[PATCH] rtc: mcp795: remove VLA usage

[PATCH] rtc: mcp795: remove VLA usage

[Stefano Manni]

In preparation to enabling -Wvla, remove VLAs and replace them with
fixed-length arrays instead.

rtc-mcp795.c uses a variable-length array declaration to contain
the command to write the rtcc; this can be replaced by a fixed-
size array of length 2 (instruction, address) + 32 (data out),
assuming a maximum data length of 32 bytes before wrap up.

This was prompted by https://lkml.org/lkml/2018/3/7/621

Signed-off-by: Stefano Manni <stefano.manni@gmail.com>
---
 drivers/rtc/rtc-mcp795.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/rtc/rtc-mcp795.c b/drivers/rtc/rtc-mcp795.c
index 77f21331ae21..a5f504e2364c 100644
--- a/drivers/rtc/rtc-mcp795.c
+++ b/drivers/rtc/rtc-mcp795.c
@@ -61,6 +61,9 @@
 
 #define SEC_PER_DAY		(24 * 60 * 60)
 
+/* Maximum length for data out in write operation to RTCC */
+#define MCP795_MAX_DATAOUT_LEN	32
+
 static int mcp795_rtcc_read(struct device *dev, u8 addr, u8 *buf, u8 count)
 {
 	struct spi_device *spi = to_spi_device(dev);
@@ -82,7 +85,10 @@ 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[2 + MCP795_MAX_DATAOUT_LEN];
+
+	if (count > MCP795_MAX_DATAOUT_LEN)
+		return -EINVAL;
 
 	tx[0] = MCP795_WRITE;
 	tx[1] = addr;
-- 
2.14.3

Re: [PATCH] rtc: mcp795: remove VLA usage

[Alexandre Belloni]

Hi,

On 13/03/2018 at 00:13:38 +0100, Stefano Manni wrote:
> In preparation to enabling -Wvla, remove VLAs and replace them with
> fixed-length arrays instead.
> 
> rtc-mcp795.c uses a variable-length array declaration to contain
> the command to write the rtcc; this can be replaced by a fixed-
> size array of length 2 (instruction, address) + 32 (data out),
> assuming a maximum data length of 32 bytes before wrap up.
> 
> This was prompted by https://lkml.org/lkml/2018/3/7/621
> 
> Signed-off-by: Stefano Manni <stefano.manni@gmail.com>
> ---
>  drivers/rtc/rtc-mcp795.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/rtc/rtc-mcp795.c b/drivers/rtc/rtc-mcp795.c
> index 77f21331ae21..a5f504e2364c 100644
> --- a/drivers/rtc/rtc-mcp795.c
> +++ b/drivers/rtc/rtc-mcp795.c
> @@ -61,6 +61,9 @@
>  
>  #define SEC_PER_DAY		(24 * 60 * 60)
>  
> +/* Maximum length for data out in write operation to RTCC */
> +#define MCP795_MAX_DATAOUT_LEN	32
> +

This is wrong, see https://marc.info/?l=linux-kernel&m=152046370320811&w=2

Also, https://git.kernel.org/pub/scm/linux/kernel/git/abelloni/linux.git/commit/?h=rtc-next&id=74ce1a932504da166cfbccf5567aa3751b6aa599

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

Re: [PATCH] rtc: mcp795: remove VLA usage

[Stefano Manni]

Hi,

2018-03-13 0:31 GMT+01:00 Alexandre Belloni
<alexandre.belloni@free-electrons.com>:
> Hi,
>
> On 13/03/2018 at 00:13:38 +0100, Stefano Manni wrote:
>> In preparation to enabling -Wvla, remove VLAs and replace them with
>> fixed-length arrays instead.
>>
>> rtc-mcp795.c uses a variable-length array declaration to contain
>> the command to write the rtcc; this can be replaced by a fixed-
>> size array of length 2 (instruction, address) + 32 (data out),
>> assuming a maximum data length of 32 bytes before wrap up.
>>
>> This was prompted by https://lkml.org/lkml/2018/3/7/621
>>
>> Signed-off-by: Stefano Manni <stefano.manni@gmail.com>
>> ---
>>  drivers/rtc/rtc-mcp795.c | 8 +++++++-
>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/rtc/rtc-mcp795.c b/drivers/rtc/rtc-mcp795.c
>> index 77f21331ae21..a5f504e2364c 100644
>> --- a/drivers/rtc/rtc-mcp795.c
>> +++ b/drivers/rtc/rtc-mcp795.c
>> @@ -61,6 +61,9 @@
>>
>>  #define SEC_PER_DAY          (24 * 60 * 60)
>>
>> +/* Maximum length for data out in write operation to RTCC */
>> +#define MCP795_MAX_DATAOUT_LEN       32
>> +
>
> This is wrong, see https://marc.info/?l=linux-kernel&m=152046370320811&w=2
>
> Also, https://git.kernel.org/pub/scm/linux/kernel/git/abelloni/linux.git/commit/?h=rtc-next&id=74ce1a932504da166cfbccf5567aa3751b6aa599
>

You sure that the right value to use is 255 + 2? mcp795_rtcc_write() just writes
into the RTCC that contains only 32 registers (table 4-1 of
datasheet). I assumed
32 as the maximum length of data to write before wrapping up (start
from reg 0x0).
Probably 32 is just a wrong assumption but why did you choose 255?

Another thing: don't we need also to check count against the array length?

if (count > MCP795_MAX_DATAOUT_LEN)
    return -EINVAL;

Re: [PATCH] rtc: mcp795: remove VLA usage

[Alexandre Belloni]

On 13/03/2018 at 09:29:33 +0100, Stefano Manni wrote:
> Hi,
> 
> 2018-03-13 0:31 GMT+01:00 Alexandre Belloni
> <alexandre.belloni@free-electrons.com>:
> > Hi,
> >
> > On 13/03/2018 at 00:13:38 +0100, Stefano Manni wrote:
> >> In preparation to enabling -Wvla, remove VLAs and replace them with
> >> fixed-length arrays instead.
> >>
> >> rtc-mcp795.c uses a variable-length array declaration to contain
> >> the command to write the rtcc; this can be replaced by a fixed-
> >> size array of length 2 (instruction, address) + 32 (data out),
> >> assuming a maximum data length of 32 bytes before wrap up.
> >>
> >> This was prompted by https://lkml.org/lkml/2018/3/7/621
> >>
> >> Signed-off-by: Stefano Manni <stefano.manni@gmail.com>
> >> ---
> >>  drivers/rtc/rtc-mcp795.c | 8 +++++++-
> >>  1 file changed, 7 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/rtc/rtc-mcp795.c b/drivers/rtc/rtc-mcp795.c
> >> index 77f21331ae21..a5f504e2364c 100644
> >> --- a/drivers/rtc/rtc-mcp795.c
> >> +++ b/drivers/rtc/rtc-mcp795.c
> >> @@ -61,6 +61,9 @@
> >>
> >>  #define SEC_PER_DAY          (24 * 60 * 60)
> >>
> >> +/* Maximum length for data out in write operation to RTCC */
> >> +#define MCP795_MAX_DATAOUT_LEN       32
> >> +
> >
> > This is wrong, see https://marc.info/?l=linux-kernel&m=152046370320811&w=2
> >
> > Also, https://git.kernel.org/pub/scm/linux/kernel/git/abelloni/linux.git/commit/?h=rtc-next&id=74ce1a932504da166cfbccf5567aa3751b6aa599
> >
> 
> You sure that the right value to use is 255 + 2? mcp795_rtcc_write() just writes
> into the RTCC that contains only 32 registers (table 4-1 of
> datasheet). I assumed
> 32 as the maximum length of data to write before wrapping up (start
> from reg 0x0).
> Probably 32 is just a wrong assumption but why did you choose 255?
> 

It is not so wrong but the plan is to be able to support reading/writing
the EEPROM later but I agree it will need to be changed anyway.

> Another thing: don't we need also to check count against the array length?
> 
> if (count > MCP795_MAX_DATAOUT_LEN)
>     return -EINVAL;

u8 count will never be greater than 255.


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