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

On 08/27/2018 02:29 AM, Richard Biener wrote:
On Sun, Aug 26, 2018 at 7:26 AM 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.


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.


PR tree-optimization/87028 - false positive -Wstringop-truncation strncpy with global variable source string

      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.


      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.

Please do not add 'cfun' references.  Note that the next stmt is also accessible
when there is no CFG.  I guess the issue is that we fold this during
where the next stmt is not yet "there" (but still in GENERIC)?

We generally do not want to have unfolded stmts in the IL when we can avoid that
which is why we fold most stmts during gimplification.  We also do that because
we now do less folding on GENERIC.

There may be the possibility to refactor gimplification time folding to what we
do during inlining - queue stmts we want to fold and perform all
folding delayed.
This of course means bigger compile-time due to cache effects.

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.

Yes.  Why not simply compare the SSA names?  Why would it be
not OK to do that when !lhsbase?

The added code handles this case:

  void f (char *d)
    __builtin_strncpy (d, "12345", 4);
    d[3] = 0;

where during forwprop we see:

  __builtin_strncpy (d_3(D), "12345", 4);
  MEM[(char *)d_3(D) + 3B] = 0;

The next statement after the strncpy is the assignment whose
lhs is the MEM_REF with a GIMPLE_NOP as an operand.  There
is no other information in the GIMPLE_NOP that I can see to
tell that the operand is d_3(D) or that it's the same as
the strncpy argument (i.e., the PARAM_DECl d).  Having to
do open-code this all the time seems so cumbersome -- is
there some API that would do this for me?  (I thought
get_addr_base_and_unit_offset was that API but clearly in
this case it doesn't do what I expect -- it just returns
the argument.)


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