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

Javier Martinez Canillas
2015-05-20T11:34:26Z

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@chromium.org>
>> >>> Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
>> >>> Tested-by: Heiko Stuebner <heiko@sntech.de>
>> >>> ---
>> >>>
>> >>> 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