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

[Lee Jones <lee.jones@linaro.org>]

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.

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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