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plumber: don't leak srvfd file descriptor into sub processes

Put OCEXEC flag on the srvfd so it wont be leaked
on sub-processes we spawn from plumb rules.

--- a/sys/src/cmd/plumb/fsys.c

+++ b/sys/src/cmd/plumb/fsys.c

@@ -99,7 +99,7 @@

 static int	ndir = NQID;

 static int		srvfd;

-static int		srvclosefd;			/* rock for end of pipe to close */

+static int		mntfd;

 static int		clockfd;

 static int		clock;

 static Fid		*fids[Nhash];

@@ -190,19 +190,34 @@

 	return atoi(buf);

 }

+/*

+ * Build pipe with OCEXEC set on second fd.

+ * Can't put it on both because we want to post one in /srv.

+ */

+static int

+cexecpipe(int *p0, int *p1)

+{

+	/* pipe the hard way to get close on exec */

+	if(bind("#|", "/mnt/temp", MREPL) == -1)

+		return -1;

+	*p0 = open("/mnt/temp/data", ORDWR);

+	*p1 = open("/mnt/temp/data1", ORDWR|OCEXEC);

+	unmount(nil, "/mnt/temp");

+	if(*p0<0 || *p1<0)

+		return -1;

+	return 0;

+}

+

 void

 startfsys(void)

 {

-	int p[2], fd;

+	int fd;

 	fmtinstall('F', fcallfmt);

 	clockfd = open("/dev/time", OREAD|OCEXEC);

 	clock = getclock();

-	if(pipe(p) < 0)

+	if(cexecpipe(&mntfd, &srvfd) < 0)

 		error("can't create pipe: %r");

-	/* 0 will be server end, 1 will be client end */

-	srvfd = p[0];

-	srvclosefd = p[1];

 	sprint(srvfile, "/srv/plumb.%s.%d", user, getpid());

 	if(putenv("plumbsrv", srvfile) < 0)

 		error("can't write $plumbsrv: %r");

@@ -209,16 +224,12 @@

 	fd = create(srvfile, OWRITE|OCEXEC|ORCLOSE, 0600);

 	if(fd < 0)

 		error("can't create /srv file: %r");

-	if(fprint(fd, "%d", p[1]) <= 0)

+	if(fprint(fd, "%d", mntfd) <= 0)

 		error("can't write /srv/file: %r");

-	/* leave fd open; ORCLOSE will take care of it */

-

 	procrfork(fsysproc, nil, Stack, RFFDG);

-

-	close(p[0]);

-	if(mount(p[1], -1, "/mnt/plumb", MREPL, "") == -1)

+	close(srvfd);

+	if(mount(mntfd, -1, "/mnt/plumb", MREPL, "") == -1)

 		error("can't mount /mnt/plumb: %r");

-	close(p[1]);

 }

 static void

@@ -229,8 +240,8 @@

 	Fid *f;

 	uchar *buf;

-	close(srvclosefd);

-	srvclosefd = -1;

+	close(mntfd);

+

 	t = nil;

 	for(;;){

 		buf = malloc(messagesize);	/* avoid memset of emalloc */