[patch][version 6] add -ftrivial-auto-var-init and variable attribute "uninitialized" to gcc

Qing Zhao qing.zhao@oracle.com
Tue Aug 10 13:39:59 GMT 2021



> On Aug 10, 2021, at 2:36 AM, Richard Biener <rguenther@suse.de> wrote:
> 
> On Mon, 9 Aug 2021, Qing Zhao wrote:
> 
>> Hi, Richard,
>> 
>> Thanks a lot for you review.
>> 
>> Although these comments are not made on the latest patch (7th version) :-), all the comments are valid since the parts you commented
>> are not changed in the 7th version.
>> 
>> 
>>> On Aug 9, 2021, at 9:09 AM, Richard Biener <rguenther@suse.de> wrote:
>>> 
>>> On Tue, 27 Jul 2021, Qing Zhao wrote:
>>> 
>>>> Hi,
>>>> 
>>>> This is the 6th version of the patch for the new security feature for GCC.
>>>> 
>>>> I have tested it with bootstrap on both x86 and aarch64, regression testing on both x86 and aarch64.
>>>> Also compile CPU2017 (running is ongoing), without any issue. (With the fix to bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101586).
>>>> 
>>>> Please take a look and let me know any issue.
>>> 
>>> +/* Handle an "uninitialized" attribute; arguments as in
>>> +   struct attribute_spec.handler.  */
>>> +
>>> +static tree
>>> +handle_uninitialized_attribute (tree *node, tree name, tree ARG_UNUSED 
>>> (args),
>>> +                               int ARG_UNUSED (flags), bool 
>>> *no_add_attrs)
>>> +{
>>> +  if (!VAR_P (*node))
>>> +    {
>>> +      warning (OPT_Wattributes, "%qE attribute ignored", name);
>>> +      *no_add_attrs = true;
>>> +    }
>>> 
>>> you are documenting this attribute for automatic variables but
>>> here you allow placement on globals as well (not sure if at this
>>> point TREE_STATIC / DECL_EXTERNAL are set correctly).
>> 
>> Right, I should warn when the attribute is placed for globals or static variables. 
>> I will try TREE_STATIC/DECL_EXTERNAL to see whether it’s work or not.
>> 
>>> 
>>> +  /* for languages that do not support BUILT_IN_CLEAR_PADDING, create the
>>> +     function node for padding initialization.  */
>>> +  if (!fn)
>>> +    {
>>> +      tree ftype = build_function_type_list (void_type_node,
>>> +                                            ptr_type_node,
>>> 
>>> the "appropriate" place to do this would be 
>>> tree.c:build_common_builtin_nodes
>> 
>> Sure, will move the creation of  function node of BUILT_IN_CLEAR_PADDING for Fortran etc. to tree.c:build_common_builtin_nodes.
>> 
>>> 
>>> You seem to marshall the is_vla argument as for_auto_init when
>>> expanding/folding the builtin and there it's used to suppress
>>> diagnostics (and make covered pieces not initialized?).
>> 
>> Yes, I added an extra argument “for_auto_init” for “BUILT_IN_CLEAR_PADDING”, this argument is added to suppress errors emitted during folding
>> BUILT_IN_CLEAR_PADDING for flexible array member . Such errors should Not be emitted when “BUILT_IN_CLEAR_PADDING” is called with compiler automatic initialization.
>> Please see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101586, comment #6 from Jakub Jelinek.
>> 
>>> I suggest
>>> to re-name is_vla/for_auto_init to something more descriptive.
>> 
>> Okay, I will. 
>>> 
>>> +   gimple_fold_builtin_clear_padding. If FOR_AUTO_INIT,
>>> +   not emit some of the error messages since doing that
>>> +   might confuse the end user.  */
>>> 
>>> doesn't explain to me whether errors still might be raised or
>>> what the actual behavior is.
>> 
>> Okay, will make this more clear in the comments.
>> 
>>> 
>>> +static gimple *
>>> +build_deferred_init (tree decl,
>>> +                    enum auto_init_type init_type,
>>> +                    bool is_vla)
>>> +{
>>> +  gcc_assert ((is_vla && TREE_CODE (decl) == WITH_SIZE_EXPR)
>>> +             || (!is_vla && TREE_CODE (decl) != WITH_SIZE_EXPR));
>>> 
>>> so the is_vla parameter looks redundant (and the assert dangerous?).
>>> Either the caller knows it deals with a VLA, then that should be
>>> passed through - constant sizes can also later appear during
>>> optimization after all - or is_vla should be determined here
>>> based on whether the size at gimplification time is constant.
>> 
>> The routine “build_deferred_init” is ONLY called during gimplification phase by the routine “gimple_add_init_for_auto_var", at this place,
>> Is_vla should be determined by the caller to check the size of the DECL. If it’s a vla, the “maybe_with_size_expr” will be applied for
>> DECL to make it to a WITH_SIZE_EXPR.  So, the assertion is purely to make sure this at gimplification phase.
>> 
>> Yes, the size of the VLA decl might become a constant later due to constant propagation, etc.  but during the gimplification phase, the assertion should be true.
>>> 
>>> +         /* If the user requests to initialize automatic variables, we
>>> +            should initialize paddings inside the variable. Add a call to
>>> +            __BUILTIN_CLEAR_PADDING (&object, 0, for_auto_init = true) to
>>> +            initialize paddings of object always to zero regardless of
>>> +            INIT_TYPE.  */
>>> +         if (opt_for_fn (current_function_decl, flag_auto_var_init)
>>> +               > AUTO_INIT_UNINITIALIZED
>>> +             && VAR_P (object)
>>> +             && !DECL_EXTERNAL (object)
>>> +             && !TREE_STATIC (object))
>>> +           gimple_add_padding_init_for_auto_var (object, false, pre_p);
>>> +         return ret;
>>> 
>>> I think you want to use either auto_var_p (object) or
>>> auto_var_in_fn_p (object, current_function_decl).  Don't you also
>>> want to check for the 'uninitialized' attribute here?  I suggest
>>> to abstract the check on whether 'object' should be subject
>>> to autoinit to a helper function.
>> 
>> Thanks for the suggestion, I will do this.
>> 
>> 
>>> 
>>> There's another path above this calling gimplify_init_constructor
>>> for the case of
>>> 
>>> const struct S x = { ... };
>>> struct S y = x;
>>> 
>>> where it will try to init 'y' from the CTOR directly, it seems you
>>> do not cover this case.
>> 
>> Yes, you are right, this case was not covered right now, and this should be covered.
>> 
>> Looks like that I need to move the “gimple_add_padding_init_for_auto_var” inside the routine “gimplify_init_constructor” to
>> Cover all the cases. 
>> 
>>> I also think that the above place applies
>>> to all aggregate assignment statements, not only to INIT_EXPRs?
>> 
>>> So don't you want to restrict clear-padding emit here?
>> 
>> You are right, I might need to restrict it Only to INIT_EXPR. 
>> Will update.
>> 
>>> 
>>> +static void
>>> +expand_DEFERRED_INIT (internal_fn, gcall *stmt)
>>> +{
>>> +  tree var = gimple_call_lhs (stmt);
>>> +  tree size_of_var = gimple_call_arg (stmt, 0);
>>> +  tree vlaaddr = NULL_TREE;
>>> +  tree var_type = TREE_TYPE (var);
>>> +  bool is_vla = (bool) TREE_INT_CST_LOW (gimple_call_arg (stmt, 2));
>>> +  enum auto_init_type init_type
>>> +    = (enum auto_init_type) TREE_INT_CST_LOW (gimple_call_arg (stmt, 1));
>>> +
>>> +  gcc_assert (init_type > AUTO_INIT_UNINITIALIZED);
>>> +
>>> +  /* if this variable is a VLA, get its SIZE and ADDR first.  */
>>> +  if (is_vla)
>>> +    {
>>> +      /* The temporary address variable for this vla should have been
>>> +        created during gimplification phase.  Refer to gimplify_vla_decl
>>> +        for details.  */
>>> +      tree var_decl = (TREE_CODE (var) == SSA_NAME) ?
>>> +                      SSA_NAME_VAR (var) : var;
>>> +      gcc_assert (DECL_HAS_VALUE_EXPR_P (var_decl));
>>> +      gcc_assert (TREE_CODE (DECL_VALUE_EXPR (var_decl)) == 
>>> INDIRECT_REF);
>>> +      /* Get the address of this vla variable.  */
>>> +      vlaaddr = TREE_OPERAND (DECL_VALUE_EXPR (var_decl), 0);
>>> 
>>> err - isn't the address of the decl represented by the LHS 
>>> regardless whether this is a VLA or not?
>> 
>> The LHS of the call to .DEFERRED_INIT is the DECL itself whatever it’s a VLA or not. 
>> 
>> In order to create a memset call, we need the Address of this DECL as the first argument. 
>> If the DECL is not a VLA, we just simply apply “build_fold_addr_expr” on this DECL to get its address,
>> However, for VLA, during gimplification phase “gimplify_vla_decl”, we have already created a temporary
>> address variable for this DECL, and recorded this address variable with “DECL_VALUE_EXPR(DECL), 
>> We should use this already created address variable  for VLAs. 
> 
> So the issue is that the LHS of the .DEFERRED_INIT call is not properly
> gimplified.  We should not have such decl there but I see we do not
> have IL verification that covers this.

Don’t quite understand here:  do you mean all the LHS of .DEFERRED_INIT call are not properly gimplified, or
Only the LHS of .DEFERRED_INIT call for VLA are not properly gimplified?

What do you mean by “such” decl? A decl whole “DECL_VALUE_EXPR(DECL)” is valid?

Qing
> 
> The gimplifier usually does this in gimplify_var_or_parm_decl,
> but you can of course substitute DECL_VALUE_EXPR yourself if the
> decl was already gimplified (was it?)
> 
>> 
>>> Looking at DECL_VALUE_EXPR
>>> looks quite fragile since that's not sth data dependence honors.
>>> It looks you only partly gimplify the build init here?  All
>>> DECL_VALUE_EXPRs should have been resolved.
>> 
>> Don’t quite understand here. you mean that all the “DECL_VALUE_EXPRs” have been resolved at the phase RTL expansion,
>> So I cannot use this to get the address variable of the VLA?
>> 
>> (However, my unit testing cases for VLAs are all looks fine).
>> 
>>> 
>>> +  if (is_vla || (!use_register_for_decl (var)))
>>> ...
>>> +  else
>>> +    {
>>> +    /* If this variable is in a register, use expand_assignment might
>>> +       generate better code.  */
>>> 
>>> you compute the patter initializer even when not needing it,
>>> that's wasteful.
>> 
>> Okay, I will restrict the pattern initializer computation when really needed. 
>> 
>>> It's also quite ugly, IMHO you should
>>> use can_native_interpret_type_p (var_type) and native_interpret
>>> a char [] array initialized to the pattern and if
>>> !can_native_interpret_type_p () go the memset route.
>> 
>> Thanks for the suggestion. 
>> 
>> Will try this. 
>> 
>>> 
>>> +  /* We will not verify the arguments for the calls to .DEFERRED_INIT.
>>> +     Such call is not a real call, just a placeholder for a later
>>> +     initialization during expand phase.
>>> +     This is mainly to avoid assertion failure for the following
>>> +     case:
>>> +
>>> +     uni_var = .DEFERRED_INIT (var_size, INIT_TYPE, is_vla);
>>> +     foo (&uni_var);
>>> +
>>> +     in the above, the uninitialized auto variable "uni_var" is
>>> +     addressable, therefore should not be in registers, resulting
>>> +     the assertion failure in the following argument verification.  */
>>> +  if (gimple_call_internal_p (stmt, IFN_DEFERRED_INIT))
>>> +    return false;
>>> +
>>>  /* ???  The C frontend passes unpromoted arguments in case it
>>>     didn't see a function declaration before the call.  So for now
>>>     leave the call arguments mostly unverified.  Once we gimplify
>>>     unit-at-a-time we have a chance to fix this.  */
>>> 
>>> -  for (i = 0; i < gimple_call_num_args (stmt); ++i)
>>> 
>>> isn't that from the time there was a decl argument to .DEFERRED_INIT?
>> 
>> You mean this issue is only there when the decl is the first argument (the old design for .DEFERRED_INIT).
>> With the new design, this issue is not there anymore?
> 
> I think so, yes - the change should no longer be needed.
> 
> Ricahrd.
> 
>>> 
>>> +  if (gimple_call_internal_p (stmt, IFN_DEFERRED_INIT))
>>> +    {
>>> +      tree size_of_arg0 = gimple_call_arg (stmt, 0);
>>> +      tree size_of_lhs = TYPE_SIZE_UNIT (TREE_TYPE (lhs));
>>> +      tree is_vla_node = gimple_call_arg (stmt, 2);
>>> +      bool is_vla = (bool) TREE_INT_CST_LOW (is_vla_node);
>>> +
>>> +      if (TREE_CODE (lhs) == SSA_NAME)
>>> +       lhs = SSA_NAME_VAR (lhs);
>>> +
>>> 
>>> 'lhs' is not looked at after this, no need to look at SSA_NAME_VAR.
>> 
>> Okay, will update this.
>> 
>>> 
>>> 
>>> Thanks and sorry for the delay in reviewing this (again).
>> 
>> Thanks again for your detailed review and suggestions.
>> 
>> I will update the patch accordingly and send the updated patch soon.
>> 
>> Qing
>>> 
>>> Richard.
>>> 
>>> 
>>>> Thanks
>>>> 
>> 
>> 
> 
> -- 
> Richard Biener <rguenther@suse.de>
> SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
> Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)



More information about the Gcc-patches mailing list