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

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Date:	Wed, 09 Sep 2015 17:56:05 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Jon Hunter <jonathanh@...dia.com>
Cc:	Ulf Hansson <ulf.hansson@...aro.org>, grundler@...gle.com,
	olofj@...omium.org, Seshagiri Holi <sholi@...dia.com>,
	linux-mmc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mmc: block: Add new ioctl to send multi commands

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?

Or would it be possible to process the user data one at a time while
going through the commands?

> +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.

	Arnd
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