]

On 12/2/19 10:06 AM, Jeff Law wrote:
> On 11/8/19 3:11 PM, Martin Sebor wrote:
>> Unless it's used with _FORTIFY_SOURCE, -Wstringop-overflow
>> doesn't consider out-of-bounds accesses to objects allocated
>> by alloca, malloc, other functions declared with attribute
>> alloc_size, or even VLAs with variable bounds.  This was
>> a known limitation of the checks (done just before expansion)
>> relying on the the object size pass when they were introduced
>> in GCC 7.
>>
>> But since its introduction in GCC 7, the warning has evolved
>> beyond some of the limitations of the object size pass.  Unlike
>> it, the warning considers non-constant offsets and stores with
>> non-constant sizes.  Attached is a simple enhancement that
>> (finally) adds the ability to also detect overflow in allocated
>> objects to the warning.
>>
>> With the patch GCC detects the overflow in code like this:
>>
>>    char* f (void)
>>    {
>>      char s[] = "12345";
>>      char *p = malloc (strlen (s));
>>      strcpy (p, s);   // warning here
>>      return p;
>>    }
>>
>> but not (yet) in something like this:
>>
>>    char* g (const char *s)
>>    {
>>      char *p = malloc (strlen (s));
>>      strcpy (p, s);   // no warning (yet)
>>      return p;
>>    }
>>
>> and quite a few other examples.  Doing better requires extending
>> the strlen pass.  I'm working on this extension and expect to
>> submit a patch before stage 1 ends.
>>
>> Martin
>>
>> PS I was originally planning to do all the allocation checking
>> in the strlen pass but it occurred to me that by also enhancing
>> the compute_objsize function, all warnings that use it will
>> benefit.  Besides -Wstringop-overflow this includes a subset
>> of -Warray-bounds, -Wformat-overflow, and -Wrestrict.  It's
>> nice when a small enhancement has such a broad positive effect.
> 
>> PR middle-end/91582 - missing heap overflow detection for strcpy
>>
>> gcc/ChangeLog:
>>
>>          * builtins.c (gimple_call_alloc_size): New function.
>>          (compute_objsize): Add argument.  Call gimple_call_alloc_size.
>>          Handle variable offsets and indices.
>>          * builtins.h (gimple_call_alloc_size): Declare.
>>          (compute_objsize): Add argument.
>>          * tree-ssa-strlen.c (handle_store): Handle calls to allocated objects.
>>
>> gcc/testsuite/ChangeLog:
>>
>>          * c-c++-common/Wstringop-truncation.c: Remove xfails.
>>          * gcc/testsuite/g++.dg/ext/attr-alloc_size.C: Suppress -Warray-bounds.
>>          * gcc.dg/Wstringop-overflow-22.c: New test.
>>          * gcc/testsuite/gcc.dg/attr-alloc_size.c: Suppress -Warray-bounds.
>>          * gcc/testsuite/gcc.dg/attr-copy-2.c: Same.
>>          * gcc.dg/builtin-stringop-chk-5.c: Remove xfails.
>>          * gcc.dg/builtin-stringop-chk-8.c: Same.  Correct the text of expected
>>          warnings.
>>          * gcc.target/i386/pr82002-2a.c: Prune expected warning.
>>          * gcc.target/i386/pr82002-2b.c: Same.
> [ ... ]
> 
> 
>> Index: gcc/builtins.c
>> ===================================================================
>> --- gcc/builtins.c      (revision 277978)
>> +++ gcc/builtins.c      (working copy)
>> @@ -3563,6 +3563,80 @@ check_access (tree exp, tree, tree, tree dstwrite,
>>     return true;
>>   }
>>
>> +/* If STMT is a call to an allocation function, returns the size
>> +   of the object allocated by the call.  */
>> +
>> +tree
>> +gimple_call_alloc_size (gimple *stmt)
>> +{
>> +  tree size = gimple_call_arg (stmt, argidx1);
>> +  tree n = argidx2 < nargs ? gimple_call_arg (stmt, argidx2) : integer_one_node;
>> +
>> +  /* To handle ranges do the math in wide_int and return the product
>> +     of the upper bounds as a constant.  Ignore anti-ranges.  */
>> +  wide_int rng1[2];
>> +  if (TREE_CODE (size) == INTEGER_CST)
>> +    rng1[0] = rng1[1] = wi::to_wide (size);
>> +  else if (TREE_CODE (size) != SSA_NAME
>> +          || get_range_info (size, rng1, rng1 + 1) != VR_RANGE)
>> +    return NULL_TREE;
>> +
>> +  wide_int rng2[2];
>> +  if (TREE_CODE (n) == INTEGER_CST)
>> +    rng2[0] = rng2[1] = wi::to_wide (n);
>> +  else if (TREE_CODE (n) != SSA_NAME
>> +          || get_range_info (n, rng2 + 1, rng2 + 1) != VR_RANGE)
>> +    return NULL_TREE;
> Should that 2nd call to get_range_info be "get_range_info (n, rng2, rng2
> + 1)?  I don't think it makes any difference in practice due to the
> implementation of get_range_info, but if it wasn't intentional let's get
> it fixed.

Yes, it should be.  It's correct in my tree but didn't post
the updated revision.

> 
> 
>> Index: gcc/builtins.h
>> ===================================================================
>> --- gcc/builtins.h      (revision 277978)
>> +++ gcc/builtins.h      (working copy)
>> @@ -134,7 +134,8 @@ extern tree fold_call_stmt (gcall *, bool);
>>   extern void set_builtin_user_assembler_name (tree decl, const char *asmspec);
>>   extern bool is_simple_builtin (tree);
>>   extern bool is_inexpensive_builtin (tree);
>> -extern tree compute_objsize (tree, int, tree * = NULL);
>> +tree gimple_call_alloc_size (gimple *);
>> +extern tree compute_objsize (tree, int, tree * = NULL, tree * = NULL);
>>
>>   extern bool readonly_data_expr (tree exp);
>>   extern bool init_target_chars (void);
> Is there a reason there's no "extern" on the gimple_call_alloc_size
> prototype?

I'm sure it was copied and pasted from the definition.  It makes
no difference either way so it didn't get caught by anything.

> 
> I think this is fine with those nits fixed.  You'll have a minor merge
> conflict with the compute_objsize changes due to recent fixes in the
> same hunk of code, but I don't think it warrants reposting/resubmission.
> 

I've fixed the nits above and committed r278983 after retesting.

This is an improvement in the buffer overflow detection but there
is still the (arguably more important) second half of it:
extending the strlen pass to detect the overflow that cannot be
caught later (e.g., all stores by MEM_REFs are only handled in
strlen): https://gcc.gnu.org/ml/gcc-patches/2019-11/msg02340.html

Martin