posix: Use posix_spawn on popen

This patch uses posix_spawn on popen instead of fork and execl.  On Linux
this has the advantage of much lower memory consumption (usually 32 Kb
minimum for the mmap stack area).

Two issues are also fixed with this change:

  * BZ#17490: although POSIX pthread_atfork description only list 'fork'
    as the function that should execute the atfork handlers, popen
    description states that:

      '[...] shall be *as if* a child process were created within the popen()
       call using the fork() function [...]'

    Other libc/system seems to follow the idea atfork handlers should not be
    executed for popen:

    libc/system	| run atfork handles   | notes
    ------------|----------------------|---------------------------------------
    Freebsd	|        no            | uses vfork
    Solaris 11	|        no            |
    MacOSX 11   |        no            | implemented through posix_spawn syscall
    ------------|----------------------|----------------------------------------

    Similar to posix_spawn and system, popen idea is to spawn a different
    binary so all the POSIX rationale to run the atfork handlers to avoid
    internal process inconsistency is not really required and in some cases
    might be unsafe.

  * BZ#22834: the described scenario, where the forked process might access
    invalid memory due an inconsistent state in multithreaded environment,
    should not happen because posix_spawn does not access the affected
    data structure (proc_file_chain).

Checked on x86_64-linux-gnu and i686-linux-gnu.

	[BZ #22834]
	[BZ #17490]
	* NEWS: Add new semantic for atfork with popen and system.
	* libio/iopopen.c (_IO_new_proc_open): use posix_spawn instead of
	fork and execl.

1 files changed, 85 insertions, 47 deletions

diff --git a/libio/iopopen.c b/libio/iopopen.c
index 2eff45b..c768295 100644
--- a/libio/iopopen.c
+++ b/libio/iopopen.c
@@ -34,7 +34,8 @@
 #include <not-cancel.h>
 #include <sys/types.h>
 #include <sys/wait.h>
-#include <kernel-features.h>
+#include <spawn.h>
+#include <paths.h>
 
 struct _IO_proc_file
 {
@@ -59,13 +60,60 @@ unlock (void *not_used)
 }
 #endif
 
+/* POSIX states popen shall ensure that any streams from previous popen()
+   calls that remain open in the parent process should be closed in the new
+   child process.
+   To avoid a race-condition between checking which file descriptors need to
+   be close (by transversing the proc_file_chain list) and the insertion of a
+   new one after a successful posix_spawn this function should be called
+   with proc_file_chain_lock acquired.  */
+static bool
+spawn_process (posix_spawn_file_actions_t *fa, FILE *fp, const char *command,
+	       int do_cloexec, int pipe_fds[2], int parent_end, int child_end,
+	       int child_pipe_fd)
+{
+
+  for (struct _IO_proc_file *p = proc_file_chain; p; p = p->next)
+    {
+      int fd = _IO_fileno ((FILE *) p);
+
+      /* If any stream from previous popen() calls has fileno
+	 child_pipe_fd, it has been already closed by the adddup2 action
+	 above.  */
+      if (fd != child_pipe_fd
+	  && __posix_spawn_file_actions_addclose (fa, fd) != 0)
+	return false;
+    }
+
+  if (__posix_spawn (&((_IO_proc_file *) fp)->pid, _PATH_BSHELL, fa, 0,
+		     (char *const[]){ (char*) "sh", (char*) "-c",
+		     (char *) command, NULL }, __environ) != 0)
+    return false;
+
+  __close_nocancel (pipe_fds[child_end]);
+
+  if (!do_cloexec)
+    /* Undo the effects of the pipe2 call which set the
+       close-on-exec flag.  */
+    __fcntl (pipe_fds[parent_end], F_SETFD, 0);
+
+  _IO_fileno (fp) = pipe_fds[parent_end];
+
+  ((_IO_proc_file *) fp)->next = proc_file_chain;
+  proc_file_chain = (_IO_proc_file *) fp;
+
+  return true;
+}
+
 FILE *
 _IO_new_proc_open (FILE *fp, const char *command, const char *mode)
 {
   int read_or_write;
+  /* These are indexes for pipe_fds.  */
   int parent_end, child_end;
   int pipe_fds[2];
-  pid_t child_pid;
+  int child_pipe_fd;
+  bool spawn_ok;
 
   int do_read = 0;
   int do_write = 0;
@@ -108,72 +156,62 @@ _IO_new_proc_open (FILE *fp, const char *command, const char *mode)
 
   if (do_read)
     {
-      parent_end = pipe_fds[0];
-      child_end = pipe_fds[1];
+      parent_end = 0;
+      child_end = 1;
       read_or_write = _IO_NO_WRITES;
+      child_pipe_fd = 1;
     }
   else
     {
-      parent_end = pipe_fds[1];
-      child_end = pipe_fds[0];
+      parent_end = 1;
+      child_end = 0;
       read_or_write = _IO_NO_READS;
+      child_pipe_fd = 0;
     }
 
-  ((_IO_proc_file *) fp)->pid = child_pid = __fork ();
-  if (child_pid == 0)
-    {
-      int child_std_end = do_read ? 1 : 0;
-      struct _IO_proc_file *p;
-
-      if (child_end != child_std_end)
-	__dup2 (child_end, child_std_end);
-      else
-	/* The descriptor is already the one we will use.  But it must
-	   not be marked close-on-exec.  Undo the effects.  */
-	__fcntl (child_end, F_SETFD, 0);
-      /* POSIX.2:  "popen() shall ensure that any streams from previous
-         popen() calls that remain open in the parent process are closed
-	 in the new child process." */
-      for (p = proc_file_chain; p; p = p->next)
-	{
-	  int fd = _IO_fileno ((FILE *) p);
+  posix_spawn_file_actions_t fa;
+  /* posix_spawn_file_actions_init does not fail.  */
+  __posix_spawn_file_actions_init (&fa);
 
-	  /* If any stream from previous popen() calls has fileno
-	     child_std_end, it has been already closed by the dup2 syscall
-	     above.  */
-	  if (fd != child_std_end)
-	    __close_nocancel (fd);
-	}
-
-      execl ("/bin/sh", "sh", "-c", command, (char *) 0);
-      _exit (127);
-    }
-  __close_nocancel (child_end);
-  if (child_pid < 0)
+  /* The descriptor is already the one the child will use.  In this case
+     it must be moved to another one otherwise, there is no safe way to
+     remove the close-on-exec flag in the child without creating a FD leak
+     race in the parent.  */
+  if (pipe_fds[child_end] == child_pipe_fd)
     {
-      __close_nocancel (parent_end);
-      return NULL;
+      int tmp = __fcntl (child_pipe_fd, F_DUPFD_CLOEXEC, 0);
+      if (tmp < 0)
+	goto spawn_failure;
+      __close_nocancel (pipe_fds[child_end]);
+      pipe_fds[child_end] = tmp;
     }
 
-  if (!do_cloexec)
-    /* Undo the effects of the pipe2 call which set the
-       close-on-exec flag.  */
-    __fcntl (parent_end, F_SETFD, 0);
+  if (__posix_spawn_file_actions_adddup2 (&fa, pipe_fds[child_end],
+      child_pipe_fd) != 0)
+    goto spawn_failure;
 
-  _IO_fileno (fp) = parent_end;
-
-  /* Link into proc_file_chain. */
 #ifdef _IO_MTSAFE_IO
   _IO_cleanup_region_start_noarg (unlock);
   _IO_lock_lock (proc_file_chain_lock);
 #endif
-  ((_IO_proc_file *) fp)->next = proc_file_chain;
-  proc_file_chain = (_IO_proc_file *) fp;
+  spawn_ok = spawn_process (&fa, fp, command, do_cloexec, pipe_fds,
+			    parent_end, child_end, child_pipe_fd);
 #ifdef _IO_MTSAFE_IO
   _IO_lock_unlock (proc_file_chain_lock);
   _IO_cleanup_region_end (0);
 #endif
 
+  __posix_spawn_file_actions_destroy (&fa);
+
+  if (!spawn_ok)
+    {
+    spawn_failure:
+      __close_nocancel (pipe_fds[child_end]);
+      __close_nocancel (pipe_fds[parent_end]);
+      __set_errno (ENOMEM);
+      return NULL;
+    }
+
   _IO_mask_flags (fp, read_or_write, _IO_NO_READS|_IO_NO_WRITES);
   return fp;
 }