/proc/sys/vm/drop_caches: add error handling

[Pavel Machek <pavel@suse.cz>]

Document that drop_caches is unsafe, and add error checking so that it
bails out on invalid inputs. [Note that this was triggered by Android
trying to use it in production, and incidentally writing invalid
value...]

Signed-off-by: Pavel Machek <pavel@suse.cz> 

diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index 3197fc8..b90f050 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -132,6 +132,10 @@ To free pagecache, dentries and inodes:
 As this is a non-destructive operation and dirty objects are not freeable, the
 user should run `sync' first.
 
+Note that calling this causes some serious latencies, and that this is
+a debug feature; it should not be used for production as it does not
+contain neccessary locking to guarantee safe operation.
+
 ==============================================================
 
 hugepages_treat_as_movable
diff --git a/fs/drop_caches.c b/fs/drop_caches.c
index 3e5637f..765dec3 100644
--- a/fs/drop_caches.c
+++ b/fs/drop_caches.c
@@ -9,8 +9,9 @@
 #include <linux/sysctl.h>
 #include <linux/gfp.h>
 
-/* A global variable is a bit ugly, but it keeps the code simple */
-int sysctl_drop_caches;
+/* A global variable is a bit ugly, and has locking problems,
+   but it keeps the code simple */
+unsigned int sysctl_drop_caches;
 
 static void drop_pagecache_sb(struct super_block *sb)
 {
@@ -65,12 +66,17 @@ static void drop_slab(void)
 int drop_caches_sysctl_handler(ctl_table *table, int write,
 	struct file *file, void __user *buffer, size_t *length, loff_t *ppos)
 {
-	proc_dointvec_minmax(table, write, file, buffer, length, ppos);
-	if (write) {
-		if (sysctl_drop_caches & 1)
-			drop_pagecache();
-		if (sysctl_drop_caches & 2)
-			drop_slab();
-	}
-	return 0;
+	int res;
+	res = proc_dointvec_minmax(table, write, file, buffer, length, ppos);
+	if (res)
+		return res;
+	if (!write)
+		return res;
+	if (sysctl_drop_caches & ~3)
+		return -EINVAL;
+	if (sysctl_drop_caches & 1)
+		drop_pagecache();
+	if (sysctl_drop_caches & 2)
+		drop_slab();
+	return res;
 }



-- 
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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html