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* [gcc(refs/users/meissner/heads/ieee)] strlen: Punt on UB reads past end of string literal [PR94187]
@ 2020-03-18 17:20 Michael Meissner
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From: Michael Meissner @ 2020-03-18 17:20 UTC (permalink / raw)
To: gcc-cvs
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;
}
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