[PATCH] avoid warning on constant strncpy until next statement is reachable (PR 87028)

[Martin Sebor]

On 08/25/2018 11:24 PM, Jeff Law wrote:
> On 08/24/2018 09:58 AM, Martin Sebor wrote:
>> The warning suppression for -Wstringop-truncation looks for
>> the next statement after a truncating strncpy to see if it
>> adds a terminating nul.  This only works when the next
>> statement can be reached using the Gimple statement iterator
>> which isn't until after gimplification.  As a result, strncpy
>> calls that truncate their constant argument that are being
>> folded to memcpy this early get diagnosed even if they are
>> followed by the nul assignment:
>>
>>   const char s[] = "12345";
>>   char d[3];
>>
>>   void f (void)
>>   {
>>     strncpy (d, s, sizeof d - 1);   // -Wstringop-truncation
>>     d[sizeof d - 1] = 0;
>>   }
>>
>> To avoid the warning I propose to defer folding strncpy to
>> memcpy until the pointer to the basic block the strnpy call
>> is in can be used to try to reach the next statement (this
>> happens as early as ccp1).  I'm aware of the preference to
>> fold things early but in the case of strncpy (a relatively
>> rarely used function that is often misused), getting
>> the warning right while folding a bit later but still fairly
>> early on seems like a reasonable compromise.  I fear that
>> otherwise, the false positives will drive users to adopt
>> other unsafe solutions (like memcpy) where these kinds of
>> bugs cannot be as readily detected.
>>
>> Tested on x86_64-linux.
>>
>> Martin
>>
>> PS There still are outstanding cases where the warning can
>> be avoided.  I xfailed them in the test for now but will
>> still try to get them to work for GCC 9.
>>
>> gcc-87028.diff
>>
>>
>> PR tree-optimization/87028 - false positive -Wstringop-truncation strncpy with global variable source string
>> gcc/ChangeLog:
>>
>> 	PR tree-optimization/87028
>> 	* gimple-fold.c (gimple_fold_builtin_strncpy): Avoid folding when
>> 	statement doesn't belong to a basic block.
>> 	* tree-ssa-strlen.c (maybe_diag_stxncpy_trunc): Handle MEM_REF on
>> 	the left hand side of assignment.
>>
>> gcc/testsuite/ChangeLog:
>>
>> 	PR tree-optimization/87028
>> 	* c-c++-common/Wstringop-truncation.c: Remove xfails.
>> 	* gcc.dg/Wstringop-truncation-5.c: New test.
>>
>> diff --git a/gcc/gimple-fold.c b/gcc/gimple-fold.c
>> index 07341eb..284c2fb 100644
>> --- a/gcc/gimple-fold.c
>> +++ b/gcc/gimple-fold.c
>> @@ -1702,6 +1702,11 @@ gimple_fold_builtin_strncpy (gimple_stmt_iterator *gsi,
>>    if (tree_int_cst_lt (ssize, len))
>>      return false;
>>
>> +  /* Defer warning (and folding) until the next statement in the basic
>> +     block is reachable.  */
>> +  if (!gimple_bb (stmt))
>> +    return false;
> I think you want cfun->cfg as the test here.  They should be equivalent
> in practice.
>
>
>> diff --git a/gcc/tree-ssa-strlen.c b/gcc/tree-ssa-strlen.c
>> index d0792aa..f1988f6 100644
>> --- a/gcc/tree-ssa-strlen.c
>> +++ b/gcc/tree-ssa-strlen.c
>> @@ -1981,6 +1981,23 @@ maybe_diag_stxncpy_trunc (gimple_stmt_iterator gsi, tree src, tree cnt)
>>  	  && known_eq (dstoff, lhsoff)
>>  	  && operand_equal_p (dstbase, lhsbase, 0))
>>  	return false;
>> +
>> +      if (code == MEM_REF
>> +	  && TREE_CODE (lhsbase) == SSA_NAME
>> +	  && known_eq (dstoff, lhsoff))
>> +	{
>> +	  /* Extract the referenced variable from something like
>> +	       MEM[(char *)d_3(D) + 3B] = 0;  */
>> +	  gimple *def = SSA_NAME_DEF_STMT (lhsbase);
>> +	  if (gimple_nop_p (def))
>> +	    {
>> +	      lhsbase = SSA_NAME_VAR (lhsbase);
>> +	      if (lhsbase
>> +		  && dstbase
>> +		  && operand_equal_p (dstbase, lhsbase, 0))
>> +		return false;
>> +	    }
>> +	}
> If you find yourself looking at SSA_NAME_VAR, you're usually barking up
> the wrong tree.  It'd be easier to suggest something here if I could see
> the gimple (with virtual operands).  BUt at some level what you really
> want to do is make sure the base of the MEM_REF is the same as what got
> passed as the destination of the strncpy.  You'd want to be testing
> SSA_NAMEs in that case.

I replied to Richard with the code that this hunk handles:

   https://gcc.gnu.org/ml/gcc-patches/2018-08/msg01697.html

I couldn't find any other way to determine that d_3(D) in

   MEM[(char *)d_3(D) + 3B] = 0;

is the same as the first argument in:

   __builtin_strncpy (d_3(D), "12345", 4);

The MEM_REF operand is an SSA_NAME whose DEF_STMT is
a GIMPLE_NOP and whose SSA_NAME_VAR is the PARAM_DECL d.
Where else can I get the variable from?

Martin