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

[Javier Martinez Canillas]

Hello Lee,

On 05/20/2015 01:33 PM, Lee Jones wrote:
> On Wed, 20 May 2015, Javier Martinez Canillas wrote:
> 
>> Hello Lee,
>> 
>> On 05/13/2015 01:37 PM, Javier Martinez Canillas wrote:
>> > 
>> > 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.
>> >
>> 
>> [snip]
>> 
>> > 
>> > [0]: https://lkml.org/lkml/2015/4/24/8
>> > 
>> 
>> You didn't answer if you agree with Gwendal that we can allocate things on
>> the stack or if you still prefer to use kmalloc/kfree. As I said I don't
>> have a strong argument on either approach but just want to agree to avoid
>> doing the same change on each revision.
> 
> I don't want you to use variable names to allocate arrays like this.
> 

Perfect, thanks a lot for the clarification.

Best regards,
Javier
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