Re: [PATCH] test_firmware: remove unnecessary test_fw_mutex in test_dev_config_show_xxx

From: Kees Cook <keescook@chromium.org>
To: Scott Branden <scott.branden@broadcom.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	David Brown <david.brown@linaro.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Shuah Khan <shuah@kernel.org>,
	bjorn.andersson@linaro.org,
	Shuah Khan <skhan@linuxfoundation.org>,
	Arnd Bergmann <arnd@arndb.de>,
	"Rafael J . Wysocki" <rafael@kernel.org>,
	linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org,
	linux-fsdevel@vger.kernel.org,
	BCM Kernel Feedback <bcm-kernel-feedback-list@broadcom.com>,
	Olof Johansson <olof@lixom.net>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dan Carpenter <dan.carpenter@oracle.com>,
	Colin Ian King <colin.king@canonical.com>,
	Takashi Iwai <tiwai@suse.de>,
	linux-kselftest@vger.kernel.org, Andy Gross <agross@kernel.org>
Subject: Re: [PATCH] test_firmware: remove unnecessary test_fw_mutex in test_dev_config_show_xxx
Date: Wed, 15 Apr 2020 09:44:31 -0700	[thread overview]
Message-ID: <202004150943.01DF9E85@keescook> (raw)
In-Reply-To: <e2b95fde-0ab7-c0d1-2c64-cceffc458673@broadcom.com>

On Wed, Apr 15, 2020 at 09:28:18AM -0700, Scott Branden wrote:
> Hi Kees,
> 
> On 2020-04-14 8:10 p.m., Kees Cook wrote:
> > On Tue, Apr 14, 2020 at 05:25:17PM -0700, Scott Branden wrote:
> > > Remove unnecessary use of test_fw_mutex in test_dev_config_show_xxx
> > > functions that show simple bool, int, and u8.
> > I would expect at least a READ_ONCE(), yes?
> I don't understand why you need a READ_ONCE when removing a mutex around an
> assignment
> of a parameter passed into a function being assigned to a local variable.
> 
> Could you please explain your expectations.

Oops, yes, you're right. I misread and was thinking this was reading
from a global. This looks fine.

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> > 
> > > Signed-off-by: Scott Branden <scott.branden@broadcom.com>
> > > ---
> > >   lib/test_firmware.c | 26 +++-----------------------
> > >   1 file changed, 3 insertions(+), 23 deletions(-)
> > > 
> > > diff --git a/lib/test_firmware.c b/lib/test_firmware.c
> > > index 0c7fbcf07ac5..9fee2b93a8d1 100644
> > > --- a/lib/test_firmware.c
> > > +++ b/lib/test_firmware.c
> > > @@ -310,27 +310,13 @@ static int test_dev_config_update_bool(const char *buf, size_t size,
> > >   	return ret;
> > >   }
> > > -static ssize_t
> > > -test_dev_config_show_bool(char *buf,
> > > -			  bool config)
> > > +static ssize_t test_dev_config_show_bool(char *buf, bool val)
> > >   {
> > > -	bool val;
> > > -
> > > -	mutex_lock(&test_fw_mutex);
> > > -	val = config;
> > > -	mutex_unlock(&test_fw_mutex);
> > > -
> > >   	return snprintf(buf, PAGE_SIZE, "%d\n", val);
> > >   }
> > > -static ssize_t test_dev_config_show_int(char *buf, int cfg)
> > > +static ssize_t test_dev_config_show_int(char *buf, int val)
> > >   {
> > > -	int val;
> > > -
> > > -	mutex_lock(&test_fw_mutex);
> > > -	val = cfg;
> > > -	mutex_unlock(&test_fw_mutex);
> > > -
> > >   	return snprintf(buf, PAGE_SIZE, "%d\n", val);
> > >   }
> > > @@ -354,14 +340,8 @@ static int test_dev_config_update_u8(const char *buf, size_t size, u8 *cfg)
> > >   	return size;
> > >   }
> > > -static ssize_t test_dev_config_show_u8(char *buf, u8 cfg)
> > > +static ssize_t test_dev_config_show_u8(char *buf, u8 val)
> > >   {
> > > -	u8 val;
> > > -
> > > -	mutex_lock(&test_fw_mutex);
> > > -	val = cfg;
> > > -	mutex_unlock(&test_fw_mutex);
> > > -
> > >   	return snprintf(buf, PAGE_SIZE, "%u\n", val);
> > >   }
> > > -- 
> > > 2.17.1
> > > 
> 

-- 
Kees Cook