[PATCH 3/3] firmware: add mutex fw_lock_fallback for race condition

[Scott Branden]

A race condition exists between _request_firmware_prepare checking
if firmware is assigned and firmware_fallback_sysfs creating a sysfs
entry (kernel trace below).  To avoid such condition add a mutex
fw_lock_fallback to protect against such condition.

misc test_firmware: Falling back to sysfs fallback for: nope-test-firmware.bin
sysfs: cannot create duplicate filename 
'/devices/virtual/misc/test_firmware/nope-test-firmware.bin'
CPU: 4 PID: 2059 Comm: test_firmware-3 Not tainted 5.3.0-rc4 #1
Hardware name: Dell Inc. OptiPlex 7010/0KRC95, BIOS A13 03/25/2013
Call Trace:
 dump_stack+0x67/0x90
 sysfs_warn_dup.cold+0x17/0x24
 sysfs_create_dir_ns+0xb3/0xd0
 kobject_add_internal+0xa6/0x2a0
 kobject_add+0x7e/0xb0
 ? _cond_resched+0x15/0x30
 device_add+0x121/0x670
 firmware_fallback_sysfs+0x15c/0x3c9
 _request_firmware+0x432/0x5a0
 ? devres_find+0x63/0xc0
 request_firmware_into_buf+0x63/0x80
 test_fw_run_batch_request+0x96/0xe0
 kthread+0xfb/0x130
 ? reset_store+0x30/0x30
 ? kthread_park+0x80/0x80
 ret_from_fork+0x3a/0x50
kobject_add_internal failed for nope-test-firmware.bin with -EEXIST, don't try 
to register things with the same name in the same directory.

Signed-off-by: Scott Branden <scott.bran...@broadcom.com>
---
 drivers/base/firmware_loader/main.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/drivers/base/firmware_loader/main.c 
b/drivers/base/firmware_loader/main.c
index bf44c79beae9..ce9896e3b782 100644
--- a/drivers/base/firmware_loader/main.c
+++ b/drivers/base/firmware_loader/main.c
@@ -88,6 +88,7 @@ static inline struct fw_priv *to_fw_priv(struct kref *ref)
 /* fw_lock could be moved to 'struct fw_sysfs' but since it is just
  * guarding for corner cases a global lock should be OK */
 DEFINE_MUTEX(fw_lock);
+DEFINE_MUTEX(fw_lock_fallback);
 
 static struct firmware_cache fw_cache;
 
@@ -758,6 +759,17 @@ _request_firmware(const struct firmware **firmware_p, 
const char *name,
        if (!firmware_p)
                return -EINVAL;
 
+       /*
+        * There is a race condition between _request_firmware_prepare checking
+        * if firmware is assigned and firmware_fallback_sysfs creating sysfs
+        * entries with duplicate names.
+        * Yet, with this lock the firmware_test locks up with cache enabled
+        * and no event used during firmware test.
+        * This points to some very racy code I don't know how to entirely fix.
+        */
+       if (opt_flags & FW_OPT_NOCACHE)
+               mutex_lock(&fw_lock_fallback);
+
        if (!name || name[0] == '\0') {
                ret = -EINVAL;
                goto out;
@@ -791,6 +803,9 @@ _request_firmware(const struct firmware **firmware_p, const 
char *name,
                fw = NULL;
        }
 
+       if (opt_flags & FW_OPT_NOCACHE)
+               mutex_unlock(&fw_lock_fallback);
+
        *firmware_p = fw;
        return ret;
 }
-- 
2.17.1