kern_shm_open: push O_CLOEXEC into caller control

The motivation for this change is to allow wrappers around shm to be written
that don't set CLOEXEC. kern_shm_open currently accepts O_CLOEXEC but sets
it unconditionally. kern_shm_open is used by the shm_open(2) syscall, which
is mandated by POSIX to set CLOEXEC, and CloudABI's sys_fd_create1().
Presumably O_CLOEXEC is intended in the latter caller, but it's unclear from
the context.

sys_shm_open() now unconditionally sets O_CLOEXEC to meet POSIX
requirements, and a comment has been dropped in to kern_fd_open() to explain
the situation and add a pointer to where O_CLOEXEC setting is maintained for
shm_open(2) correctness. CloudABI's sys_fd_create1() also unconditionally
sets O_CLOEXEC to match previous behavior.

This also has the side-effect of making flags correctly reflect the
O_CLOEXEC status on this fd for the rest of kern_shm_open(), but a
glance-over leads me to believe that it didn't really matter.

Reviewed by:	kib, markj
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D21119

1 files changed, 10 insertions, 2 deletions

diff --git a/sys/kern/uipc_shm.c b/sys/kern/uipc_shm.c
index 0105650d7f93..d393f7818387 100644
--- a/sys/kern/uipc_shm.c
+++ b/sys/kern/uipc_shm.c
@@ -729,7 +729,14 @@ kern_shm_open(struct thread *td, const char *userpath, int flags, mode_t mode,
 	fdp = td->td_proc->p_fd;
 	cmode = (mode & ~fdp->fd_cmask) & ACCESSPERMS;
 
-	error = falloc_caps(td, &fp, &fd, O_CLOEXEC, fcaps);
+	/*
+	 * shm_open(2) created shm should always have O_CLOEXEC set, as mandated
+	 * by POSIX.  We allow it to be unset here so that an in-kernel
+	 * interface may be written as a thin layer around shm, optionally not
+	 * setting CLOEXEC.  For shm_open(2), O_CLOEXEC is set unconditionally
+	 * in sys_shm_open() to keep this implementation compliant.
+	 */
+	error = falloc_caps(td, &fp, &fd, flags & O_CLOEXEC, fcaps);
 	if (error)
 		return (error);
 
@@ -844,7 +851,8 @@ int
 sys_shm_open(struct thread *td, struct shm_open_args *uap)
 {
 
-	return (kern_shm_open(td, uap->path, uap->flags, uap->mode, NULL));
+	return (kern_shm_open(td, uap->path, uap->flags | O_CLOEXEC, uap->mode,
+	    NULL));
 }
 
 int