sysfs: correctly handle short reads on PREALLOC attrs.

[ Upstream commit 65da3484d9be5664f5f7d2378e438bb2794f40b8 ]

attributes declared with __ATTR_PREALLOC use sysfs_kf_read()
which ignores the 'count' arg.
So a 1-byte read request can return more bytes than that.

This is seen with the 'dash' shell when 'read' is used on
some 'md' sysfs attributes.

So only return the 'min' of count and the attribute length.

Signed-off-by: NeilBrown <neilb@suse.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>

diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index 7c2867b44141e5df54734e7e27e3df344a976dea..9905595ef41d780517a232149386edb89334710d 100644 (file)
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c
@@ -108,6 +108,7 @@ static ssize_t sysfs_kf_read(struct kernfs_open_file *of, char *buf,
 {
        const struct sysfs_ops *ops = sysfs_file_ops(of->kn);
        struct kobject *kobj = of->kn->parent->priv;
+       size_t len;
 
        /*
         * If buf != of->prealloc_buf, we don't know how
@@ -115,7 +116,8 @@ static ssize_t sysfs_kf_read(struct kernfs_open_file *of, char *buf,
         */
        if (pos || WARN_ON_ONCE(buf != of->prealloc_buf))
                return 0;
-       return ops->show(kobj, of->kn->priv, buf);
+       len = ops->show(kobj, of->kn->priv, buf);
+       return min(count, len);
 }
 
 /* kernfs write callback for regular sysfs files */