linux-kernel - Re: [PATCH] tuners: tda8290: reduce stack usage with kasan

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Date:   Mon, 11 Dec 2017 16:17:52 -0500
From:   Michael Ira Krufky <mkrufky@...uxtv.org>
To:     Joe Perches <joe@...ches.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media <linux-media@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tuners: tda8290: reduce stack usage with kasan

On Mon, Dec 11, 2017 at 2:34 PM, Joe Perches <joe@...ches.com> wrote:
> On Mon, 2017-12-11 at 13:06 +0100, Arnd Bergmann wrote:
>> With CONFIG_KASAN enabled, we get a relatively large stack frame in one function
>>
>> drivers/media/tuners/tda8290.c: In function 'tda8290_set_params':
>> drivers/media/tuners/tda8290.c:310:1: warning: the frame size of 1520 bytes is larger than 1024 bytes [-Wframe-larger-than=]
>>
>> With CONFIG_KASAN_EXTRA this goes up to
>>
>> drivers/media/tuners/tda8290.c: In function 'tda8290_set_params':
>> drivers/media/tuners/tda8290.c:310:1: error: the frame size of 3200 bytes is larger than 3072 bytes [-Werror=frame-larger-than=]
>>
>> We can significantly reduce this by marking local arrays as 'static const', and
>> this should result in better compiled code for everyone.
> []
>> diff --git a/drivers/media/tuners/tda8290.c b/drivers/media/tuners/tda8290.c
> []
>> @@ -63,8 +63,8 @@ static int tda8290_i2c_bridge(struct dvb_frontend *fe, int close)
>>  {
>>       struct tda8290_priv *priv = fe->analog_demod_priv;
>>
>> -     unsigned char  enable[2] = { 0x21, 0xC0 };
>> -     unsigned char disable[2] = { 0x21, 0x00 };
>> +     static unsigned char  enable[2] = { 0x21, 0xC0 };
>> +     static unsigned char disable[2] = { 0x21, 0x00 };
>
> Doesn't match commit message.
>
> static const or just static?
>
>> @@ -84,9 +84,9 @@ static int tda8295_i2c_bridge(struct dvb_frontend *fe, int close)
>>  {
>>       struct tda8290_priv *priv = fe->analog_demod_priv;
>>
>> -     unsigned char  enable[2] = { 0x45, 0xc1 };
>> -     unsigned char disable[2] = { 0x46, 0x00 };
>> -     unsigned char buf[3] = { 0x45, 0x01, 0x00 };
>> +     static unsigned char  enable[2] = { 0x45, 0xc1 };
>> +     static unsigned char disable[2] = { 0x46, 0x00 };
>
> etc.
>
>


Joe is correct - they can be CONSTified. My bad -- a lot of the code I
wrote many years ago has this problem -- I wasn't so stack-conscious
back then.

The bytes in `enable` / `disable` don't get changed, but they may be
copied to another byte array that does get changed.  If would be best
to make these `static const`

Best regards,

Michael Ira Krufky