From: Arnd Bergmann <arnd@arndb.de>

Turning on -Wstringop-overflow globally exposed a misleading compiler
warning in bluetooth:

net/bluetooth/hci_event.c: In function 'hci_cc_read_class_of_dev':
net/bluetooth/hci_event.c:524:9: error: 'memcpy' writing 3 bytes into a region of size 0 overflows the destination [-Werror=stringop-overflow=]
  524 |         memcpy(hdev->dev_class, rp->dev_class, 3);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The problem here is the check for hdev being NULL in bt_dev_dbg() that
leads the compiler to conclude that hdev->dev_class might be an invalid
pointer access.

Add another explicit check for the same condition to make sure gcc sees
this cannot happen.

Fixes: a9de9248064b ("[Bluetooth] Switch from OGF+OCF to using only opcodes")
Fixes: 1b56c90018f0 ("Makefile: Enable -Wstringop-overflow globally")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 net/bluetooth/hci_event.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/bluetooth/hci_event.c b/net/bluetooth/hci_event.c
index 5b6fd625fc09..5651e96e78da 100644
--- a/net/bluetooth/hci_event.c
+++ b/net/bluetooth/hci_event.c
@@ -516,6 +516,9 @@ static u8 hci_cc_read_class_of_dev(struct hci_dev *hdev, void *data,
 {
 	struct hci_rp_read_class_of_dev *rp = data;
 
+	if (WARN_ON(!hdev))
+		return -ENXIO;
+
 	bt_dev_dbg(hdev, "status 0x%2.2x", rp->status);
 
 	if (rp->status)
-- 
2.39.2

Hi Arnd,

kernel test robot noticed the following build warnings:

url:    https://github.com/intel-lab-lkp/linux/commits/Arnd-Bergmann/Bluetooth-hci_event-shut-up-a-false-positive-warning/20231123-112143
base:   https://git.kernel.org/pub/scm/linux/kernel/git/bluetooth/bluetooth-next.git master
patch link:    https://lore.kernel.org/r/20231122221805.3139482-1-arnd%40kernel.org
patch subject: [PATCH] Bluetooth: hci_event: shut up a false-positive warning
config: i386-randconfig-141-20231123 (https://download.01.org/0day-ci/archive/20231124/202311241707.qytKwxbE-lkp@intel.com/config)
compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
reproduce: (https://download.01.org/0day-ci/archive/20231124/202311241707.qytKwxbE-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Reported-by: Dan Carpenter <error27@gmail.com>
| Closes: https://lore.kernel.org/r/202311241707.qytKwxbE-lkp@intel.com/

New smatch warnings:
net/bluetooth/hci_event.c:520 hci_cc_read_class_of_dev() warn: signedness bug returning '(-6)'

Old smatch warnings:
net/bluetooth/hci_event.c:3278 hci_conn_request_evt() warn: variable dereferenced before check 'hdev' (see line 3268)

vim +520 net/bluetooth/hci_event.c

c8992cffbe7411 Luiz Augusto von Dentz 2021-12-01  514  static u8 hci_cc_read_class_of_dev(struct hci_dev *hdev, void *data,
c8992cffbe7411 Luiz Augusto von Dentz 2021-12-01  515  				   struct sk_buff *skb)
a9de9248064bfc Marcel Holtmann        2007-10-20  516  {
c8992cffbe7411 Luiz Augusto von Dentz 2021-12-01  517  	struct hci_rp_read_class_of_dev *rp = data;
e3f3a1aea8719a Luiz Augusto von Dentz 2021-12-01  518  
5f3aa66f201253 Arnd Bergmann          2023-11-22  519  	if (WARN_ON(!hdev))
5f3aa66f201253 Arnd Bergmann          2023-11-22 @520  		return -ENXIO;

This function returns u8.

5f3aa66f201253 Arnd Bergmann          2023-11-22  521  
e3f3a1aea8719a Luiz Augusto von Dentz 2021-12-01  522  	bt_dev_dbg(hdev, "status 0x%2.2x", rp->status);
a9de9248064bfc Marcel Holtmann        2007-10-20  523  
a9de9248064bfc Marcel Holtmann        2007-10-20  524  	if (rp->status)
c8992cffbe7411 Luiz Augusto von Dentz 2021-12-01  525  		return rp->status;
a9de9248064bfc Marcel Holtmann        2007-10-20  526  
a9de9248064bfc Marcel Holtmann        2007-10-20  527  	memcpy(hdev->dev_class, rp->dev_class, 3);
a9de9248064bfc Marcel Holtmann        2007-10-20  528  
e3f3a1aea8719a Luiz Augusto von Dentz 2021-12-01  529  	bt_dev_dbg(hdev, "class 0x%.2x%.2x%.2x", hdev->dev_class[2],
e3f3a1aea8719a Luiz Augusto von Dentz 2021-12-01  530  		   hdev->dev_class[1], hdev->dev_class[0]);
c8992cffbe7411 Luiz Augusto von Dentz 2021-12-01  531  
c8992cffbe7411 Luiz Augusto von Dentz 2021-12-01  532  	return rp->status;
a9de9248064bfc Marcel Holtmann        2007-10-20  533  }

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki