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copied!<p>You can't copy raw pointers, since a pointer into kernel space is meaningless to userspace (and will segfault if dereferenced).</p> <p>The typical way of doing something like this is to ask the userspace code to allocate the memory and pass in a pointer to that memory into a system call. If the program doesn't pass in a large enough buffer, then fail with an error (e.g. <code>EFAULT</code>). If there's no way for the program to know in advance a priori how much memory it will need, then typically you'd return the amount of data needed when passed a <code>NULL</code> pointer.</p> <p>Example usage from userspace:</p> <pre><code>// Fixed-size data typedef struct { uint32_t someProperty; uint32_t numOfFruits; } ObjectCapabilities; // First query the number of fruits we need ObjectCapabilities caps; int r = sys_get_fruit(&caps, NULL, 0); if (r != 0) { /* Handle error */ } // Now allocate memory and query the fruit uint32_t *arrayOfFruits = malloc(caps.numOfFruits * sizeof(uint32_t)); r = sys_get_fruit(&caps, arrayOfFruits, caps.numOfFruits); if (r != 0) { /* Handle error */ } </code></pre> <p>And here's how the corresponding code would look in kernel space on the other side of the system call:</p> <pre><code>int sys_get_fruit(ObjectCapabilities __user *userCaps, uint32_t __user *userFruit, uint32_t numFruits) { ObjectCapabilities caps; caps.someProperty = 1024; caps.numOfFruits = 3; // Copy out fixed-size data int r = copy_to_user(userCaps, &caps, sizeof(caps)); if (r != 0) return r; uint32_t localArray[] = { FRUIT_TYPE_APPLE, FRUIT_TYPE_ORANGE, FRUIT_TYPE_BANANA }; // Attempt to copy variable-sized data. Check the size first. if (numFruits * sizeof(uint32_t) < sizeof(localArray)) return -EFAULT; return copy_to_user(userFruit, localArray, sizeof(localArray)); } </code></pre>

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