fs/quota: handle overflows of sysctl fs.quota.* and report as unsigned long

Quota statistics counted as 64-bit per-cpu counter. Reading sums per-cpu
fractions as signed 64-bit int, filters negative values and then reports
lower half as signed 32-bit int.

Result may looks like:

fs.quota.allocated_dquots = 22327
fs.quota.cache_hits = -489852115
fs.quota.drops = -487288718
fs.quota.free_dquots = 22083
fs.quota.lookups = -486883485
fs.quota.reads = 22327
fs.quota.syncs = 335064
fs.quota.writes = 3088689

Values bigger than 2^31-1 reported as negative.

All counters except "allocated_dquots" and "free_dquots" are monotonic,
thus they should be reported as is without filtering negative values.

Kernel doesn't have generic helper for 64-bit sysctl yet,
let's use at least unsigned long.

Link: https://lore.kernel.org/r/157337934693.2078.9842146413181153727.stgit@buzz
Signed-off-by: Konstantin Khlebnikov <[e-mail]>
Signed-off-by: Jan Kara <[e-mail]>

fs/quota/dquot.c

···
 static int do_proc_dqstats(struct ctl_table *table, int write,
 		     void __user *buffer, size_t *lenp, loff_t *ppos)
 {
-	unsigned int type = (int *)table->data - dqstats.stat;
+	unsigned int type = (unsigned long *)table->data - dqstats.stat;
+	s64 value = percpu_counter_sum(&dqstats.counter[type]);
+
+	/* Filter negative values for non-monotonic counters */
+	if (value < 0 && (type == DQST_ALLOC_DQUOTS ||
+			  type == DQST_FREE_DQUOTS))
+		value = 0;
 
 	/* Update global table */
-	dqstats.stat[type] =
-			percpu_counter_sum_positive(&dqstats.counter[type]);
-	return proc_dointvec(table, write, buffer, lenp, ppos);
+	dqstats.stat[type] = value;
+	return proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
 }
 
 static struct ctl_table fs_dqstats_table[] = {
 	{
 		.procname	= "lookups",
 		.data		= &dqstats.stat[DQST_LOOKUPS],
-		.maxlen		= sizeof(int),
+		.maxlen		= sizeof(unsigned long),
 		.mode		= 0444,
 		.proc_handler	= do_proc_dqstats,
 	},
 	{
 		.procname	= "drops",
 		.data		= &dqstats.stat[DQST_DROPS],
-		.maxlen		= sizeof(int),
+		.maxlen		= sizeof(unsigned long),
 		.mode		= 0444,
 		.proc_handler	= do_proc_dqstats,
 	},
 	{
 		.procname	= "reads",
 		.data		= &dqstats.stat[DQST_READS],
-		.maxlen		= sizeof(int),
+		.maxlen		= sizeof(unsigned long),
 		.mode		= 0444,
 		.proc_handler	= do_proc_dqstats,
 	},
 	{
 		.procname	= "writes",
 		.data		= &dqstats.stat[DQST_WRITES],
-		.maxlen		= sizeof(int),
+		.maxlen		= sizeof(unsigned long),
 		.mode		= 0444,
 		.proc_handler	= do_proc_dqstats,
 	},
 	{
 		.procname	= "cache_hits",
 		.data		= &dqstats.stat[DQST_CACHE_HITS],
-		.maxlen		= sizeof(int),
+		.maxlen		= sizeof(unsigned long),
 		.mode		= 0444,
 		.proc_handler	= do_proc_dqstats,
 	},
 	{
 		.procname	= "allocated_dquots",
 		.data		= &dqstats.stat[DQST_ALLOC_DQUOTS],
-		.maxlen		= sizeof(int),
+		.maxlen		= sizeof(unsigned long),
 		.mode		= 0444,
 		.proc_handler	= do_proc_dqstats,
 	},
 	{
 		.procname	= "free_dquots",
 		.data		= &dqstats.stat[DQST_FREE_DQUOTS],
-		.maxlen		= sizeof(int),
+		.maxlen		= sizeof(unsigned long),
 		.mode		= 0444,
 		.proc_handler	= do_proc_dqstats,
 	},
 	{
 		.procname	= "syncs",
 		.data		= &dqstats.stat[DQST_SYNCS],
-		.maxlen		= sizeof(int),
+		.maxlen		= sizeof(unsigned long),
 		.mode		= 0444,
 		.proc_handler	= do_proc_dqstats,
 	},

include/linux/quota.h

···
 };
 
 struct dqstats {
-	int stat[_DQST_DQSTAT_LAST];
+	unsigned long stat[_DQST_DQSTAT_LAST];
 	struct percpu_counter counter[_DQST_DQSTAT_LAST];
 };