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posix: Clear close-on-exec for posix_spawn adddup2 (BZ#23640)

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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 #23640]
	* 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
This commit is contained in:
Adhemerval Zanella
2018-09-19 12:14:34 -07:00
parent 03992356e6
commit 805334b26c
4 changed files with 75 additions and 14 deletions
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18
sysdeps/unix/sysv/linux/spawni.c
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@ -253,9 +253,21 @@ __spawni_child (void *arguments)
break;
case spawn_do_dup2:
if (__dup2 (action->action.dup2_action.fd,
action->action.dup2_action.newfd)
!= 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)
{
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;