privsep: Notify processes when dhcpcd has daemonised

This allows us to dup stdout and stderr onto stdin which is
guaranteed to be dupped to /dev/null.
This in turn avoids SIGPIPE when the privileged proccess launches
the script and it wants to write to stdout/stderr or stupidly
read from stdin.

src/dhcpcd.c

@@ -329,6 +329,36 @@ dhcpcd_ipwaited(struct dhcpcd_ctx *ctx)
 	return 1;
 }
 
+#ifndef THERE_IS_NO_FORK
+void
+dhcpcd_daemonised(struct dhcpcd_ctx *ctx)
+{
+	unsigned int logopts = loggetopts();
+
+	/*
+	 * Stop writing to stderr.
+	 * On the happy path, only the manager process writes to stderr,
+	 * so this just stops wasting fprintf calls to nowhere.
+	 * All other calls - ie errors in privsep processes or script output,
+	 * will error when printing.
+	 * If we *really* want to fix that, then we need to suck
+	 * stderr/stdout in the manager process and either discard it or pass
+	 * it to the launcher process and then to stderr.
+	 */
+	logopts &= ~LOGERR_ERR;
+	logsetopts(logopts);
+
+	/*
+	 * We need to do something with stdout/stderr to avoid SIGPIPE
+	 * We know that stdin is already mapped to /dev/null
+	 */
+	dup2(STDIN_FILENO, STDOUT_FILENO);
+	dup2(STDIN_FILENO, STDERR_FILENO);
+
+	ctx->options |= DHCPCD_DAEMONISED;
+}
+#endif
+
 /* Returns the pid of the child, otherwise 0. */
 void
 dhcpcd_daemonise(struct dhcpcd_ctx *ctx)
@@ -363,26 +393,20 @@ dhcpcd_daemonise(struct dhcpcd_ctx *ctx)
 	if (!(logopts & LOGERR_QUIET) && ctx->stderr_valid)
 		(void)fprintf(stderr,
 		    "forked to background, child pid %d\n", getpid());
+
+#ifdef PRIVSEP
+	ps_daemonised(ctx);
+#else
+	dhcpcd_daemonised(ctx);
+#endif
+
 	i = EXIT_SUCCESS;
 	if (write(ctx->fork_fd, &i, sizeof(i)) == -1)
 		logerr("write");
 	ctx->options |= DHCPCD_DAEMONISED;
 	eloop_event_delete(ctx->eloop, ctx->fork_fd);
 	close(ctx->fork_fd);
 	ctx->fork_fd = -1;
-
-	/*
-	 * Stop writing to stderr.
-	 * On the happy path, only the manager process writes to stderr,
-	 * so this just stops wasting fprintf calls to nowhere.
-	 * All other calls - ie errors in privsep processes or script output,
-	 * will error when printing.
-	 * If we *really* want to fix that, then we need to suck
-	 * stderr/stdout in the manager process and either disacrd it or pass
-	 * it to the launcher process and then to stderr.
-	 */
-	logopts &= ~LOGERR_ERR;
-	logsetopts(logopts);
 #endif
 }
 
@@ -1871,17 +1895,17 @@ dhcpcd_pidfile_timeout(void *arg)
 
 static int dup_null(int fd)
 {
+	int fd_null = open(_PATH_DEVNULL, O_WRONLY);
 	int err;
-	int fd_null = open(_PATH_DEVNULL, O_RDONLY);
 
 	if (fd_null == -1) {
 		logwarn("open %s", _PATH_DEVNULL);
 		return -1;
 	}
 
-	if ((err = dup2(fd, fd_null)) == -1)
+	if ((err = dup2(fd_null, fd)) == -1)
 		logwarn("dup2 %d", fd);
-	close(fd);
+	close(fd_null);
 	return err;
 }
 
@@ -1988,6 +2012,15 @@ main(int argc, char **argv, char **envp)
 	ctx.stdout_valid = fcntl(STDOUT_FILENO, F_GETFD) != -1;
 	ctx.stderr_valid = fcntl(STDERR_FILENO, F_GETFD) != -1;
 
+	/* Even we if we don't have input/outputs, we need to
+	 * ensure they are setup for shells. */
+	if (!ctx.stdin_valid)
+		dup_null(STDIN_FILENO);
+	if (!ctx.stdout_valid)
+		dup_null(STDOUT_FILENO);
+	if (!ctx.stderr_valid)
+		dup_null(STDERR_FILENO);
+
 	logopts = LOGERR_LOG | LOGERR_LOG_DATE | LOGERR_LOG_PID;
 	if (ctx.stderr_valid)
 		logopts |= LOGERR_ERR;

src/dhcpcd.h

@@ -265,6 +265,7 @@ extern const char *dhcpcd_default_script;
 
 int dhcpcd_ifafwaiting(const struct interface *);
 int dhcpcd_afwaiting(const struct dhcpcd_ctx *);
+void dhcpcd_daemonised(struct dhcpcd_ctx *);
 void dhcpcd_daemonise(struct dhcpcd_ctx *);
 
 void dhcpcd_signal_cb(int, void *);

src/privsep-control.c

@@ -69,8 +69,7 @@ ps_ctl_recvmsg(void *arg, unsigned short events)
 {
 	struct ps_process *psp = arg;
 
-	if (ps_recvpsmsg(psp->psp_ctx, psp->psp_fd, events,
-	    NULL, psp->psp_ctx) == -1)
+	if (ps_recvpsmsg(psp->psp_ctx, psp->psp_fd, events, NULL, NULL) == -1)
 		logerr(__func__);
 }
 

src/privsep.c

@@ -1099,6 +1099,26 @@ ps_recvmsg(int rfd, unsigned short events, uint16_t cmd, int wfd)
 	return len;
 }
 
+ssize_t
+ps_daemonised(struct dhcpcd_ctx *ctx)
+{
+	struct ps_process *psp;
+	ssize_t err = 0;
+
+	dhcpcd_daemonised(ctx);
+
+	/* Echo the message to all processes */
+	TAILQ_FOREACH(psp, &ctx->ps_processes, next) {
+		if (psp->psp_pid == getpid())
+			continue;
+		if (ps_sendcmd(psp->psp_ctx, psp->psp_fd, PS_DAEMONISED,
+		    0, NULL, 0) == -1)
+			err = -1;
+	}
+
+	return err;
+}
+
 ssize_t
 ps_recvpsmsg(struct dhcpcd_ctx *ctx, int fd, unsigned short events,
     ssize_t (*callback)(void *, struct ps_msghdr *, struct msghdr *),
@@ -1131,6 +1151,9 @@ ps_recvpsmsg(struct dhcpcd_ctx *ctx, int fd, unsigned short events,
 		if (psm.psm_hdr.ps_cmd == PS_STOP) {
 			stop = true;
 			len = 0;
+		} else if (psm.psm_hdr.ps_cmd == PS_DAEMONISED) {
+			ps_daemonised(ctx);
+			return 0;
 		}
 	}
 

src/privsep.h

@@ -55,6 +55,7 @@
 #define	PS_CTL_EOF		0x0019
 #define	PS_LOGREOPEN		0x0020
 #define	PS_STOPPROCS		0x0021
+#define	PS_DAEMONISED		0x0022
 
 /* Domains */
 #define	PS_ROOT			0x0101
@@ -203,6 +204,7 @@ int ps_stop(struct dhcpcd_ctx *);
 int ps_stopwait(struct dhcpcd_ctx *);
 int ps_entersandbox(const char *, const char **);
 int ps_managersandbox(struct dhcpcd_ctx *, const char *);
+ssize_t ps_daemonised(struct dhcpcd_ctx *);
 
 int ps_unrollmsg(struct msghdr *, struct ps_msghdr *, const void *, size_t);
 ssize_t ps_sendpsmmsg(struct dhcpcd_ctx *, int,