Subject: Re: bug#31946: 27.0.50; The NSM should warn about more TLS problems

From: Eli Zaretskii <[email-masked]>
Date: Tue, 27 Aug 2019 10:23:55 +0300

> From: Andy Moreton <[email-masked]>
> Date: Mon, 26 Aug 2019 23:56:06 +0100
> 
> > Looks like something is wrong with how I'm using gnutls_free when it
> > comes from the DLL.  I need to look into that.
> 
> Found it I think. gnutls_free is a pointer of type gnutls_free_function,
> so the value returned from GetProcAddress is a data pointer, not a
> function address. Something like the following patch appears to work
> i.e. no longer crashes with the recipe above.

Yes, thanks.  I was struggling with the same thing, but evidently
there's a limit to what I can do after 11PM at my age ;-)

> If this is the right fix then we will need to add e.g. DEF_DLL_DATA and
> LOAD_DLL_DATA macros for runtime import of data (and an equivalent to
> get_proc_addr to use the right types for runtime data imports).

I think there's no need to invent harness for a single user, we can
simply do that by hand.  Please try the patch below, it's against the
current master (it also removes the unnecessary intermediate buffer
allocation while at that).

> The patch also has "#undef gnutls_free" because gnutls.h also defines
> gnutls_free as a macro, if GNUTLS_INTERNAL_BUILD is defined. For some
> reason the MSYS2 maintainers add a patch to their build to define
> GNUTLS_INTERNAL_BUILD unconditionally: see 0001-add-missing-define.patch
> at <https://github.com/msys2/MINGW-packages/tree/master/mingw-w64-gnutls>.

I guess they want for some reason not to have the exported functions
decorated with __declspec(dllimport).  I have no idea why, maybe for
reasons of function visibility?  It's worth asking them, I think,
because this kind of kludge can be a nuisance.

diff --git a/src/gnutls.c b/src/gnutls.c
index 67d1fb9..fb75eb9 100644
--- a/src/gnutls.c
+++ b/src/gnutls.c
@@ -140,7 +140,6 @@ DEF_DLL_FN (void, gnutls_dh_set_prime_bits,
 DEF_DLL_FN (int, gnutls_dh_get_prime_bits, (gnutls_session_t));
 DEF_DLL_FN (int, gnutls_error_is_fatal, (int));
 DEF_DLL_FN (int, gnutls_global_init, (void));
-DEF_DLL_FN (void, gnutls_free, (void *));
 DEF_DLL_FN (void, gnutls_global_set_log_function, (gnutls_log_func));
 #  ifdef HAVE_GNUTLS3
 DEF_DLL_FN (void, gnutls_global_set_audit_log_function, (gnutls_audit_log_func));
@@ -291,6 +290,7 @@ DEF_DLL_FN (const char *, gnutls_ext_get_name, (unsigned int));
 #   endif
 #  endif	 /* HAVE_GNUTLS3 */
 
+static gnutls_free_function *gnutls_free_func;
 
 static bool
 init_gnutls_functions (void)
@@ -327,7 +327,6 @@ init_gnutls_functions (void)
   LOAD_DLL_FN (library, gnutls_dh_get_prime_bits);
   LOAD_DLL_FN (library, gnutls_error_is_fatal);
   LOAD_DLL_FN (library, gnutls_global_init);
-  LOAD_DLL_FN (library, gnutls_free);
   LOAD_DLL_FN (library, gnutls_global_set_log_function);
 #  ifdef HAVE_GNUTLS3
   LOAD_DLL_FN (library, gnutls_global_set_audit_log_function);
@@ -430,6 +429,13 @@ init_gnutls_functions (void)
 #   endif
 #  endif	 /* HAVE_GNUTLS3 */
 
+  /* gnutls_free is a variable inside GnuTLS, whose value is the
+     "free" function.  So it needs special handling.  */
+  gnutls_free_func = (gnutls_free_function *) GetProcAddress (library,
+							      "gnutls_free");
+  if (!gnutls_free_func)
+    return false;
+
   max_log_level = clip_to_bounds (INT_MIN, global_gnutls_log_level, INT_MAX);
   {
     Lisp_Object name = CAR_SAFE (Fget (Qgnutls, QCloaded_from));
@@ -465,7 +471,6 @@ init_gnutls_functions (void)
 #  define gnutls_global_init fn_gnutls_global_init
 #  define gnutls_global_set_audit_log_function fn_gnutls_global_set_audit_log_function
 #  define gnutls_global_set_log_function fn_gnutls_global_set_log_function
-#  define gnutls_free fn_gnutls_free
 #  define gnutls_global_set_log_level fn_gnutls_global_set_log_level
 #  define gnutls_handshake fn_gnutls_handshake
 #  define gnutls_init fn_gnutls_init
@@ -562,6 +567,11 @@ init_gnutls_functions (void)
 #   endif
 #  endif	 /* HAVE_GNUTLS3 */
 
+/* gnutls_free is a data pointer to a variable which holds a pointer
+   to the function.  We use #undef because MinGW64 defines gnutls_free
+   as a macro as well in the GnuTLS headers.  */
+#  undef gnutls_free
+#  define gnutls_free (*gnutls_free_func)
 
 /* This wrapper is called from fns.c, which doesn't know about the
    LOAD_DLL_FN stuff above.  */
@@ -1612,16 +1622,10 @@ DEFUN ("gnutls-format-certificate", Fgnutls_format_certificate,
 	     emacs_gnutls_strerror (err));
     }
 
-  char *out_buf = xmalloc ((out.size + 1) * sizeof (char));
-  memset (out_buf, 0, (out.size + 1) * sizeof (char));
-  memcpy (out_buf, out.data, out.size);
-
+  Lisp_Object result = make_string_from_bytes (out.data, out.size, out.size);
   gnutls_free (out.data);
   gnutls_x509_crt_deinit (crt);
 
-  Lisp_Object result = build_string (out_buf);
-  xfree (out_buf);
-
   return result;
 }