Disable SIGBUS handler

This handler leads to undefined behavior.  Further, it calls mmap in a
way that POSIX says must fail with EBADF.  This commit returns to
simply taking SIGBUS if it occurs and exiting.  If that happens, then
an issue can be created and a way forward discusssed.  See
https://github.com/BestImageViewer/geeqie/issues/1052 for more
context.

diff --git a/src/main.cc b/src/main.cc
index a8b7635..62cb022 100644 (file)
--- a/src/main.cc
+++ b/src/main.cc
@@ -1008,22 +1008,31 @@ void exit_program(void)
        exit_program_final();
 }
 
-/* This code is supposed to handle situation when a file mmaped by image_loader
+/* This code attempts to handle situation when a file mmaped by image_loader
  * or by exif loader is truncated by some other process.
- * This is probably not completely correct according to posix, because
- * mmap is not in the list of calls that can be used safely in signal handler,
- * but anyway, the handler is called in situation when the application would
- * crash otherwise.
- * Ideas for improvement are welcome ;)
+ * This code is incorrect according to POSIX, because:
+ *
+ *   mmap is not async-signal-safe and thus may not be called from a signal handler
+ * 
+ *   mmap must be called with a valid file descriptor.  POSIX requires that
+ *   a fildes argument of -1 must cause mmap to return EBADF.
+ *
+ * See https://github.com/BestImageViewer/geeqie/issues/1052 for discussion of
+ * an alternative approach.
  */
 /** @FIXME this probably needs some better ifdefs. Please report any compilation problems */
 
 #if defined(SIGBUS) && defined(SA_SIGINFO)
 static void sigbus_handler_cb(int UNUSED(signum), siginfo_t *info, void *UNUSED(context))
 {
-       unsigned long pagesize = sysconf(_SC_PAGE_SIZE);
-       DEBUG_1("SIGBUS %p", info->si_addr);
-       mmap((void *)(((unsigned long)info->si_addr / pagesize) * pagesize), pagesize, PROT_READ | PROT_WRITE, MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+       /*
+        * @FIXME Design and implement a POSIX-acceptable approach,
+        * after first documenting the sitations where SIGBUS occurs.
+        * See https://github.com/BestImageViewer/geeqie/issues/1052 for discussion
+        */
+
+       DEBUG_1("SIGBUS %p NOT HANDLED", info->si_addr);
+       exit(EXIT_FAILURE);
 }
 #endif
 
@@ -1196,7 +1205,10 @@ gint main(gint argc, gchar *argv[])
        /* setup random seed for random slideshow */
        srand(time(NULL));
 
+#if 0
+       /* See later comment; this handler leads to UB. */
        setup_sigbus_handler();
+#endif
 
        /* register global notify functions */
        file_data_register_notify_func(cache_notify_cb, NULL, NOTIFY_PRIORITY_HIGH);