On 6/30/23 8:01 PM, Andrei Vagin wrote:
> On Wed, Jun 28, 2023 at 02:54:23PM +0500, Muhammad Usama Anjum wrote:
>> This IOCTL, PAGEMAP_SCAN on pagemap file can be used to get and/or clear
>> the info about page table entries. The following operations are supported
>> in this ioctl:
>> - Get the information if the pages have been written-to (PAGE_IS_WRITTEN),
>> file mapped (PAGE_IS_FILE), present (PAGE_IS_PRESENT), swapped
>> (PAGE_IS_SWAPPED) or page has pfn zero (PAGE_IS_PFNZERO).
>> - Find pages which have been written-to and/or write protect the pages
>> (atomic PM_SCAN_OP_GET + PM_SCAN_OP_WP)
>>
>> This IOCTL can be extended to get information about more PTE bits. The
>> entire address range passed by user [start, end) is scanned until either
>> the user provided buffer is full or max_pages have been found.
>>
>> Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
>
> <snip>
>
>> +
>> +static long do_pagemap_scan(struct mm_struct *mm, unsigned long __arg)
>> +{
>> + struct pm_scan_arg __user *uarg = (struct pm_scan_arg __user *)__arg;
>> + unsigned long long start, end, walk_start, walk_end;
>> + unsigned long empty_slots, vec_index = 0;
>> + struct mmu_notifier_range range;
>> + struct page_region __user *vec;
>> + struct pagemap_scan_private p;
>> + struct pm_scan_arg arg;
>> + int ret = 0;
>> +
>> + if (copy_from_user(&arg, uarg, sizeof(arg)))
>> + return -EFAULT;
>> +
>> + start = untagged_addr((unsigned long)arg.start);
>> + end = untagged_addr((unsigned long)arg.end);
>> + vec = (struct page_region __user *)untagged_addr((unsigned long)arg.vec);
>> +
>> + ret = pagemap_scan_args_valid(&arg, start, end, vec);
>> + if (ret)
>> + return ret;
>> +
>> + p.max_pages = (arg.max_pages) ? arg.max_pages : ULONG_MAX;
>> + p.found_pages = 0;
>> + p.required_mask = arg.required_mask;
>> + p.anyof_mask = arg.anyof_mask;
>> + p.excluded_mask = arg.excluded_mask;
>> + p.return_mask = arg.return_mask;
>> + p.flags = arg.flags;
>> + p.flags |= ((p.required_mask | p.anyof_mask | p.excluded_mask) &
>> + PAGE_IS_WRITTEN) ? PM_SCAN_REQUIRE_UFFD : 0;
>> + p.cur_buf.start = p.cur_buf.len = p.cur_buf.flags = 0;
>> + p.vec_buf = NULL;
>> + p.vec_buf_len = PAGEMAP_WALK_SIZE >> PAGE_SHIFT;
>> +
>> + /*
>> + * Allocate smaller buffer to get output from inside the page walk
>> + * functions and walk page range in PAGEMAP_WALK_SIZE size chunks. As
>> + * we want to return output to user in compact form where no two
>> + * consecutive regions should be continuous and have the same flags.
>> + * So store the latest element in p.cur_buf between different walks and
>> + * store the p.cur_buf at the end of the walk to the user buffer.
>> + */
>> + if (IS_PM_SCAN_GET(p.flags)) {
>> + p.vec_buf = kmalloc_array(p.vec_buf_len, sizeof(*p.vec_buf),
>> + GFP_KERNEL);
>> + if (!p.vec_buf)
>> + return -ENOMEM;
>> + }
>> +
>> + if (IS_PM_SCAN_WP(p.flags)) {
>> + mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_VMA, 0,
>> + mm, start, end);
>> + mmu_notifier_invalidate_range_start(&range);
>> + }
>> +
>> + walk_start = walk_end = start;
>> + while (walk_end < end && !ret) {
>> + if (IS_PM_SCAN_GET(p.flags)) {
>> + p.vec_buf_index = 0;
>> +
>> + /*
>> + * All data is copied to cur_buf first. When more data
>> + * is found, we push cur_buf to vec_buf and copy new
>> + * data to cur_buf. Subtract 1 from length as the
>> + * index of cur_buf isn't counted in length.
>> + */
>> + empty_slots = arg.vec_len - vec_index;
>> + p.vec_buf_len = min(p.vec_buf_len, empty_slots - 1);
>> + }
>> +
>> + ret = mmap_read_lock_killable(mm);
>> + if (ret)
>> + goto return_status;
>> +
>> + walk_end = min((walk_start + PAGEMAP_WALK_SIZE) & PAGEMAP_WALK_MASK, end);
>> +
>> + ret = walk_page_range(mm, walk_start, walk_end,
>> + &pagemap_scan_ops, &p);
>> + mmap_read_unlock(mm);
>> +
>> + if (ret && ret != PM_SCAN_FOUND_MAX_PAGES &&
>> + ret != PM_SCAN_END_WALK)
>> + goto return_status;
>> +
>> + walk_start = walk_end;
>> + if (IS_PM_SCAN_GET(p.flags) && p.vec_buf_index) {
>> + if (copy_to_user(&vec[vec_index], p.vec_buf,
>> + p.vec_buf_index * sizeof(*p.vec_buf))) {
>> + /*
>> + * Return error even though the OP succeeded
>> + */
>> + ret = -EFAULT;
>> + goto return_status;
>> + }
>> + vec_index += p.vec_buf_index;
>> + }
>> + }
>> +
>> + if (p.cur_buf.len) {
>> + if (copy_to_user(&vec[vec_index], &p.cur_buf, sizeof(p.cur_buf))) {
>> + ret = -EFAULT;
>> + goto return_status;
>> + }
>> + vec_index++;
>> + }
>> +
>> + ret = vec_index;
>> +
>> +return_status:
>> + arg.start = (unsigned long)walk_end;
>
> This doesn't look right. pagemap_scan_pmd_entry can stop early. For
> example, it can happen when it hits the max_pages limit. Do I miss
> something?
The walk_page_range() calls pagemap_scan_pmd_entry(). So whatever status is
returned from pagemap_scan_pmd_entry(), walk_page_range() returns to this
function where we are handling the status code. After while loop starts,
there is only 1 return path. Hence there isn't any path missing where we'll
miss setting arg.start.
>
>> + if (copy_to_user(&uarg->start, &arg.start, sizeof(arg.start)))
>> + ret = -EFAULT;
>> +
>> + if (IS_PM_SCAN_WP(p.flags))
>> + mmu_notifier_invalidate_range_end(&range);
>> +
>> + kfree(p.vec_buf);
>> + return ret;
>> +}
>> +
--
BR,
Muhammad Usama Anjum
