linux-kernel - Re: [PATCH] rtc: remove VLA usage

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Date:   Thu, 8 Mar 2018 00:12:58 +0100
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     "Gustavo A. R. Silva" <garsilva@...eddedor.com>
Cc:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        linux-rtc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rtc: remove VLA usage

On 07/03/2018 at 17:09:22 -0600, Gustavo A. R. Silva wrote:
> 
> 
> On 03/07/2018 05:01 PM, Alexandre Belloni wrote:
> > On 07/03/2018 at 16:39:51 -0600, Gustavo A. R. Silva wrote:
> > > Hi Alexandre,
> > > 
> > > On 03/07/2018 03:25 PM, Alexandre Belloni wrote:
> > > > On 07/03/2018 at 14:11:33 -0600, Gustavo A. R. Silva wrote:
> > > > > In preparation to enabling -Wvla, remove VLA and replace it
> > > > > with a fixed-length array instead.
> > > > > 
> > > > You should probably explain what VLA is and why this is important to do.
> > > Sure. I can elaborate a little bit more.
> > > 
> > > > > Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
> > > > > ---
> > > > >    drivers/rtc/rtc-bq32k.c  | 2 +-
> > > > >    drivers/rtc/rtc-mcp795.c | 2 +-
> > > > >    2 files changed, 2 insertions(+), 2 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/rtc/rtc-bq32k.c b/drivers/rtc/rtc-bq32k.c
> > > > > index e8698e9..e4b234a 100644
> > > > > --- a/drivers/rtc/rtc-bq32k.c
> > > > > +++ b/drivers/rtc/rtc-bq32k.c
> > > > > @@ -74,7 +74,7 @@ static int bq32k_read(struct device *dev, void *data, uint8_t off, uint8_t len)
> > > > >    static int bq32k_write(struct device *dev, void *data, uint8_t off, uint8_t len)
> > > > >    {
> > > > >    	struct i2c_client *client = to_i2c_client(dev);
> > > > > -	uint8_t buffer[len + 1];
> > > > > +	uint8_t buffer[256];
> > > > You chose to change len to 255, probably because this is a uint8_t but
> > > Correct.
> > > 
> > > > this is way too much for this rtc, it only has 10 consecutive registers.
> > > In that case probably the best solution is to add the following line to the
> > > module:
> > > 
> > > #define MAX_LEN 10
> > > 
> > > and update the rest of the code as follows:
> > > 
> > > uint8_t buffer[MAX_LEN + 1];
> > > 
> > Seems better, yes.
> > 
> > > > >    	buffer[0] = off;
> > > > >    	memcpy(&buffer[1], data, len);
> > > > > diff --git a/drivers/rtc/rtc-mcp795.c b/drivers/rtc/rtc-mcp795.c
> > > > > index 79e24ea..00e11c1 100644
> > > > > --- a/drivers/rtc/rtc-mcp795.c
> > > > > +++ b/drivers/rtc/rtc-mcp795.c
> > > > > @@ -82,7 +82,7 @@ static int mcp795_rtcc_write(struct device *dev, u8 addr, u8 *data, u8 count)
> > > > >    {
> > > > >    	struct spi_device *spi = to_spi_device(dev);
> > > > >    	int ret;
> > > > > -	u8 tx[2 + count];
> > > > > +	u8 tx[257];
> > > For this particular case it seems to me that the following works just fine:
> > > 
> > > #define MAX_COUNT 7
> > > 
> > > u8 tx[MAX_COUNT + 2];
> > > 
> > > What do you think?
> > > 
> > Nope, that RTC has a section of 255 bytes that could be read at once so
> > 257 is the correct value.
> > 
> 
> I see...
> 
> I was looking into this piece of code drivers/rtc/rtc-mcp795.c:302:
> 
>         tmp[0] = (tmp[0] & 0x80) | bin2bcd(alm->time.tm_sec);
>         tmp[1] = (tmp[1] & 0x80) | bin2bcd(alm->time.tm_min);
>         tmp[2] = (tmp[2] & 0xE0) | bin2bcd(alm->time.tm_hour);
>         tmp[3] = (tmp[3] & 0x80) | bin2bcd(alm->time.tm_wday + 1);
>         /* set alarm match: seconds, minutes, hour, day, date and month */
>         tmp[3] |= (MCP795_ALM0C2_BIT | MCP795_ALM0C1_BIT |
> MCP795_ALM0C0_BIT);
>         tmp[4] = (tmp[4] & 0xC0) | bin2bcd(alm->time.tm_mday);
>         tmp[5] = (tmp[5] & 0xE0) | bin2bcd(alm->time.tm_mon + 1);
> 
>         ret = mcp795_rtcc_write(dev, MCP795_REG_ALM0_SECONDS, tmp,
> sizeof(tmp));
> 

Yeah, the 128 or 255 bytes NVRAM is not yet supported but this is on my
todo list

-- 
Alexandre Belloni, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com