bugfix: memory corruption due to sizeof (dstr)

This was reportd by Jeremy Brubaker along with a working
patch. I reworked it to a shorter fix. Jeremy is using
GCC 13.3.1, Evidently, the sizeof a struct which has a
flexible array member is not the same as the offsetof that
member, which we are relying on. I have to research more into
this because ISO C seems to require them to be the same.

* pw.c (dstr_of, dsgrow): use offsetof (struct dsr, str)
rather than sizeof (dstr) when displacing the string pointer
back to the header, and when allocating the space.

1 files changed, 3 insertions, 2 deletions

diff --git a/pw.c b/pw.c
index 29b4cb5..a2ddff0 100644
--- a/pw.c
+++ b/pw.c
@@ -26,6 +26,7 @@
 // POSSIBILITY OF SUCH DAMAGE.
 
 #include <assert.h>
+#include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdint.h>
@@ -130,7 +131,7 @@ typedef struct dstr {
   char str[];
 } dstr;
 
-#define dstr_of(str) ((dstr *) ((str) - sizeof (dstr)))
+#define dstr_of(s) ((dstr *) ((s) - offsetof (struct dstr, str)))
 
 static char *pw_name;
 static int poll_interval = 1000;
@@ -203,7 +204,7 @@ static char *dsgrow(char *str, size_t len)
 {
   dstr *ds = str ? dstr_of(str) : 0;
   int flags = str ? ds->flags : 0;
-  size_t size = sizeof *ds + len + 1;
+  size_t size = offsetof(struct dstr, str) + len + 1;
 
   assert (ds == 0 || ds->refs == 1);