[gcc(refs/users/meissner/heads/ieee)] strlen: Punt on UB reads past end of string literal [PR94187]

[gcc(refs/users/meissner/heads/ieee)] strlen: Punt on UB reads past end of string literal [PR94187]

[Michael Meissner]

https://gcc.gnu.org/g:741ff2a263fe0ddc343288331c0047c1a32af8b2

commit 741ff2a263fe0ddc343288331c0047c1a32af8b2
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Mar 17 10:43:46 2020 +0100

    strlen: Punt on UB reads past end of string literal [PR94187]
    
    The gcc.dg/pr68785.c test which contains:
    int
    foo (void)
    {
      return *(int *) "";
    }
    has UB in the program if it is ever called, but causes UB in the compiler
    as well as at least in theory non-reproduceable code generation.
    The problem is that nbytes is in this case 4, prep is the
    TREE_STRING_POINTER of a "" string literal with TREE_STRING_LENGTH of 1 and
    we do:
    4890              for (const char *p = prep; p != prep + nbytes; ++p)
    4891                if (*p)
    4892                  {
    4893                    *allnul = false;
    4894                    break;
    4895                  }
    and so read the bytes after the STRING_CST payload, which can be random.
    I think we should just punt in this case.
    
    2020-03-17  Jakub Jelinek  <jakub@redhat.com>
    
            PR tree-optimization/94187
            * tree-ssa-strlen.c (count_nonzero_bytes): Punt if
            nchars - offset < nbytes.

Diff:
---
 gcc/ChangeLog         | 4 ++++
 gcc/tree-ssa-strlen.c | 2 ++
 2 files changed, 6 insertions(+)

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index fc35ceac272..f8105ccc67d 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,5 +1,9 @@
 2020-03-17  Jakub Jelinek  <jakub@redhat.com>
 
+	PR tree-optimization/94187
+	* tree-ssa-strlen.c (count_nonzero_bytes): Punt if
+	nchars - offset < nbytes.
+
 	PR middle-end/94189
 	* builtins.c (expand_builtin_strnlen): Do return NULL_RTX if we would
 	emit a warning if it was enabled and don't depend on TREE_NO_WARNING
diff --git a/gcc/tree-ssa-strlen.c b/gcc/tree-ssa-strlen.c
index 0d70f3cc95d..ec33d7c4baf 100644
--- a/gcc/tree-ssa-strlen.c
+++ b/gcc/tree-ssa-strlen.c
@@ -4822,6 +4822,8 @@ count_nonzero_bytes (tree exp, unsigned HOST_WIDE_INT offset,
 	   of the access), set it here to the size of the string, including
 	   all internal and trailing nuls if the string has any.  */
 	nbytes = nchars - offset;
+      else if (nchars - offset < nbytes)
+	return false;
 
       prep = TREE_STRING_POINTER (exp) + offset;
     }