[C PATCH]: Add Walloc-type to warn about insufficient size in allocations

[Siddhesh Poyarekar]

On 2023-07-21 07:21, Martin Uecker via Gcc-patches wrote:
> 
> 
> This patch adds a warning for allocations with insufficient size
> based on the "alloc_size" attribute and the type of the pointer
> the result is assigned to. While it is theoretically legal to
> assign to the wrong pointer type and cast it to the right type
> later, this almost always indicates an error. Since this catches
> common mistakes and is simple to diagnose, it is suggested to
> add this warning.
>   
> 
> Bootstrapped and regression tested on x86.
> 
> 
> Martin
> 
> 
> 
> Add option Walloc-type that warns about allocations that have
> insufficient storage for the target type of the pointer the
> storage is assigned to.
> 
> gcc:
> 	* doc/invoke.texi: Document -Wstrict-flex-arrays option.
> 
> gcc/c-family:
> 
> 	* c.opt (Walloc-type): New option.
> 
> gcc/c:
> 	* c-typeck.cc (convert_for_assignment): Add Walloc-type warning.
> 
> gcc/testsuite:
> 
> 	* gcc.dg/Walloc-type-1.c: New test.
> 
> 
> diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
> index 4abdc8d0e77..8b9d148582b 100644
> --- a/gcc/c-family/c.opt
> +++ b/gcc/c-family/c.opt
> @@ -319,6 +319,10 @@ Walloca
>   C ObjC C++ ObjC++ Var(warn_alloca) Warning
>   Warn on any use of alloca.
>   
> +Walloc-type
> +C ObjC Var(warn_alloc_type) Warning
> +Warn when allocating insufficient storage for the target type of the
> assigned pointer.
> +
>   Walloc-size-larger-than=
>   C ObjC C++ LTO ObjC++ Var(warn_alloc_size_limit) Joined Host_Wide_Int
> ByteSize Warning Init(HOST_WIDE_INT_MAX)
>   -Walloc-size-larger-than=<bytes>	Warn for calls to allocation
> functions that
> diff --git a/gcc/c/c-typeck.cc b/gcc/c/c-typeck.cc
> index 7cf411155c6..2e392f9c952 100644
> --- a/gcc/c/c-typeck.cc
> +++ b/gcc/c/c-typeck.cc
> @@ -7343,6 +7343,32 @@ convert_for_assignment (location_t location,
> location_t expr_loc, tree type,
>   		    "request for implicit conversion "
>   		    "from %qT to %qT not permitted in C++", rhstype,
> type);
>   
> +      /* Warn of new allocations are not big enough for the target
> type.  */
> +      tree fndecl;
> +      if (warn_alloc_type
> +	  && TREE_CODE (rhs) == CALL_EXPR
> +	  && (fndecl = get_callee_fndecl (rhs)) != NULL_TREE
> +	  && DECL_IS_MALLOC (fndecl))
> +	{
> +	  tree fntype = TREE_TYPE (fndecl);
> +	  tree fntypeattrs = TYPE_ATTRIBUTES (fntype);
> +	  tree alloc_size = lookup_attribute ("alloc_size",
> fntypeattrs);
> +	  if (alloc_size)
> +	    {
> +	      tree args = TREE_VALUE (alloc_size);
> +	      int idx = TREE_INT_CST_LOW (TREE_VALUE (args)) - 1;
> +	      /* For calloc only use the second argument.  */
> +	      if (TREE_CHAIN (args))
> +		idx = TREE_INT_CST_LOW (TREE_VALUE (TREE_CHAIN
> (args))) - 1;
> +	      tree arg = CALL_EXPR_ARG (rhs, idx);
> +	      if (TREE_CODE (arg) == INTEGER_CST
> +		  && tree_int_cst_lt (arg, TYPE_SIZE_UNIT (ttl)))
> +		 warning_at (location, OPT_Walloc_type, "allocation of
> "
> +			     "insufficient size %qE for type %qT with
> "
> +			     "size %qE", arg, ttl, TYPE_SIZE_UNIT
> (ttl));
> +	    }
> +	}
> +

Wouldn't this be much more useful in later phases with ranger feedback 
like with the warn_access warnings?  That way the comparison won't be 
limited to constant sizes.

Thanks,
Sid