Re: [PATCH v2 04/10] mfd: cros_ec: Use a zero-length array for command data

[Javier Martinez Canillas]

Hello Lee,

On 05/13/2015 01:10 PM, Lee Jones wrote:
> On Sat, 09 May 2015, Javier Martinez Canillas wrote:
> 
>> Commit 1b84f2a4cd4a ("mfd: cros_ec: Use fixed size arrays to transfer
>> data with the EC") modified the struct cros_ec_command fields to not
>> use pointers for the input and output buffers and use fixed length
>> arrays instead.
>> 
>> This change was made because the cros_ec ioctl API uses that struct
>> cros_ec_command to allow user-space to send commands to the EC and
>> to get data from the EC. So using pointers made the API not 64-bit
>> safe. Unfortunately this approach was not flexible enough for all
>> the use-cases since there may be a need to send larger commands
>> on newer versions of the EC command protocol.
>> 
>> So to avoid to choose a constant length that it may be too big for
>> most commands and thus wasting memory and CPU cycles on copy from
>> and to user-space or having a size that is too small for some big
>> commands, use a zero-length array that is both 64-bit safe and
>> flexible. The same buffer is used for both output and input data
>> so the maximum of these values should be used to allocate it.
>> 
>> Suggested-by: Gwendal Grignou <gwendal@xxxxxxxxxxxx>
>> Signed-off-by: Javier Martinez Canillas <javier.martinez@xxxxxxxxxxxxxxx>
>> Tested-by: Heiko Stuebner <heiko@xxxxxxxxx>
>> ---
>> 
>> Changes since v1:
>>  - Add Heiko Stuebner Tested-by tag
>>  - Removed a new blank line at EOF warning. Reported by Heiko Stuebner
>>  - Use kmalloc instead of kzalloc when the message is later initialized
>>    Suggested by Gwendal Grignou
>>  - Pre-allocate struct cros_ec_command instead of dynamically allocate it
>>    whenever is possible. Suggested by Gwendal Grignou
>>  - Pre-allocate buffers for the usual cases and only allocate dynamically
>>    in the heap for bigger sizes. Suggested by Gwendal Grignou
>>  - Don't access the cros_ec_command received from user-space before doing
>>    a copy_from_user. Suggested by Gwendal Grignou
>>  - Only copy from user-space outsize bytes and copy_to_user insize bytes
>>    Suggested by Gwendal Grignou
>>  - ec_device_ioctl_xcmd() must return the numbers of bytes read and not 0
>>    on success. Suggested by Gwendal Grignou
>>  - Rename alloc_cmd_msg to alloc_lightbar_cmd_msg. Suggested by Gwendal Grignou
>> ---
>>  drivers/i2c/busses/i2c-cros-ec-tunnel.c    |  59 ++++++++---
>>  drivers/input/keyboard/cros_ec_keyb.c      |  19 ++--
>>  drivers/mfd/cros_ec.c                      |  18 ++--
>>  drivers/mfd/cros_ec_i2c.c                  |   4 +-
>>  drivers/mfd/cros_ec_spi.c                  |   2 +-
>>  drivers/platform/chrome/cros_ec_dev.c      |  66 +++++++++----
>>  drivers/platform/chrome/cros_ec_lightbar.c | 152 +++++++++++++++++++----------
>>  drivers/platform/chrome/cros_ec_lpc.c      |   8 +-
>>  drivers/platform/chrome/cros_ec_sysfs.c    |  92 +++++++++--------
>>  include/linux/mfd/cros_ec.h                |   6 +-
>>  10 files changed, 273 insertions(+), 153 deletions(-)
> 
> [...]
> 
>> diff --git a/drivers/mfd/cros_ec.c b/drivers/mfd/cros_ec.c
>> index 1574a9352a6d..ee8aa8142932 100644
>> --- a/drivers/mfd/cros_ec.c
>> +++ b/drivers/mfd/cros_ec.c
>> @@ -41,7 +41,7 @@ int cros_ec_prepare_tx(struct cros_ec_device *ec_dev,
>>  	out[2] = msg->outsize;
>>  	csum = out[0] + out[1] + out[2];
>>  	for (i = 0; i < msg->outsize; i++)
>> -		csum += out[EC_MSG_TX_HEADER_BYTES + i] = msg->outdata[i];
>> +		csum += out[EC_MSG_TX_HEADER_BYTES + i] = msg->data[i];
>>  	out[EC_MSG_TX_HEADER_BYTES + msg->outsize] = (uint8_t)(csum & 0xff);
>>  
>>  	return EC_MSG_TX_PROTO_BYTES + msg->outsize;
>> @@ -75,11 +75,13 @@ int cros_ec_cmd_xfer(struct cros_ec_device *ec_dev,
>>  	ret = ec_dev->cmd_xfer(ec_dev, msg);
>>  	if (msg->result == EC_RES_IN_PROGRESS) {
>>  		int i;
>> -		struct cros_ec_command status_msg = { };
>> +		struct cros_ec_command *status_msg;
>>  		struct ec_response_get_comms_status *status;
>> +		u8 buf[sizeof(*status_msg) + sizeof(*status)] = { };
> 
> This sort of thing is usually frowned upon.  Can you allocate and free
> buf's memory using the normal kernel helpers please?
>

The first version of this patch used kmalloc (actually kzalloc) and kfree
to allocate and free the buffers but Gwendal suggested that we could
allocate in the stack instead as an optimization [0].

I have no strong opinion on this so I'm happy to change it again when
re-spinning the patches.
 
> [...]
> 
>> diff --git a/include/linux/mfd/cros_ec.h b/include/linux/mfd/cros_ec.h
>> index 14cf522123dd..7eee38abd02a 100644
>> --- a/include/linux/mfd/cros_ec.h
>> +++ b/include/linux/mfd/cros_ec.h
>> @@ -42,8 +42,7 @@ enum {
>>   * @outsize: Outgoing length in bytes
>>   * @insize: Max number of bytes to accept from EC
>>   * @result: EC's response to the command (separate from communication failure)
>> - * @outdata: Outgoing data to EC
>> - * @indata: Where to put the incoming data from EC
>> + * @data: Where to put the incoming data from EC and outgoing data to EC
>>   */
>>  struct cros_ec_command {
>>  	uint32_t version;
>> @@ -51,8 +50,7 @@ struct cros_ec_command {
>>  	uint32_t outsize;
>>  	uint32_t insize;
>>  	uint32_t result;
>> -	uint8_t outdata[EC_PROTO2_MAX_PARAM_SIZE];
>> -	uint8_t indata[EC_PROTO2_MAX_PARAM_SIZE];
>> +	uint8_t data[0];
> 
> uint8_t *data?
>

No, as explained in the commit message the, whole idea of using a zero
length array instead of a pointer is to make the data structure 64-bit
safe since using pointers will require to have a compat ioctl support
which Alan Cox and Olof said that shouldn't be needed for newer IOCTLs.

struct cros_ec_command {
        uint32_t version;
        uint32_t command;
        uint32_t outsize;
        uint32_t insize;
        uint32_t result;
        uint8_t* data;
};

IOW, the above structure has different sizes when building with -m32 and
-m64 due pointers being self-aligned to different byte boundaries in 32
and 64 bit machines. But when using a zero length array, the structure
has the same size in both 32 and 64 bit machines.

>>  };
>>  
>>  /**
>

Best regards,
Javier

[0]: https://lkml.org/lkml/2015/4/24/8
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