linux-kernel - [PATCH v2] kernel/ucounts: expose count of inotify watches in use

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Message-Id: <20190201203959.10050-1-albertvaka@gmail.com>
Date:   Fri,  1 Feb 2019 21:39:59 +0100
From:   Albert Vaca Cintora <albertvaka@...il.com>
To:     albertvaka@...il.com, akpm@...ux-foundation.org,
        rdunlap@...radead.org, mingo@...nel.org, jack@...e.cz,
        ebiederm@...ssion.com, nsaenzjulienne@...e.de,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2] kernel/ucounts: expose count of inotify watches in use

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.

Signed-off-by: Albert Vaca Cintora <albertvaka@...il.com>
Acked-by: Jan Kara <jack@...e.cz>
Reviewed-by: Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
---
 kernel/ucount.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

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
+
 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());
+	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
+
 static __init int user_namespace_sysctl_init(void)
 {
 #ifdef CONFIG_SYSCTL
-- 
2.20.1