sysfs: correctly handle short reads on PREALLOC attrs.

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>

diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index 6c95628ea377dc0d4c07cbb7979f2a280dd3e6a8..f35523d4fa3a6d657260e7b04d5273db2dcd53b1 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 */