[v4 PATCH 1/1] init: fix race between rootfs mount and firmware loading

From: Roman Pen
Date: Fri Sep 19 2014 - 08:44:41 EST

The thing is that built-in modules are being inited before
rootfs mount. Some of the modules can request firmware loading
from another thread using async 'request_firmware_nowait' call
on inition, so we can catch this kind of race:
rootfs does not exist yet, but we are going to open and load
firmware file requesting it from the kernel thread.

Solution is simple: before any rootfs access firmware loader
must wait for rootfs mount.

There are few questions which remain unanswered, but in this
patch I do not want to complicate things and want to fix exactly
the race. So here are the questions:

1. can 'request_firmware' (sync variant) be called on module inition
directly, without any threads? if can, that means firmware will
never be loaded for the driver at first try in case of built-in
module, because rootfs will be mounted only at the end of kernel
init sequence.

2. in case of separated '/lib' mount point we still have the problem:
firmware will be loaded only on further tries, but the first attempt
will always fail, because '/lib' mount point does not exist. Seems
to me this can't be simply fixed, and firmware_request logic needs
some refactoring.

Probably, there are other good questions which I do not see because
of shallow understanding of init and firmware loading sequences.

Signed-off-by: Roman Pen <r.peniaev@xxxxxxxxx>
Cc: Ming Lei <ming.lei@xxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
drivers/base/firmware_class.c | 6 ++++++
include/linux/init.h | 1 +
init/main.c | 33 +++++++++++++++++++++++++++++++--
3 files changed, 38 insertions(+), 2 deletions(-)

diff --git a/drivers/base/firmware_class.c b/drivers/base/firmware_class.c
index bf42430..450c4ae 100644
--- a/drivers/base/firmware_class.c
+++ b/drivers/base/firmware_class.c
@@ -29,6 +29,7 @@
#include <linux/syscore_ops.h>
#include <linux/reboot.h>
#include <linux/security.h>
+#include <linux/init.h>

#include <generated/utsrelease.h>

@@ -324,6 +325,11 @@ static int fw_get_filesystem_firmware(struct device *device,
int rc = -ENOENT;
char *path = __getname();

+ /* Before any file access we have to wait for rootfs.
+ In case of built-in module we can race with kernel init
+ thread, which has not mounted rootfs yet */
+ wait_for_rootfs();
for (i = 0; i < ARRAY_SIZE(fw_path); i++) {
struct file *file;

diff --git a/include/linux/init.h b/include/linux/init.h
index 2df8e8d..83233ae 100644
--- a/include/linux/init.h
+++ b/include/linux/init.h
@@ -152,6 +152,7 @@ void setup_arch(char **);
void prepare_namespace(void);
void __init load_default_modules(void);
int __init init_rootfs(void);
+void wait_for_rootfs(void);

extern void (*late_time_init)(void);

diff --git a/init/main.c b/init/main.c
index bb1aed9..2cfe187 100644
--- a/init/main.c
+++ b/init/main.c
@@ -973,6 +973,33 @@ static int __ref kernel_init(void *unused)
"See Linux Documentation/init.txt for guidance.");

+static DECLARE_WAIT_QUEUE_HEAD(rootfs_waitq);
+static bool rootfs_mounted;
+void wait_for_rootfs(void)
+ /* Here we try to protect from a few things:
+ * 1. Avoid waiting for ourselves, when init thread has not
+ * mounted rootfs yet.
+ * 2. Avoid warning if call chain was initiated from userspace
+ * /sbin/init. For example when /sbin/init loads the driver,
+ * which, in turn, wants to access rootfs (e.g. firmware loading),
+ * thus it has to be sure, that rootfs has been successfully
+ * mounted.
+ */
+ if (rootfs_mounted || WARN_ON(is_global_init(current)))
+ return;
+ else
+ wait_event(rootfs_waitq, rootfs_mounted);
+static inline void wake_up_rootfs_waiters(void)
+ rootfs_mounted = true;
+ wake_up_all(&rootfs_waitq);
static noinline void __init kernel_init_freeable(void)
@@ -1025,9 +1052,11 @@ static noinline void __init kernel_init_freeable(void)

* Ok, we have completed the initial bootup, and
- * we're essentially up and running. Get rid of the
- * initmem segments and start the user-mode stuff..
+ * we're essentially up and running. Wake up the
+ * rootfs mount waiters, get rid of the initmem
+ * segments, and start the user-mode stuff..
+ wake_up_rootfs_waiters();

/* rootfs is available now, try loading default modules */

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