On Thu, 05 Oct 2023, Greg Kroah-Hartman wrote: > On Thu, Oct 05, 2023 at 10:03:11AM +0100, Lee Jones wrote: > > On Wed, 04 Oct 2023, Greg Kroah-Hartman wrote: > > > > > On Wed, Oct 04, 2023 at 01:57:04PM +0100, Lee Jones wrote: > > > > On Wed, 04 Oct 2023, Greg Kroah-Hartman wrote: > > > > > > > > > On Wed, Oct 04, 2023 at 09:57:20AM +0100, Lee Jones wrote: > > > > > > On Wed, 04 Oct 2023, Greg Kroah-Hartman wrote: > > > > > > > > > > > > > On Wed, Oct 04, 2023 at 05:59:09AM +0000, Starke, Daniel wrote: > > > > > > > > > Daniel, any thoughts? > > > > > > > > > > > > > > > > Our application of this protocol is only with specific modems to enable > > > > > > > > circuit switched operation (handling calls, selecting/querying networks, > > > > > > > > etc.) while doing packet switched communication (i.e. IP traffic over PPP). > > > > > > > > The protocol was developed for such use cases. > > > > > > > > > > > > > > > > Regarding the issue itself: > > > > > > > > There was already an attempt to fix all this by switching from spinlocks to > > > > > > > > mutexes resulting in ~20% performance loss. However, the patch was reverted > > > > > > > > as it did not handle the T1 timer leading into sleep during atomic within > > > > > > > > gsm_dlci_t1() on every mutex lock there. > > > > > > > > > > > > That's correct. When I initially saw this report, my initial thought > > > > > > was to replace the spinlocks with mutexts, but having read the previous > > > > > > accepted attempt and it's subsequent reversion I started to think of > > > > > > other ways to solve this issue. This solution, unlike the last, does > > > > > > not involve adding sleep inducing locks into atomic contexts, nor > > > > > > should it negatively affect performance. > > > > > > > > > > > > > > There was also a suggestion to fix this in do_con_write() as > > > > > > > > tty_operations::write() appears to be documented as "not allowed to sleep". > > > > > > > > The patch for this was rejected. It did not fix the issue within n_gsm. > > > > > > > > > > > > > > > > Link: https://lore.kernel.org/all/20221203215518.8150-1-pchelkin@xxxxxxxxx/ > > > > > > > > Link: https://lore.kernel.org/all/20221212023530.2498025-1-zengheng4@xxxxxxxxxx/ > > > > > > > > Link: https://lore.kernel.org/all/5a994a13-d1f2-87a8-09e4-a877e65ed166@xxxxxxxxxx/ > > > > > > > > > > > > > > Ok, I thought I remembered this, I'll just drop this patch from my > > > > > > > review queue and wait for a better solution if it ever comes up as this > > > > > > > isn't a real issue that people are seeing on actual systems, but just a > > > > > > > syzbot report. > > > > > > > > > > > > What does the "better solution" look like? > > > > > > > > > > One that actually fixes the root problem here (i.e. does not break the > > > > > recursion loop, or cause a performance decrease for normal users, or > > > > > prevent this from being bound to the console). > > > > > > > > Does this solution break the recursion loop or affect performance? > > > > > > This solution broke the recursion by returning an error, right? > > > > This is the part I was least sure about. > > > > If this was considered valid and we were to go forward with a solution > > like this, what would a quality improvement look like? Should we have > > stayed in this function and waited for the previous occupant to leave > > before continuing through ->write()? > > This isn't valid, as it obviously never shows up in real use. > > The real solution should be to prevent binding a console to this line > discipline as it can not handle the recursion that consoles require for > the write path. Would something like this tick that box? diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c index 1f3aba607cd51..5c1d2fcd5d9e2 100644 --- a/drivers/tty/n_gsm.c +++ b/drivers/tty/n_gsm.c @@ -3716,6 +3716,10 @@ static ssize_t gsmld_write(struct tty_struct *tty, struct file *file, if (!gsm) return -ENODEV; + /* The GSM line discipline does not support binding to console */ + if (strncmp(tty->name, "tty", 3)) + return -EINVAL; + ret = -ENOBUFS; spin_lock_irqsave(&gsm->tx_lock, flags); space = tty_write_room(tty); > Then, if consoles are really needed, the code can be fixed up to handle > such recursion. That's not a trivial thing to do, as can be seen by the > crazy gyrations that the n_tty line discipline does in its write path... -- Lee Jones [李琼斯]
