On Mon, Sep 20, 2021 at 07:53:16AM -1000, Tejun Heo wrote: > Hello, > > On Fri, Sep 17, 2021 at 10:04:27PM -0700, Luis Chamberlain wrote: > > If try_module_get() fails we fail the operation on the kernfs node. > > > > We use a try method as a full lock means we'd then make our sysfs > > attributes busy us out from possible module removal, and so userspace > > could force denying module removal, a silly form of "DOS" against module > > removal. A try lock on the module removal ensures we give priority to > > module removal and interacting with sysfs attributes only comes second. > > Using a full lock could mean for instance that if you don't stop poking > > at sysfs files you cannot remove a module. > > I find this explanation odd because there's no real equivalent to locking > the module (as opposed to try locking) Actually there is, __module_get() but I suspect some of these users are probably incorrect and should be be moved to try. The documentation about "rmmod --wait" for __module_get() is also outdated as that option is no longer supported. I'll send an update for that later. > because you can't wait for the > removal to finish and then grant the lock, so any operation which increases > the reference *has* to be a try method unless the caller already holds a > reference to the same module and thus knows that the module is already > pinned. Right, the reason I mention the alternative is that we technically don't need to use try in this case since during a kernfs op it is implied the module will be pinned, but we have further motivations to use a try method here: to avoid a possible DOS from module removal by userspace mucking with ops. > The code isn't wrong, so maybe just drop the related paragraphs in > the commit message? Does it make sense to clarify the above a bit more somehow? Or do think its not needed? > > static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root, > > struct kernfs_node *parent, > > const char *name, umode_t mode, > > + struct module *owner, > > kuid_t uid, kgid_t gid, > > unsigned flags) > > Is there a particular reason why @owner is added between @mode and @uid? > Sitting between two fs attributes seems a bit awkward. Maybe it can just be > the last one? No, I just picked an arbitrary place. Sure I'll move it to the end. Luis
