Messages in this thread | | | Date | Thu, 25 Apr 2019 13:06:57 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH v2] kernel/ucounts: expose count of inotify watches in use |
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On Fri, 1 Feb 2019 21:39:59 +0100 Albert Vaca Cintora <albertvaka@gmail.com> wrote:
> Adds a readonly 'current_inotify_watches' entry to the user sysctl table. > The handler for this entry is a custom function that ends up calling > proc_dointvec. Said sysctl table already contains 'max_inotify_watches' > and it gets mounted under /proc/sys/user/. > > Inotify watches are a finite resource, in a similar way to available file > descriptors. The motivation for this patch is to be able to set up > monitoring and alerting before an application starts failing because > it runs out of inotify watches.
Matthias said "Albert found this problem while working on montitoring software, so it fixes a real problem out there", so please include full details of the problem which you encountered so that we are better able to understand the value of the patch.
> > ... > > kernel/ucount.c | 29 +++++++++++++++++++++++++++++
Documentation, please. Documentation/filesystems/inotify.txt and/or Documentation/filesystems/proc.txt.
Also, max_inotify_instances (at least) also appears to be undocumented, so it would be good to address this as well while you're in there.
> > diff --git a/kernel/ucount.c b/kernel/ucount.c > index f48d1b6376a4..d8b11e53f098 100644 > --- a/kernel/ucount.c > +++ b/kernel/ucount.c > @@ -57,6 +57,11 @@ static struct ctl_table_root set_root = { > .permissions = set_permissions, > }; > > +#ifdef CONFIG_INOTIFY_USER > +int proc_read_inotify_watches(struct ctl_table *table, int write, > + void __user *buffer, size_t *lenp, loff_t *ppos); > +#endif
The ifdefs aren't really needed. And this should be in a header file if it is indeed to be non-static.
But it should be static, in which case the ifdef will be needed to prevent a warning. It's kinda irksome and perhaps it would be better to move proc_read_inotify_watches() to be ahead of user_table[].
> static int zero = 0; > static int int_max = INT_MAX; > #define UCOUNT_ENTRY(name) \ > @@ -79,6 +84,12 @@ static struct ctl_table user_table[] = { > #ifdef CONFIG_INOTIFY_USER > UCOUNT_ENTRY("max_inotify_instances"), > UCOUNT_ENTRY("max_inotify_watches"), > + { > + .procname = "current_inotify_watches", > + .maxlen = sizeof(int), > + .mode = 0444, > + .proc_handler = proc_read_inotify_watches, > + }, > #endif > { } > }; > @@ -226,6 +237,24 @@ void dec_ucount(struct ucounts *ucounts, enum ucount_type type) > put_ucounts(ucounts); > } > > +#ifdef CONFIG_INOTIFY_USER > +int proc_read_inotify_watches(struct ctl_table *table, int write, > + void __user *buffer, size_t *lenp, loff_t *ppos) > +{ > + struct ucounts *ucounts; > + struct ctl_table fake_table; > + int count; > + > + ucounts = get_ucounts(current_user_ns(), current_euid());
get_ucounts() can return NULL. The kernel will crash.
> + count = atomic_read(&ucounts->ucount[UCOUNT_INOTIFY_WATCHES]); > + put_ucounts(ucounts); > + > + fake_table.data = &count; > + fake_table.maxlen = sizeof(count); > + return proc_dointvec(&fake_table, write, buffer, lenp, ppos); > +} > +#endif
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