From 44609bfc974bdafc0316d069aabf5e2903efa805 Mon Sep 17 00:00:00 2001 From: pali <7141871+pali@users.noreply.github.com> Date: Tue, 9 Aug 2022 11:20:15 +0200 Subject: [PATCH] pppd: Workaround for generating ppp unit id on Linux (#355) MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit Linux kernel has nasty bug / feature. If PPPIOCNEWUNIT is called with negative ppp unit id (which is default option when command line argument "unit" is not specified; and tells kernel to choose some free ppp unit id) and the lowest unused/free ppp unit id is present in some existing network interface name prefixed by "ppp" string then this PPPIOCNEWUNIT ioctl fails. In this case kernel is basically unable to create a new ppp interface via PPPIOCNEWUNIT ioctl when user does not specify some unused and non-conflicted unit id. Linux kernel should be fixed to choose usable ppp unit id when was requested via PPPIOCNEWUNIT parameter -1. Until this happens, add a workaround for pppd to help choosing some random ppp unit id when kernel returns this error. Simple test case (run on system when there is no ppp interface): sudo ./pppd ifname ppp1 nodefaultroute noauth nolock local nodetach pty "./pppd nodefaultroute noauth nolock local nodetach notty" Second pppd process without this patch prints into syslog following error: pppd 2.4.10-dev started by pali, uid 0 Couldn't create new ppp unit: File exists Exit. With this patch it falls back to random ppp unit id and succeeds: pppd 2.4.10-dev started by pali, uid 0 Using interface ppp1361 Connect: ppp1361 <--> /dev/pts/14 ... Signed-off-by: Pali Rohár --- pppd/sys-linux.c | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/pppd/sys-linux.c b/pppd/sys-linux.c index e7f851c..c5ba49d 100644 --- a/pppd/sys-linux.c +++ b/pppd/sys-linux.c @@ -878,6 +878,11 @@ static int make_ppp_unit(void) ifunit = -1; x = ioctl(ppp_dev_fd, PPPIOCNEWUNIT, &ifunit); } + if (x < 0 && errno == EEXIST) { + srand(time(NULL) * getpid()); + ifunit = rand() % 10000; + x = ioctl(ppp_dev_fd, PPPIOCNEWUNIT, &ifunit); + } if (x < 0) error("Couldn't create new ppp unit: %m"); -- 2.39.2