[PATCH v2 2/3] posix: Use posix_spawn on popen

[Adhemerval Zanella <adhemerval dot zanella at linaro dot org>]

Changed from previous version:

  - Added note about BZ#17490 fix.
  - Use proc_file_chain_lock to access proc_file_chain (BZ#22834).

---

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 where should issue the atfork handlers and
    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
    issue for popen:

    libc/system	| run atfork handles   | notes
    ------------|----------------------|---------------------------------------
    freebsd	|        no            | uses vfork
    solaris 11	|        no            |
    MacOSX 11i  |        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: now that proc_file_chain is not copied on another process, it
    just require to access is through the proc_file_chain_lock.

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

	[BZ #22834]
	[BZ #17490]
	* libio/iopopen.c (_IO_new_proc_open): use posix_spawn instead of
	fork and execl.
---
 ChangeLog       |   4 ++
 libio/iopopen.c | 105 ++++++++++++++++++++++++++++++------------------
 2 files changed, 70 insertions(+), 39 deletions(-)

diff --git a/libio/iopopen.c b/libio/iopopen.c
index 2eff45b4c8..a2867abb02 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
 {
@@ -63,9 +64,8 @@ FILE *
 _IO_new_proc_open (FILE *fp, const char *command, const char *mode)
 {
   int read_or_write;
-  int parent_end, child_end;
   int pipe_fds[2];
-  pid_t child_pid;
+  int op;
 
   int do_read = 0;
   int do_write = 0;
@@ -108,59 +108,86 @@ _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];
+      op = 0;
       read_or_write = _IO_NO_WRITES;
     }
   else
     {
-      parent_end = pipe_fds[1];
-      child_end = pipe_fds[0];
+      op = 1;
       read_or_write = _IO_NO_READS;
     }
 
-  ((_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);
-	}
+    /* The descriptor is already in 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[1 - op] == 1 - op)
+      {
+	int tmp = __fcntl (1 - op, F_DUPFD_CLOEXEC, 0);
+	if (tmp < 0)
+	  goto spawn_failure;
+	__close_nocancel (pipe_fds[1 - op]);
+	pipe_fds[1 - op] = tmp;
+      }
 
-      execl ("/bin/sh", "sh", "-c", command, (char *) 0);
-      _exit (127);
-    }
-  __close_nocancel (child_end);
-  if (child_pid < 0)
+    if (__posix_spawn_file_actions_adddup2 (&fa, pipe_fds[1 - op], 1 - op)
+	!= 0)
+      goto spawn_failure;
+
+    /* 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." */
+#ifdef _IO_MTSAFE_IO
+     _IO_cleanup_region_start_noarg (unlock);
+     _IO_lock_lock (proc_file_chain_lock);
+#endif
+    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_send, it has been already closed by the dup2 syscall
+	   above.  */
+	if (fd != 1 - op
+	    && __posix_spawn_file_actions_addclose (&fa, fd) != 0)
+	  goto spawn_failure;
+      }
+#ifdef _IO_MTSAFE_IO
+      _IO_lock_unlock (proc_file_chain_lock);
+      _IO_cleanup_region_end (0);
+#endif
+
+    if (__posix_spawn (&((_IO_proc_file *) fp)->pid, _PATH_BSHELL, &fa, 0,
+		     (char *const[]){ (char*) "sh", (char*) "-c",
+		     (char *) command, NULL }, __environ) != 0)
+      {
+      spawn_failure:
+	__posix_spawn_file_actions_destroy (&fa);
+	__close_nocancel (pipe_fds[1 - op]);
+	__set_errno (ENOMEM);
+	return NULL;
+      }
+
+    __posix_spawn_file_actions_destroy (&fa);
+  }
+  __close_nocancel (pipe_fds[1 - op]);
+  if (((_IO_proc_file *) fp)->pid < 0)
     {
-      __close_nocancel (parent_end);
+      __close_nocancel (pipe_fds[op]);
       return NULL;
     }
 
   if (!do_cloexec)
     /* Undo the effects of the pipe2 call which set the
        close-on-exec flag.  */
-    __fcntl (parent_end, F_SETFD, 0);
+    __fcntl (pipe_fds[op], F_SETFD, 0);
 
-  _IO_fileno (fp) = parent_end;
+  _IO_fileno (fp) = pipe_fds[op];
 
   /* Link into proc_file_chain. */
 #ifdef _IO_MTSAFE_IO
-- 
2.17.1