On Fri, 1 Feb 2019 21:39:59 +0100 Albert Vaca Cintora <[email protected]>
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
