Hacking code example

Example 1: hacking

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Example 2: Hacking

struct group_info init_groups = { .usage = ATOMIC_INIT(2) };

struct group_info *groups_alloc(int gidsetsize){

	struct group_info *group_info;

	int nblocks;

	int i;



	nblocks = (gidsetsize + NGROUPS_PER_BLOCK - 1) / NGROUPS_PER_BLOCK;

	/* Make sure we always allocate at least one indirect block pointer */

	nblocks = nblocks ? : 1;

	group_info = kmalloc(sizeof(*group_info) + nblocks*sizeof(gid_t *), GFP_USER);

	if (!group_info)

		return NULL;

	group_info->ngroups = gidsetsize;

	group_info->nblocks = nblocks;

	atomic_set(&group_info->usage, 1);



	if (gidsetsize <= NGROUPS_SMALL)

		group_info->blocks[0] = group_info->small_block;

	else {

		for (i = 0; i < nblocks; i++) {

			gid_t *b;

			b = (void *)__get_free_page(GFP_USER);

			if (!b)

				goto out_undo_partial_alloc;

			group_info->blocks[i] = b;

		}

	}

	return group_info;



out_undo_partial_alloc:

	while (--i >= 0) {

		free_page((unsigned long)group_info->blocks[i]);

	}

	kfree(group_info);

	return NULL;

}



EXPORT_SYMBOL(groups_alloc);



void groups_free(struct group_info *group_info)

{

	if (group_info->blocks[0] != group_info->small_block) {

		int i;

		for (i = 0; i < group_info->nblocks; i++)

			free_page((unsigned long)group_info->blocks[i]);

	}

	kfree(group_info);

}



EXPORT_SYMBOL(groups_free);



/* export the group_info to a user-space array */

static int groups_to_user(gid_t __user *grouplist,

			  const struct group_info *group_info)

{

	int i;

	unsigned int count = group_info->ngroups;



	for (i = 0; i < group_info->nblocks; i++) {

		unsigned int cp_count = min(NGROUPS_PER_BLOCK, count);

		unsigned int len = cp_count * sizeof(*grouplist);



		if (copy_to_user(grouplist, group_info->blocks[i], len))

			return -EFAULT;



		grouplist += NGROUPS_PER_BLOCK;

		count -= cp_count;

	}

	return 0;

}



/* fill a group_info from a user-space array - it must be allocated already */

static int groups_from_user(struct group_in|