Re: [PATCH 2/2] mm,fork: introduce MADV_WIPEONFORK

On Fri, 18 Aug 2017 12:28:29 -0400 Rik van Riel [off-list ref] wrote:

On Thu, 2017-08-17 at 15:50 -0700, Andrew Morton wrote:

quoted

On Tue, 15 Aug 2017 22:18:19 -0400 Rik van Riel [off-list ref]
wrote:

quoted

quoted

quoted

--- a/mm/madvise.c
+++ b/mm/madvise.c

@@ -80,6 +80,17 @@ static long madvise_behavior(struct

vm_area_struct *vma,
__		}
__		new_flags &= ~VM_DONTCOPY;
__		break;
+	case MADV_WIPEONFORK:
+		/* MADV_WIPEONFORK is only supported on
anonymous
memory. */
+		if (vma->vm_file || vma->vm_flags & VM_SHARED)
{
+			error = -EINVAL;
+			goto out;
+		}
+		new_flags |= VM_WIPEONFORK;
+		break;
+	case MADV_KEEPONFORK:
+		new_flags &= ~VM_WIPEONFORK;
+		break;
__	case MADV_DONTDUMP:
__		new_flags |= VM_DONTDUMP;
__		break;

It seems odd to permit MADV_KEEPONFORK against other-than-anon
vmas?

Given that the only way to set VM_WIPEONFORK is through
MADV_WIPEONFORK, calling MADV_KEEPONFORK on an
other-than-anon vma would be equivalent to a noop.

If new_flags == vma->vm_flags, madvise_behavior() will
immediately exit.

Yes, but calling MADV_WIPEONFORK against an other-than-anon vma is
presumably a userspace bug.____A bug which will probably result in
userspace having WIPEONFORK memory which it didn't want.____The kernel
can trivially tell userspace that it has this bug so why not do so?

Uh, what?

Braino.  I meant MADV_KEEPONFORK.  Calling MADV_KEEPONFORK against an
other-than-anon vma is a presumptive userspace bug and the kernel
should report that.