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Re: [PATCH 2/2] posix: Clear close-on-exec for posix_spawn adddup2 (BZ#23640)


My mail client ate the original submission, so responding to your ping.

On 13/12/18 2:54 AM, Adhemerval Zanella wrote:
Ping (x2).

On 29/10/2018 16:32, Adhemerval Zanella wrote:
Ping.

On 19/09/2018 19:46, adhemerval.zanella@linaro.org wrote:
From: Adhemerval Zanella <adhemerval.zanella@linaro.org>

Austin Group issue #411 [1] proposes that posix_spawn file action
posix_spawn_file_actions_adddup2 resets the close-on-exec when
source and destination refer to same file descriptor.

It solves the issue on multi-thread applications which uses
close-on-exec as default, and want to hand-chose specifically
file descriptor to purposefully inherited into a child process.
Current approach to achieve this scenario is to use two adddup2 file
actions and a temporary file description which do not conflict with
any other, coupled with a close file action to avoid leaking the
temporary file descriptor.  This approach, besides being complex,
may fail with EMFILE/ENFILE file descriptor exaustion.

This can be more easily accomplished with an in-place removal of
FD_CLOEXEC.  Although the resulting adddup2 semantic is slight
different than dup2 (equal file descriptors should be handled as
no-op), the proposed possible solution are either more complex
(fcntl action which a limited set of operations) or results in
unrequired operations (dup3 which also returns EINVAL for same
file descriptor).

Checked on aarch64-linux-gnu.


BZ # on ChangeLog.

	* posix/tst-spawn.c (do_prepare, handle_restart, do_test): Add
	posix_spawn_file_actions_adddup2 test to check O_CLOCEXEC reset.
	* sysdeps/unix/sysv/linux/spawni.c (__spawni_child): Add
	close-on-exec reset for adddup2 file action.
	* sysdeps/posix/spawni.c (__spawni_child): Likewise.

[1] http://austingroupbugs.net/view.php?id=411
---
  ChangeLog                        |  6 ++++
  posix/tst-spawn.c                | 47 +++++++++++++++++++++++++-------
  sysdeps/posix/spawni.c           | 21 ++++++++++++--
  sysdeps/unix/sysv/linux/spawni.c | 21 ++++++++++++--
  4 files changed, 79 insertions(+), 16 deletions(-)

diff --git a/posix/tst-spawn.c b/posix/tst-spawn.c
index 638ba1ba55..2354417020 100644
--- a/posix/tst-spawn.c
+++ b/posix/tst-spawn.c
@@ -40,17 +40,19 @@ static int restart;
  static char *name1;
  static char *name2;
  static char *name3;
+static char *name5;
/* Descriptors for the temporary files. */
  static int temp_fd1 = -1;
  static int temp_fd2 = -1;
  static int temp_fd3 = -1;
+static int temp_fd5 = -1;
/* The contents of our files. */
  static const char fd1string[] = "This file should get closed";
  static const char fd2string[] = "This file should stay opened";
  static const char fd3string[] = "This file will be opened";
-
+static const char fd5string[] = "This file should stay opened (O_CLOEXEC)";
/* We have a preparation function. */
  static void
@@ -63,24 +65,31 @@ do_prepare (int argc, char *argv[])
    TEST_VERIFY_EXIT ((temp_fd1 = create_temp_file ("spawn", &name1)) != -1);
    TEST_VERIFY_EXIT ((temp_fd2 = create_temp_file ("spawn", &name2)) != -1);
    TEST_VERIFY_EXIT ((temp_fd3 = create_temp_file ("spawn", &name3)) != -1);
+  TEST_VERIFY_EXIT ((temp_fd5 = create_temp_file ("spawn", &name5)) != -1);
+
+  int flags;
+  TEST_VERIFY_EXIT ((flags = fcntl (temp_fd5, F_GETFD, &flags)) != -1);
+  TEST_VERIFY_EXIT (fcntl (temp_fd5, F_SETFD, flags | FD_CLOEXEC) == 0);
  }
  #define PREPARE do_prepare
static int
  handle_restart (const char *fd1s, const char *fd2s, const char *fd3s,
-		const char *fd4s, const char *name)
+		const char *fd4s, const char *name, const char *fd5s)
  {
    char buf[100];
    int fd1;
    int fd2;
    int fd3;
    int fd4;
+  int fd5;
/* First get the descriptors. */
    fd1 = atol (fd1s);
    fd2 = atol (fd2s);
    fd3 = atol (fd3s);
    fd4 = atol (fd4s);
+  fd5 = atol (fd5s);
/* Sanity check. */
    TEST_VERIFY_EXIT (fd1 != fd2);
@@ -89,6 +98,7 @@ handle_restart (const char *fd1s, const char *fd2s, const char *fd3s,
    TEST_VERIFY_EXIT (fd2 != fd3);
    TEST_VERIFY_EXIT (fd2 != fd4);
    TEST_VERIFY_EXIT (fd3 != fd4);
+  TEST_VERIFY_EXIT (fd4 != fd5);
/* First the easy part: read from the file descriptor which is
       supposed to be open.  */
@@ -116,6 +126,10 @@ handle_restart (const char *fd1s, const char *fd2s, const char *fd3s,
    TEST_VERIFY_EXIT (read (fd1, buf, sizeof buf) == strlen (fd1string));
    TEST_VERIFY_EXIT (memcmp (fd1string, buf, strlen (fd1string)) == 0);
+ TEST_VERIFY_EXIT (lseek (fd5, 0, SEEK_SET) == 0);
+  TEST_VERIFY_EXIT (read (fd5, buf, sizeof buf) == strlen (fd5string));
+  TEST_VERIFY_EXIT (memcmp (fd5string, buf, strlen (fd5string)) == 0);
+
    return 0;
  }
@@ -131,6 +145,7 @@ do_test (int argc, char *argv[])
    char fd2name[18];
    char fd3name[18];
    char fd4name[18];
+  char fd5name[18];
    char *name3_copy;
    char *spargv[12];
    int i;
@@ -141,21 +156,24 @@ do_test (int argc, char *argv[])
         + "--library-path"	optional
         + the library path	optional
         + the application name
-     - five parameters left if called through re-execution
+     - six parameters left if called through re-execution
         + file descriptor number which is supposed to be closed
         + the open file descriptor
         + the newly opened file descriptor
-       + thhe duped second descriptor
+       + the duped second descriptor
         + the name of the closed descriptor
+       + the duped fourth dile descriptor which O_CLOEXEC should be
+	 remove by adddup2.
    */
-  if (argc != (restart ? 6 : 2) && argc != (restart ? 6 : 5))
+  if (argc != (restart ? 7 : 2) && argc != (restart ? 7 : 5))
      FAIL_EXIT1 ("wrong number of arguments (%d)", argc);
if (restart)
-    return handle_restart (argv[1], argv[2], argv[3], argv[4], argv[5]);
+    return handle_restart (argv[1], argv[2], argv[3], argv[4], argv[5],
+			   argv[6]);
- /* Prepare the test. We are creating two files: one which file descriptor
-     will be marked with FD_CLOEXEC, another which is not.  */
+  /* Prepare the test.  We are creating four files: two which file descriptor
+     will be marked with FD_CLOEXEC, another which is not  */
/* Write something in the files. */
    TEST_VERIFY_EXIT (write (temp_fd1, fd1string, strlen (fd1string))
@@ -164,6 +182,8 @@ do_test (int argc, char *argv[])
  		    == strlen (fd2string));
    TEST_VERIFY_EXIT (write (temp_fd3, fd3string, strlen (fd3string))
  		    == strlen (fd3string));
+  TEST_VERIFY_EXIT (write (temp_fd5, fd5string, strlen (fd5string))
+		    == strlen (fd5string));
/* Close the third file. It'll be opened by `spawn'. */
    close (temp_fd3);
@@ -183,17 +203,23 @@ do_test (int argc, char *argv[])
    memset (name3_copy, 'X', strlen (name3_copy));
/* We dup the second descriptor. */
-  fd4 = MAX (2, MAX (temp_fd1, MAX (temp_fd2, temp_fd3))) + 1;
+  fd4 = MAX (2, MAX (temp_fd1, MAX (temp_fd2, MAX (temp_fd3, temp_fd5)))) + 1;
    TEST_VERIFY_EXIT (posix_spawn_file_actions_adddup2 (&actions, temp_fd2,
  						      fd4) == 0);
+ /* We clear the O_CLOEXEC on fourth descriptor, so it should be
+     stay open on child.  */
+  TEST_VERIFY_EXIT (posix_spawn_file_actions_adddup2 (&actions, temp_fd5,
+						      temp_fd5) == 0);
+
    /* Now spawn the process.  */
    snprintf (fd1name, sizeof fd1name, "%d", temp_fd1);
    snprintf (fd2name, sizeof fd2name, "%d", temp_fd2);
    snprintf (fd3name, sizeof fd3name, "%d", temp_fd3);
    snprintf (fd4name, sizeof fd4name, "%d", fd4);
+  snprintf (fd5name, sizeof fd5name, "%d", temp_fd5);
- for (i = 0; i < (argc == (restart ? 6 : 5) ? 4 : 1); i++)
+  for (i = 0; i < (argc == (restart ? 7 : 5) ? 4 : 1); i++)
      spargv[i] = argv[i + 1];
    spargv[i++] = (char *) "--direct";
    spargv[i++] = (char *) "--restart";
@@ -202,6 +228,7 @@ do_test (int argc, char *argv[])
    spargv[i++] = fd3name;
    spargv[i++] = fd4name;
    spargv[i++] = name1;
+  spargv[i++] = fd5name;
    spargv[i] = NULL;
TEST_VERIFY_EXIT (posix_spawn (&pid, argv[1], &actions, NULL, spargv,
diff --git a/sysdeps/posix/spawni.c b/sysdeps/posix/spawni.c
index b138ab4393..d322db5c19 100644
--- a/sysdeps/posix/spawni.c
+++ b/sysdeps/posix/spawni.c
@@ -204,10 +204,25 @@ __spawni_child (void *arguments)
  	      break;
case spawn_do_dup2:
-	      if (__dup2 (action->action.dup2_action.fd,
-			  action->action.dup2_action.newfd)
+	      /* Austin Group issue #411 requires adddup2 action with source
+		 and destination being equal to remove close-on-exec flag.  */
+	      if (action->action.dup2_action.fd
  		  != action->action.dup2_action.newfd)
-		goto fail;
+		{
+		  if (__dup2 (action->action.dup2_action.fd,
+			     action->action.dup2_action.newfd)
+		      != action->action.dup2_action.newfd)
+		    goto fail;
+		}
+	      else
+		{
+		  int fd = action->action.dup2_action.newfd;
+		  int flags = __fcntl (fd, F_GETFD, 0);
+		  if (flags == -1)
+		    goto fail;
+		  if (__fcntl (fd, F_SETFD, flags & ~FD_CLOEXEC) == -1)
+		    goto fail;
+		}

It might be simpler to write this as follows to flatten out the nesting a bit. It also makes the location of the comment a lot more relevant because the clearing of the flag happens right under it:

    if (action->action.dup2_action.fd ==
        action->action.dup2_action.newfd)
      {
	int fd = action->action.dup2_action.newfd;
	int flags = __fcntl (fd, F_GETFD, 0);
	if (flags == -1)
	  goto fail;
	if (__fcntl (fd, F_SETFD, flags & ~FD_CLOEXEC) == -1)
	  goto fail;
      }
    else if (__dup2 (action->action.dup2_action.fd,
		     action->action.dup2_action.newfd)
	     != action->action.dup2_action.newfd)
      goto fail;

  	      break;
  	    }
  	}
diff --git a/sysdeps/unix/sysv/linux/spawni.c b/sysdeps/unix/sysv/linux/spawni.c
index 85239cedbf..b2304ffe60 100644
--- a/sysdeps/unix/sysv/linux/spawni.c
+++ b/sysdeps/unix/sysv/linux/spawni.c
@@ -253,10 +253,25 @@ __spawni_child (void *arguments)
  	      break;
case spawn_do_dup2:
-	      if (__dup2 (action->action.dup2_action.fd,
-			  action->action.dup2_action.newfd)
+	      /* Austin Group issue #411 requires adddup2 action with source
+		 and destination being equal to remove close-on-exec flag.  */
+	      if (action->action.dup2_action.fd
  		  != action->action.dup2_action.newfd)
-		goto fail;
+		{
+		  if (__dup2 (action->action.dup2_action.fd,
+			     action->action.dup2_action.newfd)
+		      != action->action.dup2_action.newfd)
+		    goto fail;
+		}
+	      else
+		{
+		  int fd = action->action.dup2_action.newfd;
+		  int flags = __fcntl (fd, F_GETFD, 0);
+		  if (flags == -1)
+		    goto fail;
+		  if (__fcntl (fd, F_SETFD, flags & ~FD_CLOEXEC) == -1)
+		    goto fail;
+		}
  	      break;
  	    }
  	}


Likewise.

Siddhesh


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