[PATCH 01/10] nptl: Perform signal initialization upon pthread_create

[Florian Weimer <fweimer@redhat.com>]

Install signal handlers and unblock signals before pthread_create
creates the first thread.
---
 nptl/Versions         |  5 ++-
 nptl/nptl-init.c      | 75 ------------------------------------
 nptl/pthreadP.h       |  6 +++
 nptl/pthread_cancel.c | 88 ++++++++++++++++++++++++++++++++++++++-----
 nptl/pthread_create.c | 45 +++++++++++++++++++++-
 5 files changed, 131 insertions(+), 88 deletions(-)

diff --git a/nptl/Versions b/nptl/Versions
index e7883cbb49..d96b830d05 100644
--- a/nptl/Versions
+++ b/nptl/Versions
@@ -367,8 +367,6 @@ libc {
     tss_set;
   }
   GLIBC_PRIVATE {
-     __nptl_create_event;
-     __nptl_death_event;
     __default_pthread_attr;
     __default_pthread_attr_lock;
     __futex_abstimed_wait64;
@@ -386,11 +384,14 @@ libc {
     __lll_trylock_elision;
     __lll_unlock_elision;
     __mutex_aconf;
+    __nptl_create_event;
     __nptl_deallocate_stack;
     __nptl_deallocate_tsd;
+    __nptl_death_event;
     __nptl_free_tcb;
     __nptl_nthreads;
     __nptl_setxid_sighandler;
+    __nptl_sigcancel_handler;
     __nptl_stack_list_add;
     __nptl_stack_list_del;
     __pthread_attr_copy;
diff --git a/nptl/nptl-init.c b/nptl/nptl-init.c
index f4b86fbfaf..bc4831ac89 100644
--- a/nptl/nptl-init.c
+++ b/nptl/nptl-init.c
@@ -44,84 +44,9 @@ size_t __static_tls_align_m1;
 /* Version of the library, used in libthread_db to detect mismatches.  */
 static const char nptl_version[] __attribute_used__ = VERSION;
 
-/* For asynchronous cancellation we use a signal.  This is the handler.  */
-static void
-sigcancel_handler (int sig, siginfo_t *si, void *ctx)
-{
-  /* Safety check.  It would be possible to call this function for
-     other signals and send a signal from another process.  This is not
-     correct and might even be a security problem.  Try to catch as
-     many incorrect invocations as possible.  */
-  if (sig != SIGCANCEL
-      || si->si_pid != __getpid()
-      || si->si_code != SI_TKILL)
-    return;
-
-  struct pthread *self = THREAD_SELF;
-
-  int oldval = THREAD_GETMEM (self, cancelhandling);
-  while (1)
-    {
-      /* We are canceled now.  When canceled by another thread this flag
-	 is already set but if the signal is directly send (internally or
-	 from another process) is has to be done here.  */
-      int newval = oldval | CANCELING_BITMASK | CANCELED_BITMASK;
-
-      if (oldval == newval || (oldval & EXITING_BITMASK) != 0)
-	/* Already canceled or exiting.  */
-	break;
-
-      int curval = THREAD_ATOMIC_CMPXCHG_VAL (self, cancelhandling, newval,
-					      oldval);
-      if (curval == oldval)
-	{
-	  /* Set the return value.  */
-	  THREAD_SETMEM (self, result, PTHREAD_CANCELED);
-
-	  /* Make sure asynchronous cancellation is still enabled.  */
-	  if ((newval & CANCELTYPE_BITMASK) != 0)
-	    /* Run the registered destructors and terminate the thread.  */
-	    __do_cancel ();
-
-	  break;
-	}
-
-      oldval = curval;
-    }
-}
-
-
-/* When using __thread for this, we do it in libc so as not
-   to give libpthread its own TLS segment just for this.  */
-extern void **__libc_dl_error_tsd (void) __attribute__ ((const));
-
-
 void
 __pthread_initialize_minimal_internal (void)
 {
-  struct sigaction sa;
-  __sigemptyset (&sa.sa_mask);
-
-  /* Install the cancellation signal handler.  If for some reason we
-     cannot install the handler we do not abort.  Maybe we should, but
-     it is only asynchronous cancellation which is affected.  */
-  sa.sa_sigaction = sigcancel_handler;
-  sa.sa_flags = SA_SIGINFO;
-  (void) __libc_sigaction (SIGCANCEL, &sa, NULL);
-
-  /* Install the handle to change the threads' uid/gid.  */
-  sa.sa_sigaction = __nptl_setxid_sighandler;
-  sa.sa_flags = SA_SIGINFO | SA_RESTART;
-  (void) __libc_sigaction (SIGSETXID, &sa, NULL);
-
-  /* The parent process might have left the signals blocked.  Just in
-     case, unblock it.  We reuse the signal mask in the sigaction
-     structure.  It is already cleared.  */
-  __sigaddset (&sa.sa_mask, SIGCANCEL);
-  __sigaddset (&sa.sa_mask, SIGSETXID);
-  INTERNAL_SYSCALL_CALL (rt_sigprocmask, SIG_UNBLOCK, &sa.sa_mask,
-			 NULL, __NSIG_BYTES);
-
   /* Get the size of the static and alignment requirements for the TLS
      block.  */
   size_t static_tls_align;
diff --git a/nptl/pthreadP.h b/nptl/pthreadP.h
index f93806e540..497c2ad3d9 100644
--- a/nptl/pthreadP.h
+++ b/nptl/pthreadP.h
@@ -571,6 +571,12 @@ libc_hidden_proto (__pthread_attr_setsigmask_internal)
 extern __typeof (pthread_attr_getsigmask_np) __pthread_attr_getsigmask_np;
 libc_hidden_proto (__pthread_attr_getsigmask_np)
 
+/* The cancellation signal handler defined alongside with
+   pthread_cancel.  This is included in statically linked binaries
+   only if pthread_cancel is linked in.  */
+void __nptl_sigcancel_handler (int sig, siginfo_t *si, void *ctx);
+libc_hidden_proto (__nptl_sigcancel_handler)
+
 /* Special versions which use non-exported functions.  */
 extern void __pthread_cleanup_push (struct _pthread_cleanup_buffer *buffer,
 				    void (*routine) (void *), void *arg);
diff --git a/nptl/pthread_cancel.c b/nptl/pthread_cancel.c
index e4ad602900..802c691874 100644
--- a/nptl/pthread_cancel.c
+++ b/nptl/pthread_cancel.c
@@ -26,6 +26,63 @@
 #include <unwind-link.h>
 #include <stdio.h>
 #include <gnu/lib-names.h>
+#include <sys/single_threaded.h>
+
+/* For asynchronous cancellation we use a signal.  This is the core
+   logic of the signal handler.  */
+static void
+sigcancel_handler (void)
+{
+  struct pthread *self = THREAD_SELF;
+
+  int oldval = THREAD_GETMEM (self, cancelhandling);
+  while (1)
+    {
+      /* We are canceled now.  When canceled by another thread this flag
+	 is already set but if the signal is directly send (internally or
+	 from another process) is has to be done here.  */
+      int newval = oldval | CANCELING_BITMASK | CANCELED_BITMASK;
+
+      if (oldval == newval || (oldval & EXITING_BITMASK) != 0)
+	/* Already canceled or exiting.  */
+	break;
+
+      int curval = THREAD_ATOMIC_CMPXCHG_VAL (self, cancelhandling, newval,
+					      oldval);
+      if (curval == oldval)
+	{
+	  /* Set the return value.  */
+	  THREAD_SETMEM (self, result, PTHREAD_CANCELED);
+
+	  /* Make sure asynchronous cancellation is still enabled.  */
+	  if ((newval & CANCELTYPE_BITMASK) != 0)
+	    /* Run the registered destructors and terminate the thread.  */
+	    __do_cancel ();
+
+	  break;
+	}
+
+      oldval = curval;
+    }
+}
+
+/* This is the actually installed SIGCANCEL handler.  It adds some
+   safety checks before performing the cancellation.  */
+void
+__nptl_sigcancel_handler (int sig, siginfo_t *si, void *ctx)
+{
+  /* Safety check.  It would be possible to call this function for
+     other signals and send a signal from another process.  This is not
+     correct and might even be a security problem.  Try to catch as
+     many incorrect invocations as possible.  */
+  if (sig != SIGCANCEL
+      || si->si_pid != __getpid()
+      || si->si_code != SI_TKILL)
+    return;
+
+  sigcancel_handler ();
+}
+libc_hidden_def (__nptl_sigcancel_handler)
 
 int
 __pthread_cancel (pthread_t th)
@@ -72,14 +129,23 @@ __pthread_cancel (pthread_t th)
 						    oldval))
 	    goto again;
 
-	  /* The cancellation handler will take care of marking the
-	     thread as canceled.  */
-	  pid_t pid = __getpid ();
-
-	  int val = INTERNAL_SYSCALL_CALL (tgkill, pid, pd->tid,
-					   SIGCANCEL);
-	  if (INTERNAL_SYSCALL_ERROR_P (val))
-	    result = INTERNAL_SYSCALL_ERRNO (val);
+	  if (pd == THREAD_SELF)
+	    /* This is not merely an optimization: An application may
+	       call pthread_cancel (pthread_self ()) without calling
+	       pthread_create, so the signal handler may not have been
+	       set up for a self-cancel.  */
+	    sigcancel_handler ();
+	  else
+	    {
+	      /* The cancellation handler will take care of marking the
+		 thread as canceled.  */
+	      pid_t pid = __getpid ();
+
+	      int val = INTERNAL_SYSCALL_CALL (tgkill, pid, pd->tid,
+					       SIGCANCEL);
+	      if (INTERNAL_SYSCALL_ERROR_P (val))
+		result = INTERNAL_SYSCALL_ERRNO (val);
+	    }
 
 	  break;
 	}
@@ -106,4 +172,8 @@ versioned_symbol (libc, __pthread_cancel, pthread_cancel, GLIBC_2_34);
 compat_symbol (libpthread, __pthread_cancel, pthread_cancel, GLIBC_2_0);
 #endif
 
-PTHREAD_STATIC_FN_REQUIRE (__pthread_create)
+/* Ensure that the unwinder is always linked in (the __pthread_unwind
+   reference from __do_cancel is weak).  Use ___pthread_unwind_next
+   (three underscores) to produce a strong reference to the same
+   file.  */
+PTHREAD_STATIC_FN_REQUIRE (___pthread_unwind_next)
diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
index 770656453d..772b5efcc6 100644
--- a/nptl/pthread_create.c
+++ b/nptl/pthread_create.c
@@ -56,6 +56,43 @@ static struct rtld_global *__nptl_rtld_global __attribute_used__
   = &_rtld_global;
 #endif
 
+/* This performs the initialization necessary when going from
+   single-threaded to multi-threaded mode for the first time.  */
+static void
+late_init (void)
+{
+  struct sigaction sa;
+  __sigemptyset (&sa.sa_mask);
+
+  /* Install the cancellation signal handler (in static builds only if
+     pthread_cancel has been linked in).  If for some reason we cannot
+     install the handler we do not abort.  Maybe we should, but it is
+     only asynchronous cancellation which is affected.  */
+#ifndef SHARED
+  extern __typeof (__nptl_sigcancel_handler) __nptl_sigcancel_handler
+    __attribute__ ((weak));
+  if (__nptl_sigcancel_handler != NULL)
+#endif
+    {
+      sa.sa_sigaction = __nptl_sigcancel_handler;
+      sa.sa_flags = SA_SIGINFO;
+      (void) __libc_sigaction (SIGCANCEL, &sa, NULL);
+    }
+
+  /* Install the handle to change the threads' uid/gid.  */
+  sa.sa_sigaction = __nptl_setxid_sighandler;
+  sa.sa_flags = SA_SIGINFO | SA_RESTART;
+  (void) __libc_sigaction (SIGSETXID, &sa, NULL);
+
+  /* The parent process might have left the signals blocked.  Just in
+     case, unblock it.  We reuse the signal mask in the sigaction
+     structure.  It is already cleared.  */
+  __sigaddset (&sa.sa_mask, SIGCANCEL);
+  __sigaddset (&sa.sa_mask, SIGSETXID);
+  INTERNAL_SYSCALL_CALL (rt_sigprocmask, SIG_UNBLOCK, &sa.sa_mask,
+			 NULL, __NSIG_BYTES);
+}
+
 /* Code to allocate and deallocate a stack.  */
 #include "allocatestack.c"
 
@@ -459,9 +496,13 @@ __pthread_create_2_1 (pthread_t *newthread, const pthread_attr_t *attr,
 {
   STACK_VARIABLES;
 
-  /* Avoid a data race in the multi-threaded case.  */
+  /* Avoid a data race in the multi-threaded case, and call the
+     deferred initialization only once.  */
   if (__libc_single_threaded)
-    __libc_single_threaded = 0;
+    {
+      late_init ();
+      __libc_single_threaded = 0;
+    }
 
   const struct pthread_attr *iattr = (struct pthread_attr *) attr;
   union pthread_attr_transparent default_attr;
-- 
2.31.1