[PATCH 2/2] kobject: add kernel/uevent_features sysfs file

[Peter Rajnoha]

We can use extended format when writing /sys/.../uevent files to
generate synthetic uevents, introduced with commit f36776fafbaa
("kobject: support passing in variables for synthetic uevents").

Before using this extended format, we need to know if it's supported
and kernel version check may not be appropriate in all cases - there
are possible differences from upstream kernel in distributions with
backports.

This patch adds /sys/kernel/uevent_features file which currently lists
'synthargs' string to denote that the kernel is able to recognize the
extended synthetic uevent arguments. Userspace can easily check for
the feature then.

Signed-off-by: Peter Rajnoha <prajnoha@xxxxxxxxxx>
---
 kernel/ksysfs.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/ksysfs.c b/kernel/ksysfs.c
index 46ba853656f6..d893d7442f61 100644
--- a/kernel/ksysfs.c
+++ b/kernel/ksysfs.c
@@ -37,6 +37,13 @@ static ssize_t uevent_seqnum_show(struct kobject *kobj,
 }
 KERNEL_ATTR_RO(uevent_seqnum);
 
+static ssize_t uevent_features_show(struct kobject *kobj,
+				    struct kobj_attribute *attr, char *buf)
+{
+	return sprintf(buf, "synthargs\n");
+}
+KERNEL_ATTR_RO(uevent_features);
+
 #ifdef CONFIG_UEVENT_HELPER
 /* uevent helper program, used during early boot */
 static ssize_t uevent_helper_show(struct kobject *kobj,
@@ -213,6 +220,7 @@ EXPORT_SYMBOL_GPL(kernel_kobj);
 static struct attribute * kernel_attrs[] = {
 	&fscaps_attr.attr,
 	&uevent_seqnum_attr.attr,
+	&uevent_features_attr.attr,
 #ifdef CONFIG_UEVENT_HELPER
 	&uevent_helper_attr.attr,
 #endif
-- 
2.19.2