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

Jeff Law law@redhat.com
Mon Dec 2 17:06:00 GMT 2019


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.


> 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 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.

Jeff



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