support: Use dlerror to detect NULL symbols in xdlsym

2 files changed, 15 insertions, 10 deletions

diff --git a/ChangeLog b/ChangeLog
index f244457..66fa654 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,10 @@
 2019-02-06  Florian Weimer  <fweimer@redhat.com>
 
+	* support/xdlfcn.c (xdlopen, xdlclose): Do not call dlerror.
+	(xdlsym): Use dlerror to detect a NULL symbol.
+
+2019-02-06  Florian Weimer  <fweimer@redhat.com>
+
 	* sysdeps/unix/sysv/linux/i386/lowlevellock.h: Do not include
 	<stap-probe.h>.
 	* sysdeps/unix/sysv/linux/x86_64/lowlevellock.h: Likewise.
diff --git a/support/xdlfcn.c b/support/xdlfcn.c
index 2f2ac76..b2e5c21 100644
--- a/support/xdlfcn.c
+++ b/support/xdlfcn.c
@@ -28,22 +28,25 @@ xdlopen (const char *filename, int flags)
   if (dso == NULL)
     FAIL_EXIT1 ("error: dlopen: %s\n", dlerror ());
 
-  /* Clear any errors.  */
-  dlerror ();
-
   return dso;
 }
 
 void *
 xdlsym (void *handle, const char *symbol)
 {
+  /* Clear any pending errors.  */
+  dlerror ();
+
   void *sym = dlsym (handle, symbol);
 
   if (sym == NULL)
-    FAIL_EXIT1 ("error: dlsym: %s\n", dlerror ());
-
-  /* Clear any errors.  */
-  dlerror ();
+    {
+      const char *error = dlerror ();
+      if (error != NULL)
+        FAIL_EXIT1 ("error: dlsym: %s\n", error);
+      /* If there was no error, we found a NULL symbol.  Return the
+         NULL value in this case.  */
+    }
 
   return sym;
 }
@@ -53,7 +56,4 @@ xdlclose (void *handle)
 {
   if (dlclose (handle) != 0)
     FAIL_EXIT1 ("error: dlclose: %s\n", dlerror ());
-
-  /* Clear any errors.  */
-  dlerror ();
 }