Re: [PATCH] mmc: block: Add new ioctl to send multi commands

Jon Hunter
2015-09-10T08:24:19Z

Hi Arnd,

On 09/09/15 21:22, Arnd Bergmann wrote:
> On Wednesday 09 September 2015 17:44:54 Jon Hunter wrote:
>>
>> On 09/09/15 16:56, Arnd Bergmann wrote:
>>> On Wednesday 09 September 2015 16:06:01 Jon Hunter wrote:
>>>> +
>>>> + idata = kcalloc(mcci.num_of_cmds, sizeof(*idata), GFP_KERNEL);
>>>> + if (!idata) {
>>>> + err = -ENOMEM;
>>>> + goto cmd_err;
>>>> + }
>>>> +
>>>> + cmds = (struct mmc_ioc_cmd __user *)(unsigned long)mcci.cmds_ptr;
>>>> + for (n_cmds = 0; n_cmds < mcci.num_of_cmds; n_cmds++) {
>>>> + idata[n_cmds] = mmc_blk_ioctl_copy_from_user(&cmds[n_cmds]);
>>>> + if (IS_ERR(idata[n_cmds])) {
>>>> + err = PTR_ERR(idata[n_cmds]);
>>>> + goto cmd_err;
>>>> + }
>>>> + }
>>>> +
>>>
>>> You have no upper bound on the number of commands, which means you end
>>> up catching overly large arguments only through -ENOMEM. Can you come
>>> up with an upper bound that is guaranteed to succeed with the allocation?
>>
>> The uint8 type would limit you to 256 commands (if you have the memory),
>> although admittedly that is probably overkill.
>
> Good point.
>
> Please note a few details here:
>
> - in uabi headers, we need to use __u8 instead of uint8, because we cannot
> rely on libc header file inclusion for kernel headers.

Ok.

> - you have some implicit padding after the structure and should replace that
> with explictit pad bytes to extend the structure to a multiple of its
> alignment (8 bytes).

Would padding with __u32 at the end be sufficient here? I assume the
__u32 would be 32-bit aligned. However, was not sure if this would
always be the case.

>>>> +struct mmc_ioc_multi_cmd {
>>>> + __u64 cmds_ptr;
>>>> + uint8_t num_of_cmds;
>>>> +};
>>>
>>> complex commands are always nasty in one way or another. Can you describe
>>> in the patch description why you picked an indirect pointer over something
>>> like
>>>
>>> struct mmc_ioc_multi_cmd {
>>> __u64 num_of_cmds;
>>> struct mmc_ioc_cmd cmds[0];
>>> };
>>>
>>> as I said, both are ugly. My first choice would have been the other one,
>>> but I'm sure you have some reasons yourself.
>>
>> It was a suggestion from Olof to ensure the structure size is constant for
>> both 32-bit and 64-bit userspaces. I am not sure if it is worth adding a
>> macro similar to the below for this?
>>
>> #define mmc_ioc_cmd_set_data(ic, ptr) ic.data_ptr = (__u64)(unsigned long) ptr
>>
>> However, yes can update the changelog.
>
> I was not referring to the use of an __u64 variable to pass a pointer, that
> is expected (and the macro would make it harder to understand).
>
> What I meant instead was the use of a pointer to an array as opposed to
> passing the array itself. With the definition I gave above, the size would
> still be the same on all architectures (you can replace the __u64 with
> an __u8 plus padding if you like), as sizeof(struct mmc_ioc_multi_cmd)
> is just '8' here.

Do you have any strong preference here? I guess I don't and agree
neither are ideal.

> Alternatively, you could just use an array of struct mmc_ioc_cmd by
> itself and encode the length in the ioctl command:
>
> #define MMC_COMBO_IOC_CMD(n) _IOC(_IOC_READ|_IOC_WRITE, 1, sizeof(struct mmc_ioc_cmd) * (n))
>
> This is of course also ugly because the ioctl command number is not
> fixed, and because the limit for the number of mmc command blocks
> is architecture dependent, depending on the definition of the _IOC
> macro that can have either 13 or 14 bits to encode the argument length
> in bytes.

Interesting idea. However, given your comments above, I think that I
would rather place the size in the structure.

Cheers
Jon