mm/mmap.c:2039 expand_downwards() warn: unsigned 'address' is never less than zero.

[kernel test robot <lkp@intel.com>]

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
head:   e55e5df193d247a38a5e1ac65a5316a0adcc22fa
commit: 8b35ca3e45e35a26a21427f35d4093606e93ad0a arm/mm: Convert to using lock_mm_and_find_vma()
date:   6 days ago
config: i386-randconfig-m021-20230629 (https://download.01.org/0day-ci/archive/20230630/202306302333.ay5rrRON-lkp@intel.com/config)
compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
reproduce: (https://download.01.org/0day-ci/archive/20230630/202306302333.ay5rrRON-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202306302333.ay5rrRON-lkp@intel.com/

smatch warnings:
mm/mmap.c:2039 expand_downwards() warn: unsigned 'address' is never less than zero.

vim +/address +2039 mm/mmap.c

  2027	
  2028	/*
  2029	 * vma is the first one with address < vma->vm_start.  Have to extend vma.
  2030	 */
  2031	int expand_downwards(struct vm_area_struct *vma, unsigned long address)
  2032	{
  2033		struct mm_struct *mm = vma->vm_mm;
  2034		MA_STATE(mas, &mm->mm_mt, vma->vm_start, vma->vm_start);
  2035		struct vm_area_struct *prev;
  2036		int error = 0;
  2037	
  2038		address &= PAGE_MASK;
> 2039		if (address < mmap_min_addr || address < FIRST_USER_ADDRESS)
  2040			return -EPERM;
  2041	
  2042		/* Enforce stack_guard_gap */
  2043		prev = mas_prev(&mas, 0);
  2044		/* Check that both stack segments have the same anon_vma? */
  2045		if (prev && !(prev->vm_flags & VM_GROWSDOWN) &&
  2046				vma_is_accessible(prev)) {
  2047			if (address - prev->vm_end < stack_guard_gap)
  2048				return -ENOMEM;
  2049		}
  2050	
  2051		if (mas_preallocate(&mas, GFP_KERNEL))
  2052			return -ENOMEM;
  2053	
  2054		/* We must make sure the anon_vma is allocated. */
  2055		if (unlikely(anon_vma_prepare(vma))) {
  2056			mas_destroy(&mas);
  2057			return -ENOMEM;
  2058		}
  2059	
  2060		/*
  2061		 * vma->vm_start/vm_end cannot change under us because the caller
  2062		 * is required to hold the mmap_lock in read mode.  We need the
  2063		 * anon_vma lock to serialize against concurrent expand_stacks.
  2064		 */
  2065		anon_vma_lock_write(vma->anon_vma);
  2066	
  2067		/* Somebody else might have raced and expanded it already */
  2068		if (address < vma->vm_start) {
  2069			unsigned long size, grow;
  2070	
  2071			size = vma->vm_end - address;
  2072			grow = (vma->vm_start - address) >> PAGE_SHIFT;
  2073	
  2074			error = -ENOMEM;
  2075			if (grow <= vma->vm_pgoff) {
  2076				error = acct_stack_growth(vma, size, grow);
  2077				if (!error) {
  2078					/*
  2079					 * We only hold a shared mmap_lock lock here, so
  2080					 * we need to protect against concurrent vma
  2081					 * expansions.  anon_vma_lock_write() doesn't
  2082					 * help here, as we don't guarantee that all
  2083					 * growable vmas in a mm share the same root
  2084					 * anon vma.  So, we reuse mm->page_table_lock
  2085					 * to guard against concurrent vma expansions.
  2086					 */
  2087					spin_lock(&mm->page_table_lock);
  2088					if (vma->vm_flags & VM_LOCKED)
  2089						mm->locked_vm += grow;
  2090					vm_stat_account(mm, vma->vm_flags, grow);
  2091					anon_vma_interval_tree_pre_update_vma(vma);
  2092					vma->vm_start = address;
  2093					vma->vm_pgoff -= grow;
  2094					/* Overwrite old entry in mtree. */
  2095					mas_set_range(&mas, address, vma->vm_end - 1);
  2096					mas_store_prealloc(&mas, vma);
  2097					anon_vma_interval_tree_post_update_vma(vma);
  2098					spin_unlock(&mm->page_table_lock);
  2099	
  2100					perf_event_mmap(vma);
  2101				}
  2102			}
  2103		}
  2104		anon_vma_unlock_write(vma->anon_vma);
  2105		khugepaged_enter_vma(vma, vma->vm_flags);
  2106		mas_destroy(&mas);
  2107		return error;
  2108	}
  2109	

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