Clobber strings with 0xf1f1f1f1 before writing to them to check buffer

lengths are correct.  Attempts to pstrcpy into an fstring or allocated
string should fail in developer builds.

This builds on abartlet's earlier overflow probe for safe_strcpy, but
by clobbering the whole string with a nonzero value is more likely to
find overflows on the stack.

This is only used in -DDEVELOPER mode.

Reviewed by abartlet, tpot.
(This used to be commit 8d915e266cd8ccc8b27e9c7ea8e9d003d05f8182)

1 files changed, 33 insertions, 8 deletions

diff --git a/source3/lib/util_str.c b/source3/lib/util_str.c
index 070c59c1b2..924cf9d921 100644
--- a/source3/lib/util_str.c
+++ b/source3/lib/util_str.c
@@ -430,6 +430,27 @@ BOOL str_is_all(const char *s,char c)
 	return True;
 }
 
+
+/**
+ * In developer builds, clobber a region of memory.
+ *
+ * If we think a string buffer is longer than it really is, this ought
+ * to make the failure obvious, by segfaulting (if in the heap) or by
+ * killing the return address (on the stack), or by trapping under a
+ * memory debugger.
+ *
+ * This is meant to catch possible string overflows, even if the
+ * actual string copied is not big enough to cause an overflow.
+ **/
+void clobber_region(char *dest, size_t len)
+{
+#ifdef DEVELOPER
+	/* F1 is odd and 0xf1f1f1f1 shouldn't be a valid pointer */
+	memset(dest, 0xF1, len);
+#endif
+}
+
+
 /**
  Safe string copy into a known length string. maxlength does not
  include the terminating zero.
@@ -444,13 +465,7 @@ char *safe_strcpy(char *dest,const char *src, size_t maxlength)
 		return NULL;
 	}
 
-#ifdef DEVELOPER
-	/* We intentionally write out at the extremity of the destination
-	 * string.  If the destination is too short (e.g. pstrcpy into mallocd
-	 * or fstring) then this should cause an error under a memory
-	 * checker. */
-	dest[maxlength] = '\0';
-#endif
+	clobber_region(dest, maxlength+1);
 
 	if (!src) {
 		*dest = 0;
@@ -490,6 +505,8 @@ char *safe_strcat(char *dest, const char *src, size_t maxlength)
 	src_len = strlen(src);
 	dest_len = strlen(dest);
 
+	clobber_region(dest + dest_len, maxlength + 1 - dest_len);
+	
 	if (src_len + dest_len > maxlength) {
 		DEBUG(0,("ERROR: string overflow by %d in safe_strcat [%.50s]\n",
 			 (int)(src_len + dest_len - maxlength), src));
@@ -499,7 +516,7 @@ char *safe_strcat(char *dest, const char *src, size_t maxlength)
 		dest[maxlength] = 0;
 		return NULL;
 	}
-	
+
 	memcpy(&dest[dest_len], src, src_len);
 	dest[dest_len + src_len] = 0;
 	return dest;
@@ -516,6 +533,8 @@ char *alpha_strcpy(char *dest, const char *src, const char *other_safe_chars, si
 {
 	size_t len, i;
 
+	clobber_region(dest, maxlength);
+
 	if (!dest) {
 		DEBUG(0,("ERROR: NULL dest in alpha_strcpy\n"));
 		return NULL;
@@ -554,8 +573,12 @@ char *alpha_strcpy(char *dest, const char *src, const char *other_safe_chars, si
 char *StrnCpy(char *dest,const char *src,size_t n)
 {
 	char *d = dest;
+
+	clobber_region(dest, n+1);
+	
 	if (!dest)
 		return(NULL);
+	
 	if (!src) {
 		*dest = 0;
 		return(dest);
@@ -576,6 +599,8 @@ char *strncpyn(char *dest, const char *src, size_t n, char c)
 	char *p;
 	size_t str_len;
 
+	clobber_region(dest, n+1);
+
 	p = strchr_m(src, c);
 	if (p == NULL) {
 		DEBUG(5, ("strncpyn: separator character (%c) not found\n", c));