[PATCH 2/2] dlopen: Do not block signals

[Florian Weimer <fweimer at redhat dot com>]

Blocking signals causes issues with certain anti-malware solutions
which rely on an unblocked SIGSYS signal for system calls they
intercept.

This reverts commit a2e8aa0d9ea648068d8be52dd7b15f1b6a008e23
("Block signals during the initial part of dlopen") and adds
comments related to async signal safety to active_nodelete and
its caller.

Note that this does not make lazy binding async-signal-safe with regards
to dlopen.  It merely avoids introducing new async-signal-safety hazards
as part of the NODELETE changes.
---
 elf/dl-open.c | 37 +++++++++++--------------------------
 1 file changed, 11 insertions(+), 26 deletions(-)

diff --git a/elf/dl-open.c b/elf/dl-open.c
index c23341be58..5a1c5b5326 100644
--- a/elf/dl-open.c
+++ b/elf/dl-open.c
@@ -34,7 +34,6 @@
 #include <atomic.h>
 #include <libc-internal.h>
 #include <array_length.h>
-#include <internal-signals.h>
 
 #include <dl-dst.h>
 #include <dl-prop.h>
@@ -53,10 +52,6 @@ struct dl_open_args
   /* Namespace ID.  */
   Lmid_t nsid;
 
-  /* Original signal mask.  Used for unblocking signal handlers before
-     running ELF constructors.  */
-  sigset_t original_signal_mask;
-
   /* Original value of _ns_global_scope_pending_adds.  Set by
      dl_open_worker.  Only valid if nsid is a real namespace
      (non-negative).  */
@@ -446,6 +441,9 @@ activate_nodelete (struct link_map *new)
 	  _dl_debug_printf ("activating NODELETE for %s [%lu]\n",
 			    l->l_name, l->l_ns);
 
+	/* The flag can already be true at this point, e.g. a signal
+	   handler may have triggered lazy binding and set NODELETE
+	   status immediately.  */
 	l->l_nodelete_active = true;
 
 	/* This is just a debugging aid, to indicate that
@@ -520,16 +518,12 @@ dl_open_worker (void *a)
   if (new == NULL)
     {
       assert (mode & RTLD_NOLOAD);
-      __libc_signal_restore_set (&args->original_signal_mask);
       return;
     }
 
   if (__glibc_unlikely (mode & __RTLD_SPROF))
-    {
-      /* This happens only if we load a DSO for 'sprof'.  */
-      __libc_signal_restore_set (&args->original_signal_mask);
-      return;
-    }
+    /* This happens only if we load a DSO for 'sprof'.  */
+    return;
 
   /* This object is directly loaded.  */
   ++new->l_direct_opencount;
@@ -565,7 +559,6 @@ dl_open_worker (void *a)
 
       assert (_dl_debug_initialize (0, args->nsid)->r_state == RT_CONSISTENT);
 
-      __libc_signal_restore_set (&args->original_signal_mask);
       return;
     }
 
@@ -712,6 +705,12 @@ dl_open_worker (void *a)
      All memory allocations for new objects must have happened
      before.  */
 
+  /* Finalize the NODELETE status first.  This comes before
+     update_scopes, so that lazy binding will not see pending NODELETE
+     state for newly loaded objects.  There is a compiler barrier in
+     update_scopes which ensures that the changes from
+     activate_nodelete are visible before new objects show up in the
+     local scope.  */
   activate_nodelete (new);
 
   /* Second stage after resize_scopes: Actually perform the scope
@@ -745,10 +744,6 @@ dl_open_worker (void *a)
   if (mode & RTLD_GLOBAL)
     add_to_global_resize (new);
 
-  /* Unblock signals.  Data structures are now consistent, and
-     application code may run.  */
-  __libc_signal_restore_set (&args->original_signal_mask);
-
   /* Run the initializer functions of new objects.  Temporarily
      disable the exception handler, so that lazy binding failures are
      fatal.  */
@@ -838,10 +833,6 @@ no more namespaces available for dlmopen()"));
   args.argv = argv;
   args.env = env;
 
-  /* Recursive lazy binding during manipulation of the dynamic loader
-     structures may result in incorrect behavior.  */
-  __libc_signal_block_all (&args.original_signal_mask);
-
   struct dl_exception exception;
   int errcode = _dl_catch_exception (&exception, dl_open_worker, &args);
 
@@ -882,16 +873,10 @@ no more namespaces available for dlmopen()"));
 
 	  _dl_close_worker (args.map, true);
 
-	  /* Restore the signal mask.  In the success case, this
-	     happens inside dl_open_worker.  */
-	  __libc_signal_restore_set (&args.original_signal_mask);
-
 	  /* All l_nodelete_pending objects should have been deleted
 	     at this point, which is why it is not necessary to reset
 	     the flag here.  */
 	}
-      else
-	__libc_signal_restore_set (&args.original_signal_mask);
 
       assert (_dl_debug_initialize (0, args.nsid)->r_state == RT_CONSISTENT);
 
-- 
2.23.0