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    <p>The user needs to provide enough space for all the data being copied out. Ideally he'll tell you how much space he provided, and you check that everything fits.</p> <p>The copied-out data should (in general) not include any pointers, since they're "local" to a different "process" (the kernel can be viewed as a separate process, as it were, and kernel / user interactions involve process-to-process IPC, similar to sending stuff over local or even Internet-connected sockets).</p> <p>Since the kernel has pretty intimate knowledge of a process, you can skirt these rules somewhat, e.g., you could compute what the user's pointer will be, and copy out a copy of the original data, with the pointer modified appropriately. But that's kind of wasteful. Or, you can copy a kernel pointer and just not use it in the user code, but now you're "leaking data" that "bad guys" can sometimes leverage in various ways. In security-people-speak you've left a wide-open "covert channel".</p> <p>In the end, then, the "right" way to do this tends to be something like this:</p> <pre><code>struct user_interface_version_of_struct { int property; int count; int data[]; /* of size "count" */ }; </code></pre> <p>The user code <code>malloc</code>s (or otherwise arranges to have sufficient space) the "user interface version" and makes some system call to the kernel (<code>read</code>, <code>receive</code>, <code>rcvmsg</code>, <code>ioctl</code>, whatever, as long as it involves doing a "read"-type operation) and tells the kernel: "here's the memory holding the struct, and here's how big it is" (in bytes, or the maximum <code>count</code> value, or whatever: user and kernel simply need to agree on the protocol). The kernel-side code then verifies the user's values in some appropriate manner, and either does the copy-out however is most convenient, or returns an error.</p> <p>"Most convenient" is sometimes two separate copy ops, or some <code>put_user</code> calls, e.g., if the kernel side has the data structure you showed, you might do:</p> <pre><code>/* let's say ulen is the user supplied length in bytes, and uaddr is the user-supplied address */ struct user_interface_version_of_struct *p; needed = sizeof(*p) + 3 * sizeof(int); if (needed &gt; ulen) return -ENOMEM; /* user did not supply enough space */ p = uaddr; error = put_user(1024, &amp;p-&gt;property); if (error == 0) error = put_user(3, &amp;p-&gt;count); if (error == 0 &amp;&amp; copy_to_user(&amp;p-&gt;data, localArray, 3 * sizeof(int)) error = -EFAULT; </code></pre> <p>You may have a situation where you must conform to some not-very-nice interface, though.</p> <hr> <p>Edit: if you're adding your own system call (rather than tying in to <code>read</code> or <code>ioctl</code> for instance), you can separate the header and data, as in <a href="https://stackoverflow.com/a/18520855/1256452">Adam Rosenfield's answer</a>.</p>
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