[bug report] mm/pgtable: allow pte_offset_map[_lock]() to fail

[Dan Carpenter <dan.carpenter@linaro.org>]

Hello Hugh Dickins,

The patch 0d940a9b270b: "mm/pgtable: allow pte_offset_map[_lock]() to
fail" from Jun 8, 2023, leads to the following Smatch static checker
warning:

	mm/userfaultfd.c:321 mfill_atomic_pte_poison()
	error: uninitialized symbol 'ptl'.

mm/userfaultfd.c
    292 static int mfill_atomic_pte_poison(pmd_t *dst_pmd,
    293                                    struct vm_area_struct *dst_vma,
    294                                    unsigned long dst_addr,
    295                                    uffd_flags_t flags)
    296 {
    297         int ret;
    298         struct mm_struct *dst_mm = dst_vma->vm_mm;
    299         pte_t _dst_pte, *dst_pte;
    300         spinlock_t *ptl;
    301 
    302         _dst_pte = make_pte_marker(PTE_MARKER_POISONED);
    303         dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
                                                                         ^^^^
The __pte_offset_map_lock() function does not initialize ptl if it
returns NULL.

    304 
    305         if (mfill_file_over_size(dst_vma, dst_addr)) {
    306                 ret = -EFAULT;
    307                 goto out_unlock;
    308         }
    309 
    310         ret = -EEXIST;
    311         /* Refuse to overwrite any PTE, even a PTE marker (e.g. UFFD WP). */
    312         if (!pte_none(*dst_pte))
    313                 goto out_unlock;
    314 
    315         set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
    316 
    317         /* No need to invalidate - it was non-present before */
    318         update_mmu_cache(dst_vma, dst_addr, dst_pte);
    319         ret = 0;
    320 out_unlock:
--> 321         pte_unmap_unlock(dst_pte, ptl);
    322         return ret;
    323 }

regards,
dan carpenter