Re: [PATCH] mm: Enforce a minimal stack gap even against inaccessible VMAs

[Lorenzo Stoakes]

On Wed, Oct 09, 2024 at 06:45:08AM +0800, kernel test robot wrote:
> Hi Jann,
>
> kernel test robot noticed the following build errors:
>
> [auto build test ERROR on 8cf0b93919e13d1e8d4466eb4080a4c4d9d66d7b]
>
> url:    https://github.com/intel-lab-lkp/linux/commits/Jann-Horn/mm-Enforce-a-minimal-stack-gap-even-against-inaccessible-VMAs/20241008-065733
> base:   8cf0b93919e13d1e8d4466eb4080a4c4d9d66d7b
> patch link:    https://lore.kernel.org/r/20241008-stack-gap-inaccessible-v1-1-848d4d891f21%40google.com
> patch subject: [PATCH] mm: Enforce a minimal stack gap even against inaccessible VMAs
> config: parisc-randconfig-r072-20241009 (https://download.01.org/0day-ci/archive/20241009/202410090632.brLG8w0b-lkp@xxxxxxxxx/config)
> compiler: hppa-linux-gcc (GCC) 14.1.0
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20241009/202410090632.brLG8w0b-lkp@xxxxxxxxx/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@xxxxxxxxx>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202410090632.brLG8w0b-lkp@xxxxxxxxx/
>
> All errors (new ones prefixed by >>):
>
>    mm/mmap.c: In function 'expand_upwards':
> >> mm/mmap.c:1069:39: error: 'prev' undeclared (first use in this function)
>     1069 |                 if (vma_is_accessible(prev))

Suspect this is just a simple typo and should be next rather than prev :>)

>          |                                       ^~~~
>    mm/mmap.c:1069:39: note: each undeclared identifier is reported only once for each function it appears in
>
>
> vim +/prev +1069 mm/mmap.c
>
>   1036
>   1037	#if defined(CONFIG_STACK_GROWSUP)
>   1038	/*
>   1039	 * PA-RISC uses this for its stack.
>   1040	 * vma is the last one with address > vma->vm_end.  Have to extend vma.
>   1041	 */
>   1042	static int expand_upwards(struct vm_area_struct *vma, unsigned long address)
>   1043	{
>   1044		struct mm_struct *mm = vma->vm_mm;
>   1045		struct vm_area_struct *next;
>   1046		unsigned long gap_addr;
>   1047		int error = 0;
>   1048		VMA_ITERATOR(vmi, mm, vma->vm_start);
>   1049
>   1050		if (!(vma->vm_flags & VM_GROWSUP))
>   1051			return -EFAULT;
>   1052
>   1053		/* Guard against exceeding limits of the address space. */
>   1054		address &= PAGE_MASK;
>   1055		if (address >= (TASK_SIZE & PAGE_MASK))
>   1056			return -ENOMEM;
>   1057		address += PAGE_SIZE;
>   1058
>   1059		/* Enforce stack_guard_gap */
>   1060		gap_addr = address + stack_guard_gap;
>   1061
>   1062		/* Guard against overflow */
>   1063		if (gap_addr < address || gap_addr > TASK_SIZE)
>   1064			gap_addr = TASK_SIZE;
>   1065
>   1066		next = find_vma_intersection(mm, vma->vm_end, gap_addr);
>   1067		if (next && !(next->vm_flags & VM_GROWSUP)) {
>   1068			/* see comments in expand_downwards() */
> > 1069			if (vma_is_accessible(prev))
>   1070				return -ENOMEM;
>   1071			if (address == next->vm_start)
>   1072				return -ENOMEM;
>   1073		}
>   1074
>   1075		if (next)
>   1076			vma_iter_prev_range_limit(&vmi, address);
>   1077
>   1078		vma_iter_config(&vmi, vma->vm_start, address);
>   1079		if (vma_iter_prealloc(&vmi, vma))
>   1080			return -ENOMEM;
>   1081
>   1082		/* We must make sure the anon_vma is allocated. */
>   1083		if (unlikely(anon_vma_prepare(vma))) {
>   1084			vma_iter_free(&vmi);
>   1085			return -ENOMEM;
>   1086		}
>   1087
>   1088		/* Lock the VMA before expanding to prevent concurrent page faults */
>   1089		vma_start_write(vma);
>   1090		/*
>   1091		 * vma->vm_start/vm_end cannot change under us because the caller
>   1092		 * is required to hold the mmap_lock in read mode.  We need the
>   1093		 * anon_vma lock to serialize against concurrent expand_stacks.
>   1094		 */
>   1095		anon_vma_lock_write(vma->anon_vma);
>   1096
>   1097		/* Somebody else might have raced and expanded it already */
>   1098		if (address > vma->vm_end) {
>   1099			unsigned long size, grow;
>   1100
>   1101			size = address - vma->vm_start;
>   1102			grow = (address - vma->vm_end) >> PAGE_SHIFT;
>   1103
>   1104			error = -ENOMEM;
>   1105			if (vma->vm_pgoff + (size >> PAGE_SHIFT) >= vma->vm_pgoff) {
>   1106				error = acct_stack_growth(vma, size, grow);
>   1107				if (!error) {
>   1108					/*
>   1109					 * We only hold a shared mmap_lock lock here, so
>   1110					 * we need to protect against concurrent vma
>   1111					 * expansions.  anon_vma_lock_write() doesn't
>   1112					 * help here, as we don't guarantee that all
>   1113					 * growable vmas in a mm share the same root
>   1114					 * anon vma.  So, we reuse mm->page_table_lock
>   1115					 * to guard against concurrent vma expansions.
>   1116					 */
>   1117					spin_lock(&mm->page_table_lock);
>   1118					if (vma->vm_flags & VM_LOCKED)
>   1119						mm->locked_vm += grow;
>   1120					vm_stat_account(mm, vma->vm_flags, grow);
>   1121					anon_vma_interval_tree_pre_update_vma(vma);
>   1122					vma->vm_end = address;
>   1123					/* Overwrite old entry in mtree. */
>   1124					vma_iter_store(&vmi, vma);
>   1125					anon_vma_interval_tree_post_update_vma(vma);
>   1126					spin_unlock(&mm->page_table_lock);
>   1127
>   1128					perf_event_mmap(vma);
>   1129				}
>   1130			}
>   1131		}
>   1132		anon_vma_unlock_write(vma->anon_vma);
>   1133		vma_iter_free(&vmi);
>   1134		validate_mm(mm);
>   1135		return error;
>   1136	}
>   1137	#endif /* CONFIG_STACK_GROWSUP */
>   1138
>
> --
> 0-DAY CI Kernel Test Service
> https://github.com/intel/lkp-tests/wiki