krb5 commit: Add finalization safety check to com_err

[Greg Hudson]

https://github.com/krb5/krb5/commit/9d654aa05e26bbf22f140abde3436afeff2fdf8d
commit 9d654aa05e26bbf22f140abde3436afeff2fdf8d
Author: Jiri Sasek <Jiri.Sasek at Oracle.COM>
Date:   Fri Mar 13 19:02:58 2020 +0100

    Add finalization safety check to com_err
    
    If the linker erroneously runs the libkrb5 finalizer after the
    libcom_err finalizer, the consequent remove_error_table() calls could
    crash due to accessing a destroyed mutex or an invalid et_list
    pointer.  Add an unsynchronized check on finalized in
    remove_error_table(), and set et_list to null in com_err_terminate()
    after destroying the list.
    
    [ghudson at mit.edu: minimized code hanges; rewrote comment and commit
    message]
    
    ticket: 8890 (new)

 src/util/et/error_message.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/src/util/et/error_message.c b/src/util/et/error_message.c
index d7069a9..7dc02a3 100644
--- a/src/util/et/error_message.c
+++ b/src/util/et/error_message.c
@@ -26,7 +26,7 @@
 
 static struct et_list *et_list;
 static k5_mutex_t et_list_lock = K5_MUTEX_PARTIAL_INITIALIZER;
-static int terminated = 0;      /* for debugging shlib fini sequence errors */
+static int terminated = 0;      /* for safety and finalization debugging */
 
 MAKE_INIT_FUNCTION(com_err_initialize);
 MAKE_FINI_FUNCTION(com_err_terminate);
@@ -69,6 +69,7 @@ void com_err_terminate(void)
         enext = e->next;
         free(e);
     }
+    et_list = NULL;
     k5_mutex_unlock(&et_list_lock);
     k5_mutex_destroy(&et_list_lock);
     terminated = 1;
@@ -280,6 +281,10 @@ remove_error_table(const struct error_table *et)
 {
     struct et_list **ep, *e;
 
+    /* Safety check in case libraries are finalized in the wrong order. */
+    if (terminated)
+        return ENOENT;
+
     if (CALL_INIT_FUNCTION(com_err_initialize))
         return 0;
     k5_mutex_lock(&et_list_lock);