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Subject: Re: [PATCH v30 08/12] landlock: Add syscall implementations
From:   =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= <mic@digikod.net>
To:     Kees Cook <keescook@chromium.org>
Cc:     James Morris <jmorris@namei.org>, Jann Horn <jannh@google.com>,
        "Serge E . Hallyn" <serge@hallyn.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
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        =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= <mic@linux.microsoft.com>
References: <20210316204252.427806-1-mic@digikod.net>
 <20210316204252.427806-9-mic@digikod.net> <202103191157.CF13C34@keescook>
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Date:   Wed, 24 Mar 2021 16:03:36 +0100
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On 19/03/2021 22:53, Mickaël Salaün wrote:
> 
> On 19/03/2021 20:06, Kees Cook wrote:
>> On Tue, Mar 16, 2021 at 09:42:48PM +0100, Mickaël Salaün wrote:
>>> From: Mickaël Salaün <mic@linux.microsoft.com>

[...]

>>> +/**
>>> + * sys_landlock_create_ruleset - Create a new ruleset
>>> + *
>>> + * @attr: Pointer to a &struct landlock_ruleset_attr identifying the scope of
>>> + *        the new ruleset.
>>> + * @size: Size of the pointed &struct landlock_ruleset_attr (needed for
>>> + *        backward and forward compatibility).
>>> + * @flags: Must be 0.
>>> + *
>>> + * This system call enables to create a new Landlock ruleset, and returns the
>>> + * related file descriptor on success.
>>> + *
>>> + * Possible returned errors are:
>>> + *
>>> + * - EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time;
>>> + * - EINVAL: @flags is not 0, or unknown access, or too small @size;
>>> + * - E2BIG or EFAULT: @attr or @size inconsistencies;
>>> + * - ENOMSG: empty &landlock_ruleset_attr.handled_access_fs.
>>> + */
>>> +SYSCALL_DEFINE3(landlock_create_ruleset,
>>> +		const struct landlock_ruleset_attr __user *const, attr,
>>> +		const size_t, size, const __u32, flags)
>>> +{
>>> +	struct landlock_ruleset_attr ruleset_attr;
>>> +	struct landlock_ruleset *ruleset;
>>> +	int err, ruleset_fd;
>>> +
>>> +	/* Build-time checks. */
>>> +	build_check_abi();
>>> +
>>> +	if (!landlock_initialized)
>>> +		return -EOPNOTSUPP;
>>> +
>>> +	/* No flag for now. */
>>> +	if (flags)
>>> +		return -EINVAL;
>>> +
>>> +	/* Copies raw user space buffer. */
>>> +	err = copy_min_struct_from_user(&ruleset_attr, sizeof(ruleset_attr),
>>> +			offsetofend(typeof(ruleset_attr), handled_access_fs),
>>
>> The use of offsetofend() here appears to be kind of the "V1", "V2", ...
>> sizes used in other extensible syscall implementations?
> 
> ruleset_attr is an extensible argument.

offsetofen() is used to set the minimum size of a valid argument. This
code will then not change with future extended ruleset_attr.